understanding oxygen sensors.docx

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Understanding oxygen sensors Since the early 1980s, oxygen sensors (O2S) and heated oxygen sensors (HO2S) have played a key role in the efficient operation of electronic fuel injected vehicles. In a modern vehicle, the powertrain control module (PCM) relies on information from the oxygen sensor to achieve optimum air/fuel ratio, good engine performance and control exhaust emissions. Understanding fundamentals of oxygen sensor operation, as well as new changes in technology, can help technicians quickly test and diagnose this increasingly important sensor. Burning gasoline in the combustion chamber of an engine is a chemical reaction with fairly predictable results. Cylinder misfire, poor engine efficiency and high exhaust emissions can be the end result of too much or too little fuel in the combustion chamber. An oxygen sensor can effectively measure these combustion results. Changes in air-to-fuel ratio affect the amount of oxygen (O2) consumed during the combustion process. The best air/fuel ratio for complete combustion and emissions is a stoichiometric 14:7:1 ratio. A rich (or excessive fuel) air/fuel ratio will consume most of the oxygen during the combustion process, resulting in low exhaust oxygen content. Leaner air/fuel ratios will result in somewhat higher exhaust oxygen content. By monitoring oxygen content of the engine exhaust, the PCM can determine the ideal air/fuel ratio and adjust fuel delivery accordingly.

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Page 1: Understanding oxygen sensors.docx

Understanding oxygen sensors

Since the early 1980s, oxygen sensors (O2S) and heated oxygen sensors (HO2S) have played a key role in the efficient operation of electronic fuel injected vehicles. In a modern vehicle, the powertrain control module (PCM) relies on information from the oxygen sensor to achieve optimum air/fuel ratio, good engine performance and

control exhaust emissions. Understanding fundamentals of oxygen sensor operation, as well as new changes in technology, can help technicians quickly test and diagnose this increasingly important sensor.

Burning gasoline in the combustion chamber of an engine is a chemical reaction with fairly predictable results. Cylinder misfire, poor engine efficiency and high exhaust emissions can be the end result of too much or too little fuel in the combustion chamber. An oxygen sensor can effectively measure these combustion results. Changes in air-to-fuel ratio affect the amount of oxygen (O2) consumed during the combustion process. The best air/fuel ratio for complete combustion and emissions is a stoichiometric 14:7:1 ratio. A rich (or excessive fuel) air/fuel ratio will consume most of the oxygen during the combustion process, resulting in low exhaust oxygen content. Leaner air/fuel ratios will result in somewhat higher exhaust oxygen content. By monitoring oxygen content of the engine exhaust, the PCM can determine the ideal air/fuel ratio and adjust fuel delivery accordingly.

 

Oxygen sensors are typically located in the exhaust manifold and/or exhaust system. While earlier fuel injection systems used one or possibly two oxygen sensors, on-board diagnostics II (OBD-II) system emission regulations have warranted the use of multiple oxygen sensors on most vehicles. OBD-II vehicles typically have at least one oxygen sensor located ahead of the catalytic converter (upstream) and an additional sensor located just after the catalyst (downstream).

Using upstream and downstream oxygen sensors enables the PCM to measure efficiency of both engine combustion and catalyst operation.

Vehicles with dual exhaust systems may also have pre- and post-catalyst oxygen sensors for each bank of engine cylinders. The exact placement and number of oxygen sensors varies with engine configuration, vehicle design and manufacturer.

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One of the most common types of oxygen sensors is the zirconium dioxide oxygen sensor. The O2 sensing component uses a solid-state electrolyte made up of a zirconic ceramic material that acts like a galvanic battery electrolyte under certain conditions. When the sensing element is cold, the zirconia material behaves similar to an insulator. At elevated temperatures, the zirconia material performs more like a semiconductor, and can generate a characteristic voltage output on the sensor connections.

In construction of the zirconia sensing element, a porous platinum electrode material covers the inner and outer surfaces of the zirconia solid-state electrolyte. The inner surface of the sensing element is exposed to an outside air reference, while hot gases in the exhaust stream surround the sensor's outer portion. Oxygen content of outside air is approximately 21 percent, while exhaust gases have much lower oxygen content - between 1 percent and 3 percent.

Differences in the two oxygen levels, and the electrolytic properties existing between the two platinum electrodes, allow ion transfer to take place and generate a small electrical charge. Oxygen ions are electrically charged particles that flow through the zirconia sensing element when there is a disparity in oxygen levels. The greater the ion flow, the higher the voltage produced. Once the zirconia sensor element reaches an operating temperature of 572 degrees Fahrenheit to 680 degrees Fahrenheit, signal voltage output can range from near zero to 1 volt -

depending on the oxygen content of the exhaust gases.

Basically, the zirconium O2 sensor compares the oxygen content of exhaust gases with oxygen from outside air. Voltage produced by the O2 sensor depends on the amount of oxygen in the exhaust. If exhaust oxygen content is low, such as a rich air/fuel ratio, the voltage output from the sensor may be as high as 1 volt. A lean air/fuel ratio increases the exhaust oxygen content, resulting in a low voltage from the sensor.

In normal operation, O2 signal voltage is routinely varying from almost zero to 1 volt. An O2 sensor signal voltage above approximately 0.45 volts is recognized by the PCM as a rich exhaust; below 0.45 volts as a lean exhaust. The goal of the PCM is to keep O2 voltage moving across the 0.45 volt rich/lean switch point for optimum fuel efficiency and emissions.

The PCM will set an O2 sensor diagnostic code if the sensor does not produce a voltage signal, stays rich too long, stays lean

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too long, does not switch rich/lean (center too long), or does not switch rich/lean fast enough. OBD-II vehicles may also run PCM diagnostic tests called monitors, which compare and analyze sensor readings to verify proper component operation.

Since OBD-II vehicles may have multiple oxygen sensors located some distance from the engine exhaust ports, these sensors are generally heated to speed the warm-up time period. The HO2S incorporates an internal electric heating element to bring the O2 sensor up to operating temperature quickly (under 35 seconds). Internal heating elements usually operate continuously while the engine is running to maintain an operating temperature of approximately 1292 degrees Fahrenheit to 1472 degrees Fahrenheit. Heated O2 sensors operate at a more consistent temperature and allow greater flexibility of placement locations in the exhaust system.

 

There are three common methods of controlling the heating element in oxygen sensors. The first method provides a power source to the heater from the ignition switch or a relay anytime the ignition is turned to the run position. This method was used on many pre-OBD-II vehicles without heater diagnostics. A second method supplies power to the heater through a PCM controlled relay. By controlling the heater power relay with the PCM, the circuit can be checked during key-off/engine-off periods. The third method is limited to newer vehicles equipped with Fast Light Off (FLO) oxygen sensors. These sensors have a larger heater for quick sensor warm-up and are current flow limited through the PCM. Note that due to heater design and current draw differences, FLO oxygen sensors cannot be interchanged with other types. Inside the PCM is a switching transistor that pulse-width modulates the power supply, thus controlling current flow in the heater circuit. Using this type of PCM control, the FLO oxygen sensors can reach full operating temperature in as little as five seconds after startup.

One aspect of OBD-II vehicle diagnostics is the ability of the PCM to periodically test the HO2S for possible heater failure. As the name implies, the HO2S heater monitor (or test) is used to check the operation of the internal heater. Because the O2 sensor may be warmed by exhaust with the engine operating, a PCM actuated heater monitor typically runs after a predetermined ignition key-off/engine-off period. Specific enabling factors for this monitor may vary between manufacturers. When the heater monitor is running, the PCM measures the internal resistance of the sensor element as it heats up. Remember, the zirconia material changes conductivity with temperature. By energizing the HO2S heater element, and simultaneously monitoring the sensor signal circuit, the PCM should see the internal resistance of the sensor signal circuit go down as the temperature increases. This monitor fundamentally checks the

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integrity of the O2 heater element and its related circuits, as well as the O2 sensor signal circuit.

Zirconia oxygen sensors can have one, two, three or four wires depending on the vehicle application. One or two wire O2 sensors are not electrically heated and will have a signal wire and possibly a ground wire. Heated O2 sensors usually have three or four wires: two wires for the heating element, one signal wire and possibly a ground wire. An electrical wiring schematic can be helpful to positively identify connector pin locations and wire colors. Oxygen sensors that are not equipped with a ground wire must have a well-grounded exhaust system to complete the sensing circuit. Basic electrical wiring circuit checks should be made to determine if the vehicle's wiring harness has good continuity and is free from short circuits.

Testing and diagnosis of the O2 sensor heater and circuit is a relatively simple task. Most heaters are a positive temperature coefficient element, meaning the resistance will go up as the element heats up. In normal operation, the increased resistance of a hot sensor will naturally limit current flow in the circuit. Use a digital multimeter to check the sensor heater element for continuity. Exact heater resistance specifications may vary depending on the vehicle and sensor location. Heater element resistance should be about 4 to 7 ohms for a sensor at ambient temperature. Expect somewhat lower resistance values for FLO type sensors.

Another fundamental diagnostic test is checking the vehicle's wiring harness for power and ground to the O2 sensor heater. Take into account that PCM-controlled heater circuits may require the engine running before the circuit will be powered up. Connecting a 12-volt test light between the power supply and ground can determine if the heater circuit is operational. Some technicians prefer to check heater circuit current flow using an ammeter. To measure current flow, connect a digital multimeter in series between the vehicle wiring harness and the sensor heater. This testing method ensures proper heater and circuit performance throughout the temperature range of the sensor. Amperage values can be from approximately 1.5 amps at ambient temperature to 200 milliamps at full operating temperature.

If engine performance checks or scan tool data suggest a potential O2 sensor malfunction, don't forget the basics. Always conduct a visual inspection of the sensor, electrical connections and wires. Electrical connections should be clean and tight. Be sure the wiring harness is routed away from high voltage ignition components and hot exhaust pipes. Inspect the exhaust system for leaks or holes that may affect the O2 sensor readings. The O2 sensor measures oxygen, so a tightly sealed exhaust system is very important for accurate sensor readings. Determining if the O2 sensor is truly defective or another electronic engine control component has failed is perhaps the most difficult obstacle for an accurate diagnosis. Careful review of scan tool

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information and sensor readings can usually identify any other problems or conditions.

 

David W. Gilbert is an Associate Professor of Automotive Technology at Southern Illinois University Carbondale. He holds a Master of Science degree from Oklahoma State University and is also ASE certified as a master automotive technician, master engine machinist and advanced engine performance specialist (L1).

Understanding CAT - Catalytic Converters

Catalytic Converters:Regardless of how perfect the engine is operating, there will always be some harmful by-products of combustion. This is what necessitates the use of a Three-Way Catalytic (TWC) Converter. This device is located in-line with the exhaust system and is used to cause a desirable chemical reaction to take place in the exhaust flow. Essentially, the catalytic converter is used to complete the oxidation process for hydrocarbon (HC) and carbon monoxide (CO), in addition to reducing oxides of nitrogen (NOx) back to simple nitrogen and carbon dioxide. TWC Construction: Two different types of Three-Way Catalytic Converters have been used on fuel injected vehicles. Some early EFI vehicles used a pelletized TWC that was constructed of catalyst coated pellets tightly packed in a sealed shell, while later model vehicles are equipped with a monolith type TWC that uses a honeycomb shaped catalyst element. While both types operate similarly, the monolith design creates less exhaust backpressure, while providing ample surface area to efficiently convert feed gases. 

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The diagram above shows the chemical reaction that takes place inside the converter. The Three-Way Catalyst, which is responsible for performing the actual feed gas conversion, is created by coating the internal converter substrate with the following key materials: o Platinum/Palladium; Oxidizing catalysts for HC and CO o Rhodium; Reducing catalyst for NOx o Cerium; Promotes oxygen storage to improve oxidation efficiency 

Three Way CAT Operation:

As engine exhaust gases flow through the converter passageways, they contact the coated surface which initiate the catalytic process. As exhaust and catalyst temperatures rise, the following reaction occurs: o Oxides of nitrogen ( NOx) are reduced into simple nitrogen (N2) and carbon dioxide (CO2) o Hydrocarbons (HC) and carbon monoxide (CO) are oxidized to create water (H2O) and carbon dioxide (CO2) Catalyst operating efficiency is greatly affected by two factors; operating temperature and feed gas composition. The catalyst begins to operate at around 550' F.(300' C.). However, efficient purification does not take place until the catalyst reaches at least 415'C. (750' F.) . Also, the converter feed gasses (engine-out exhaust gases) must alternate rapidly between high CO content, to reduce NOx emissions, and high O2 content, to oxidize HC and CO emissions. Effects of Closed Loop Control on Three Way CAT Operation: To ensure that the catalytic converter has the feed gas composition it needs, the closed loop control system is designed to rapidly alternate the air/fuel ratio slightly

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rich, then slightly lean of Stoichiometric. By doing this, the carbon monoxide and oxygen content of the exhaust gas also alternates with the air/fuel ratio. In short, the converter works as follows:     o When the A/F ratio is leaner than Stoichiometric, the oxygen content of the exhaust stream rises and the carbon monoxide content falls. This provides a high efficiency operating environment for the oxidizing catalysts (platinum and palladium). During this lean cycle, the catalyst (by using cerium) also stores excess oxygen which will be released to promote better oxidation during the rich cycle.     o When the A/F ratio is richer than Stoichiometric, the carbon monoxide content of the exhaust rises and the oxygen content falls. This provides a high efficiency operating environment for the reducing catalyst (rhodium). The oxidizing catalyst maintains its efficiency as stored oxygen is released. Precise closed loop control relies on accurate feedback information provided from the exhaust oxygen sensor. The sensor acts like a switch as the air/fuel ratio passes through stoichiometry. Closed loop fuel control effectively satisfies the three way catalyst's requirement for ample supplies of both carbon monoxide and oxygen. Generally speaking, if the closed loop control system is functioning normally, and fuel trim is relatively neutral, you can be assured that the air induction and fuel delivery sub-systems are also operating normally. If the closed loop control system is not working properly, the impact on catalytic converter efficiency, and ultimately emissions, can be significant. Effects of (the exhaust) Oxygen Sensor Degradation:Since the oxygen sensor is the heart of the closed loop control system, proper operation is critical to efficient emission control. There are several factors which can cause the oxygen sensor signal to degrade and they include the following:     o Silicon contamination from chemical additives, some RTV sealers, and contaminated fuel. o Lead contamination can be found in certain additives and leaded motor fuels.     o Carbon contamination is caused by excessive short trip driving and/or malfunctions resulting in an excessively rich mixture.The effects of sensor degradation can range from a subtle shift in air/fuel ratio to a totally inoperative closed loop system. With respect to drivability and emissions diagnosis, a silicon contaminated sensor will cause the most trouble. When silicon burns in the combustion chamber, it causes a silicon dioxide glaze to form on the oxygen sensor. This glaze causes the sensor to become sluggish when switching from rich to lean, and in some cases, increases the sensor minimum voltage on the lean switch. This causes the fuel system to spend excessive time delivering a lean mixture. 

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It is often difficult to identify a sensor which is marginally degraded, and in many cases, vehicle drivability may not be effected significantly. With the advent of MOT emissions testing however, marginal sensor degradation may cause some vehicles to fail the NOx portion of the loaded mode test. The impact of a slightly lean mixture has a dual effect on emissions. A leaner mixture means higher combustion temperatures so more NOx is produced during combustion. Additionally, because less carbon monoxide is available in catalyst feed gas, the reducing catalyst efficiency falls off dramatically. The end result is a vehicle which may fail an MOT test for excessive NOx. As previously mentioned, the O2S signal voltage must fluctuate above and below 0.45 volts at least 8 times in 10 seconds at 2500 rpm with the engine at operating temperature. During the rich swing, voltage should exceed 550 mv and during the lean swing should fall below 400 mv. O2S signal checks can be made using the Autoprobe feature of the Diagnostic Tester, digital multimeter, or 02S/RPM check using the Diagnostic Tester. Refer back to the oxygen sensor tests in the closed loop control section for specific test procedures. Effects of CAT Degradation:Now that we understand the effects of O2S degradation on catalyst efficiency, let's look at the effects of a catalytic converter failure. Keep in mind, there are many different factors that can cause its demise.    o Poor engine performance as a result of a restricted converter. Symptoms of a restricted converter include; loss of power at higher engine speeds, hard to start, poor acceleration and fuel economy.    o A red hot converter indicates exposure to raw fuel causing the substrate to overheat. This symptom is usually caused by an excessive rich air/fuel mixture or engine misfire. If the problem is not corrected, the substrate may melt, resulting in a restricted converter.    o Rotten egg odor results from excessive hydrogen sulfide production and is typically caused by high fuel sulfur content or air/fuel mixture imbalance. If the problem is severe and not corrected, converter meltdown and/or restriction may result.    o MOT emission test failure may occur if catalyst performance falls below its designed efficiency level. Perform additional tests to confirm that the problem is in fact converter efficiency and not the result of engine or emission sub-system failure. Never use an emission test failure as the only factor in replacing a catalytic converter! If you do, you may not be fixing the actual cause of the emission failure. Causes of CAT Contamination:Like the oxygen sensor, the most common cause of catalytic converter failure is contamination. Examples of converter contaminants include:

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    o Overly rich air/fuel mixtures will cause the converter to overheat causing substrate meltdown.    o Leaded fuels, even as little as one tank full, may coat the catalyst element and render the converter useless.    o Silicone from sealants (RTV, etc.) or engine coolant that has leaked into the exhaust, may also coat the catalyst and render it useless. There are other external factors that can cause the converter to degrade and require replacement. Thermal shock occurs when a hot converter is quickly exposed to cold temperature (snow, cold fuel, etc.), causing it to physically distort and eventually disintegrate. Converters that have sustained physical damage (seam cracks, shell puncture, etc.) should also be replaced as necessary. 

CAT Functional Checks:

Before a converter is condemned and replaced, it is crucial that any problem(s) that may have contributed to the damage and failure of the converter is identified and repaired. If not, the replacement converter will soon fail! Also, in order to accurately check catalytic converters, all engine mechanical, engine control systems, and emission sub-systems must be in proper working order or your results will be inaccurate. Remember, the converter relies on a narrow feed gas margin or efficiency suffers.There are a number of tests that can be performed on catalytic converters; however, no one test should be used to verify the complete integrity and conversion efficiency of the converter. The following are examples of typical TWC checks: Visual Inspection:The first check, and the easiest, is to perform a thorough visual inspection of the converter and related hardware. Many converter problems have obvious symptoms that are easily identified during a visual inspection. Look for the following; pinched exhaust pipe, physical damage to the insulator or converter shell, cracked or broken seams, excessive rust damage, mud or ice in the tailpipe, etc. Rattle Test:Perform a rattle test by firmly hitting the converter shell with the center of your palm (avoid hitting it too hard or you may damage it!) If the substrate is OK it should sound solid. If it rattles, the substrate has disintegrated and the converter should be replaced. Use Pre and Post CAT Emissions to determine the CAT Efficiency

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 Follow the Pro-Gas analyser "in program"  instructions to conduct the two (pre & post cat) tests and establish the CAT Efficiency! You will have to have (or make) access to the exhaust gases "before" and "after" the CAT!NOTE: Before any catalyst efficiency tests are performed, it is important that both the engine and converter are properly preconditioned. Remember, proper feed gas conversion cannot take place until the closed loop control system is actively maintaining ideal mixture and the catalyst has reached operating temperature. To ensure these conditions are met, particularly during cold ambient conditions, operate the engine off-idle until the CAT is sufficiently heated. This will ensure optimal catalyst conversion efficiency. Restricted Exhaust System Check: Drivability comments like "lacks power under load" or "difficult to start, acts flooded and also lacks power" may indicate a restricted exhaust. In extreme cases the exhaust may be so restrictive that the engine will not start. Generally speaking, here's how to test for a restricted exhaust system:o Attach a vacuum gauge to an intake manifold vacuum source.o Allow the engine to reach operating temperature.o From idle, raise engine speed to approximately 2000 rpm.o Note: The vacuum reading should be close to normal idle reading.o Next, quickly release the throttle.Note: The vacuum reading should momentarily rise then smoothly drop back to a normal idle reading. If the vacuum rises slowly or does not quickly return to normal level, the exhaust system may be restricted. If the catalyst has disintegrated, it is likely that contamination has also restricted the muffler. Don't overlook that possibility. If the engine will not start, try disconnecting the exhaust system at the manifold and see if the engine will start. Lead Contamination Check: A common cause of converter contamination is lead poisoning. As mentioned, lead reduces converter efficiency by coating the catalyst element. Special lead detecting test paper (or paste) is available from aftermarket suppliers that checks for the presence of lead in the tailpipe. Follow the specific instructions provided by the test paper manufacturer. 

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CAT Efficiency Quick Check (Some Vehicles)

On some vehicles equipped with sub-O2 sensors, a quick check of TWC operation can be made by comparing the signal activity of the main oxygen sensor with the sub-oxygen sensor. Since the main O2S in located upstream of the converter and the sub-O2S is located downstream, a signal comparison would indicate whether a catalytic reaction is taking place inside the converter. If the catalyst is operating, the main O2S signal should normally toggle rich/lean, while the sub-O2 sensor should react very slowly (similar to a bad main O2S signal.) Main and sub O2S signals can be observed using the graphing display of the Diagnostic Tester (OBD-II) or a Dual Channel Scope.

NOTE: Before any catalyst efficiency tests are performed, it is important that both the engine and converter are properly preconditioned. Remember, proper feed gas conversion cannot take place until the closed loop control system is actively maintaining ideal mixture and the catalyst has reached operating temperature. To ensure these conditions are met, particularly during cold ambient conditions, operate the engine off-idle until the CAT is sufficiently heated. This will ensure optimal catalyst conversion efficiency. 

Pre-Catalyst Versus Post-Catalyst Testing

When using an exhaust analyzer as a diagnostic tool, it is important to remember that

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combustion takes place twice before reaching the tailpipe. First, primary combustion takes place in the engine. This determines the composition of catalyst feed gas, which dramatically effects catalyst efficiency. When the exhaust gases reach the three-way catalytic converter, two chemical processes occur.Catalyst Reduction:First, nitrogen oxide gives up its oxygen. This only occurs when a sufficient amount of carbon monoxide is available for the oxygen to bond with. This chemical reaction results in reduction of nitrogen oxide to pure nitrogen and oxidation of the carbon monoxide to form carbon dioxide.Catalyst Oxidation:Second, hydrocarbon and carbon monoxide continue to burn. This occurs only if there a sufficient amount of oxygen available for the hydrogen and carbon to bond with. This chemical reaction results in oxidation of hydrogen and carbon to form water vapor (H2O) and carbon dioxide (CO2). 

Examples of Deceiving Post-Catalytic Analysis

When troubleshooting an emissions failure, your primary concern will be what comes out of the tailpipe. In other words, it doesn't matter whether the efficient burn occurred in the engine or the catalyst. However, when troubleshooting a driveability concern, the catalytic converter may mask important diagnostic clues which can be gathered with your exhaust analyzer. The following are examples of situations where post-catalyst reading may be deceiving.

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     o Example 1: A minor misfire under load is causing a vehicle to surge. The exhaust gas from the engine would show an increase in HC and O2, and a reduction in CO2. However, once this exhaust gas reaches the catalytic converter, especially a relatively new and efficient catalyst, the oxidation process will continue. The excess HC will be oxidized, causing HC and O2 to fall, and CO2 to increase. At the tailpipe, the exhaust readings may look perfectly normal. In this example, it is interesting to note that NOx readings will increase because of the reduced carbon monoxide and increased oxygen levels in the catalyst feed gas. This could only be detected with a five gas analyzer. 

    o Example 2: A small exhaust leak upstream of the exhaust oxygen sensor is causing a false lean indication to the ECM. This resulted in excessively rich fuel delivery to bring oxygen sensor voltage back to normal operating range. The customer concern is a sudden decrease of 20% in fuel economy. 

    o Example 3: A restriction in the fuel return line elevates pressure causing an excessively rich air/fuel ratio and a 20% decrease in fuel economy. Although carbon monoxide emissions from the engine are elevated as a result of this rich air/fuel ratio, the catalytic converter is able to oxidize most of it into carbon dioxide. The resulting

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tailpipe readings appear to be normal, except for Oxygen, which is extremely low for two reasons. First, the increase in CO caused a proportionate decrease in O2 in the converter feed gas. Second, the little oxygen left over was totally consumed oxidizing the CO into CO2.Based on this example, you can see that Oxygen is a better indicator of lean or rich air/fuel ratios than carbon monoxide when testing Post Catalytic Converters.

 

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Five Gas Exhaust Analysis Theory

             Use of a four or five Gas Exhaust Analyser can be helpful in troubleshooting both emissions and driveability concerns. Presently, shop grade analysers are capable of measuring from as few as two exhaust gasses, HC and CO, to as many as five. The five gasses measured (for petrol emissions) by the latest technology exhaust analysers are: HC, CO, CO2, O2 and NOx. All five of these gasses, especially O2 and CO2, are excellent troubleshooting tools. Use of an exhaust gas analyser will allow you to narrow down the potential cause of driveability and emissions concerns, focus your troubleshooting tests in the area(s) most likely to be causing the concern, and save diagnostic time. In addition to helping you focus your troubleshooting, an exhaust gas analyzer also gives you the ability to measure the effectiveness of repairs by comparing before and after exhaust readings. In troubleshooting, always remember the combustion chemistry equation: Fuel (hydrogen, carbon, sulphur) + Air (nitrogen, oxygen) = Carbon dioxide + water vapour + oxygen + carbon monoxide + hydrocarbon + oxides of nitrogen + sulphur oxides.

             When we do exhaust analysis, we are being a detectives. We look at what came out of the exhaust and figure out what could have happened before to create those emissions. What happened in the combustion chamber, or before the combustion chamber, to create these results?

      We can use clues and patterns of exhaust readings to figure out if we have a problem in one of the following areas:

Air/Fuel Ratio Combustion Ignition Emission Control Devices

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       Then we know where to start our diagnosis with visual and functional tests. If we know that the combustion in our engine is OK and efficient, there isn't much left to worry about. But how do we know good combustion from a bad one? Let's find out...

Complete (Good) Combustion:

       Let's start by reviewing good combustion. The idea is to properly burn up all the petrol and not have any "leftovers". Into the combustion chamber we put petrol, symbolized by 'HC' for hydrocarbons. These are combinations of hydrogen and carbon atoms. We also add lots of air, which contains oxygen, symbolized by 'O2'. Normal air is about 20.7% oxygen, and if your workshop gas analyser doesn't show about this when reading the air inside your shop, you could have a bad oxygen sensor in your gas analyser( those are chemical sensors and have expected life of about one year), or a serious problem with the air in your shop, or the planet has a problem... Back to combustion. The air we add to the combustion chamber is mainly nitrogen, about 78%. (No, that's not nitrous, but related.) This doesn't burn, it just goes along for the ride and expands with the heat, helping to push down the piston.

Coming out of the combustion chamber we have carbon dioxide, water and nitrogen. The carbon dioxide is symbolized CO2. (One carbon atom combined with two oxygen atoms) It's good, in that plants like it and it doesn't hurt us, but is blamed too much for global warming. The water is symbolized by H2O, two hydrogen atoms combined with one oxygen atom. Did you realize that for every gallon of petrol we burn, the tailpipe puts out about a gallon of water? And then good combustion also puts out all the nitrogen that came in.

Good combustion is simply put this way: HC + O2 + N2 = H2O + CO2 + N2. Ideally, what we want is to convert all the Fuel and Air that enters the engine in to Water and Nitrogen!

We want an ideal mixture of 14.7 pounds of air to 1 pound of gasoline for the cleanest burning. (14.7:1 Stoichiometric ratio, is the air to fuel ratio at which there is just enough air to burn certain amount of fuel completely.)

There are a few other exhaust components which impact driveability and/or emissions diagnosis, that are not measured by workshop Gas analyzers. They are:

Water vapour (H2O) Sulphur Dioxide (SO2) Hydrogen (HO) Particulate carbon soot (C)

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Sulphur dioxide (SO2) is sometimes created during the combustion process from the small amount of sulphur present in gasoline. During certain conditions the catalyst oxidizes sulphur dioxide to make SO3, which then reacts with water to make H2SO4 or sulphuric acid. Finally, when sulphur and hydrogen react, it forms hydrogen sulphide gas. This process creates the rotten egg odour you sometimes smell when following vehicles on the highway. Particulate carbon soot is the visible black "smoke you see from the tailpipe of a vehicle that's running very rich.

Incomplete ( BAD) Combustion:

Now for Bad Combustion. This is where the wrong things happen, and the by-products of combustion produce gases which contribute to air pollution or other problems. The Complete Combustion is impossible to obtain even with the best tuned engines! So in practice we are left with Incomplete Combustion and an obsolete engine design. ( When was it invented,.... anyone? too young to remember...) One example of Incomplete Combustion is raw gasoline (HC) which goes in, then comes out, and isn't burnt up in the process. Another example is Carbon Monoxide (CO). It doesn't create smog, but it's deadly, so you don't want it around. A third example is Oxides of Nitrogen (NOx). It helps create out brown smog. These are all a problem and we are soon going to talk about them in more detail. But first, look at what it takes to create photochemical smog:

HC + NOx + Still air + Sunlight = Smog. Get the idea? The HC and NOx are what it takes to create smog, so if we prevent them from coming out of the tailpipe, we can cut down on the smog.

In any diagnosis of emission or driveability related concern, ask yourself the following questions:

What is the symptom? What are the "baseline" exhaust readings? At idle, 2500 rpm, acceleration, deceleration, light

load cruise, etc. Which sub-system(s) or component(s) could cause the combination of exhaust gas readings

measured?

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The following major factors contribute to the overall increase in exhaust emissions levels and degraded vehicle driveability:

Lack of scheduled maintenance:1. Sub-system failures2. Combination of multiple marginal sub-systems

Tampering with the engine emissions system and sub-systems ( control unit, sensors, actuators etc. etc.)

1. Removal of emissions sub-system equipment2. Modification of engine/emissions sub-systems3. Use of leaded fuels or incompatible additives in closed loop control systems

A word to all engine tune-up boys out there. When tuning a ordinary modern engine always keep in mind that the factory that produce it already spend millions to get it right! Your best bet is to bring it back to factory specifications, or waste your time and effort trying to "over tune" it! Believe me, it's a long and lonely road with few and far in between real rewards.

Gas Analyser   Measurements:

Print this picture and stick it next to your gas analyser. It makes life easier ;-))

Look closely at this diagram. It represents the Exhaust Gases relationship to the Air/Fuel ratio that enters the engine and the Power output of the engine. As they say a picture says a thousand words. Ideal Air/Fuel Ratio is 14.71/1 (for petrol) and not consequently this is where the CO2 is at about it's highest percentage and the HC at it's lowest. The Power graph highest point is also almost there, so these should be your guide lines. Also notice that the CO and O2 have almost the same (low) values at the "ideal" point ( the gray area on the diagram). Don't forget that all gases relate to one another so if one is out of range it will reflect on the others too.

So to recap: The highest CO2 with CO and O2 with same low values and the lowest HC you can get and you have almost the perfect picture...pardon, engine combustion! Enjoy!

 

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We need to know   what the Gas Analyser measures .

These are the gases that the 4 or 5-gas Gas analyser sees in a petrol engine:

HC = Hydrocarbons, concentration of the exhaust in parts per million (ppm). = Unburned Petrol, represents the amount of unburned fuel due to incomplete combustion exiting through the exhaust. This is a necessary evil. We don't want it so try to keep it as low as possible. An approximate relationship between the percentage of wasted fuel through incomplete combustion and the ppm of HC is about 1/200 ( 1.0% partially burned fuel produces 200 ppm HC, 10%=2000 ppm HC, 0.1%=20 ppm HC )

CO = Carbon Oxide, concentration of the exhaust in percent of the total sample. = Partially Burned Petrol, This is the petrol that has combusted, but not completely. This gas is formed in the cylinders when there is incomplete combustion and an excess of fuel. Therefore excessive CO contents are always a sign of an overly rich mixture preparation. ( The CO should have become CO2 but did not have the time or enough O2 to became real CO2 so it is exhausted as CO instead.) CO is HIGHLY POISONOUS ODORLESS GAS! Always work in well ventilated areas!

CO2 = Carbon Dioxide, concentration of the exhaust in percent of the total sample. = Completely Burned Petrol, represents how well the air/fuel mixture is burned in the engine ( efficiency ). This gas gives a direct indication of combustion efficiency. It is generally 1-2% higher at 2500 RPM than at idle. This is due to improved gas flow resulting in better combustion efficiency. Maximum is around 16%. At night the trees convert CO2 in to Oxygen. Preserve them!

O2 = Oxygen, concentration of the exhaust in percent of the total sample. Free O2 occurs in the exhaust when there is an excess of air in the mixture. The O2 content increases sharply as soon as Lambda rises above 1. Taken with the CO2 maximum, the oxygen content is a clear indicator of the transition from rich to lean mixture range, or leaks in the manifold or exhaust systems or combustion failures. With rich mixture most of the oxygen is burned during combustion. Whit very lean mixture more O2 escapes "un-combusted" so the level rises.

NOx = Oxides of Nitrogen (This is only seen by a 5-gas analyser) Only seen with dynamometer or engine under load. NOx emissions rise and fall in a reverse pattern to HC emissions. As the mixture becomes leaner more of the HC's are burnt, but at high temperatures and pressures (under load) in the combustion chamber there will be excess O2 molecules which combine with

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the nitrogen to create NOx. NOx increases in proportion to the ignition timing advance, irrespective of variations in A/F ratio. This gas is related to the exhaust gas detoxification systems ( in conjunction with Co and HC) , exhaust gas recirculation systems. Those systems bring some of the inert (processed) exhaust gas back in to the engine to be burned again. This time around this gas has no O2 extra molecules and prevents high combustion temperatures and further increase in NOx formation. NOx is Very Dangerous Lethal Gas and air pollutant!

A/F ratio or Lambda = Calculated Air/Fuel Ratio or Lambda value based on the HC, CO, CO2 and O2 concentrations. Remember the ideal (Stoichiometric) A/F is 14.7 liters air to 1 liter fuel or 14.7/1. The ideal Lambda value is 1(one) below that the A/F mixture is rich and above - lean. For example, lambda=0.8 corresponds to an air/fuel ratio of (0.8x14.7):1=11.76:1 ( e.g. lambda 0.8 = A/F ratio of 11.76/1 or very rich air fuel mixture )

General Rules of Emission Analysis

If CO goes up, O2 goes down, and conversely if O2 goes up, CO goes down. Remember, CO readings are an indicator of a rich running engine and O2 readings are an indicator of a lean running engine.

If HC increases as a result of a lean misfire, O2 will also increase CO2 will decrease in any of the above cases because of an air/fuel imbalance or misfire An increase in CO does not necessarily mean there will be an increase in HC. Additional HC will

only be created at the point where rich misfire begins (3% to 4% CO) High HC, low CO, and high O2 at same time indicates a misfire due to lean or EGR diluted

mixture High HC, high CO, and high O2 at same time indicates a misfire due to excessively rich mixture. High HC, Normal to marginally low CO, high O2, indicates a misfire due to a mechanical engine

problem or ignition misfire Normal to marginally high HC, Normal to marginally low CO, and high O2 indicates a misfire due

to false air or marginally lean mixture

Evaporative Emissions

       Up to now, we've only discussed the creation and causes of tailpipe or exhaust emission output. However, it should be noted that hydrocarbon (HC) emissions come from the tailpipe, as well as other evaporative sources, like the crankcase, fuel tank and evaporative emissions recovery system. In fact, studies indicate that as much as 20% of all HC emissions from automobiles comes from the fuel tank and carburettor (on carburetted vehicle, of course). Because hydrocarbon emissions are Volatile Organic Compounds (VOCs) which contribute to smog production, it is just as important that evaporative emission controls are in as good a working order as combustion emission controls. Fuel injected vehicles use an evaporative emissions system to store fuel vapours from the fuel tank and burn them in the engine when it is

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running. When this system is in good operating order, fuel vapour cannot escape from the vehicle unless the fuel cap is removed.

And finally remember: In nature nothing is lost or gained, only converted! Same rule applies to emission analysis.

 

Air to Fuel Ratio ( AFR )

This is the relationship ratio between the Air needed to completely burn 1 litre of Fuel in the combustion process. It's also called Stoichiometric ratio. In other words, how

many litres of Air is needed to completely burn 1 litre of Fuel during the engine combustion. 

The Relationship between Lambda and A/F ratio:

Because Lambda = 1.000 when the oxygen and combustibles are in perfect Stoichiometric balance, Lambda can easily be used to calculate A/F Ratio for

particular fuels. The actual A/F ratio is simply the calculated Lambda times the Stoichiometric A/F ratio for the specific fuel used. (14.71 for petrol). This method is

far superior to other approaches which use only one gas (CO or Oxygen) to approximate A/F ratio, as this method uses all of the oxygen and carbon-bearing gases to calculate the ratio of air to fuel. Now, bear in mind that the fuel quality is extremely important and very closely involved in both methods of calculating AFR and Lambda!

Due to the fact that fuels differ in composition from country to country (even from source to source) you may find that different gas analysers will yield different AFR Lambda results! For the results to be correct, one has to know the real fuel composition used in it's country (as a standard - Hydrogen, Carbon and Oxygen fractions), and then have a way to change this in the equation the gas analyser uses. Different fuels (Petrol, CNG, LPG, Diesel etc.etc.) have different ratios. Too bad if your gas analyser simply don't know about South African fuels!

There are of coarse, straight forward (without having to calculate Lambda first) methods of calculating AFR, one of which is using the Spindt formula published by the S.A.E. in the USA. Spindt method of calculating AFR is as complex as the Brettshneider method of calculating Lambda. Both methods ( first Lambda and then AFR, or straight AFR ) yield very similar results, so it really doesn't matter much which one your gas analyser uses, as long as it's done properly.

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 Providing a uniform method to relate the specific exhaust gas constituents to Air/Fuel balance (independent of the quality of the combustion process or the power produced) makes the engine tuner’s job much easier and easier to understand as well. Calculated Lambda value is based on the measurement of HC, CO, CO2, O2, NOx and other contents concentrations ( see Lambda for more info...).

Remember the ideal (Stoichiometric) A/F Ratio is 14.71 litres air to 1 litre fuel or 14.71/1. (for petrol only, other fuels have different values!) The ideal Lambda value is 1.000 (one) below that the A/F mixture is rich and above - lean.

For example, lambda=0.8 corresponds to an Air/Fuel Ratio of (0.8x14.7) to 1= 11.76 to 1 ( e.g. lambda 0.8 = A/F ratio of 11.76 to 1 or very rich air/fuel mixture ) Low Lambda = Low AFR = Rich Mixture

Please Remember that Lambda=1 and AFR=14.71/1 are ONLY TRUE  when ALL Gas Values are Within Specs!!!

You may get Lambda=1 even if say HC=1530 and CO=4.5! Lambda and AFR are Air Fuel balance, they DO NOT represent Correctly Burning A/F Mixture! CEF ( Engine Combustion Efficiency), however is another story. That is why we need to understand ALL gases and only make use of Lambda and AFR as confirmation points.  

 

 

Lambda

  Oxygen/Combustibles balance ( Lambda ), is calculated from the measured values of O2, CO, CO2, HC, NOx and Water Vapour in the exhaust gas. This is a direct measurement of Air/Fuel ratio, and may be easily used to assess fuel mixture balance. The Lambda calculation compares all of the Oxygen in the exhaust gases to all of the Carbon and Hydrogen in the gases. ( Water,

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which contains both Hydrogen and Oxygen, is determined by estimation using the fraction of the sum of CO to CO2 in the exhaust.)

The result of the calculation is ‘Lambda’, a dimensionless term that relates nicely to the Stoichiometric value of air to fuel. At the Stoichiometric point, Lambda = 1.000. A Lambda value of 1.050 is 5.0% lean, and a Lambda value of 0.950 is 5.0% rich.  Once Lambda is calculated, A/F Ratio can be easily determined by simply multiplying Lambda times the Stoichiometric A/F ratio for the fuel used - e.g. 14.71 for petrol - gasoline.

Details of the Lambda Calculation:

The Brettshneider equation is the de-facto standard method used to calculate the normalised Air/Fuel Balance (Lambda) for domestic and International Inspection Programs. It's derived from a paper written by Dr. J. Brettshneider in 1979. He established a method to calculate Lambda ( Balance of Oxygen to Fuel ) by comparing the ratio of Oxygen molecules to Carbon molecules in the exhaust.

Although this equation is very complex, the result of it is relatively easy to use in practice. Lambda directly reflects the ‘degree of lean-ness’ of the air/fuel mixture and is independent how efficiently the fuel is oxidized, a very important factor to consider when dealing specifically with air / fuel balance issues. The manner in which this equation is to be used is strictly a function of the application though, and it is an excellent replacement for "old" commonly used conventions, such as CO measurement for rich-side applications (performance tuning), ‘wide range lambda sensors’, which are not only very non-linear, but also very sensitive to combustibles in the exhaust stream. The only dependable air/fuel ratio measurement that we have found to date is one that first makes an accurate measure of the constituent gases in the exhaust stream (at least the four gases of HC, CO, CO2 and O2) and calculates the oxygen and combustibles content and then the Lambda and A/F value.

Using Lambda as an Diagnostic Aid

It is important to actually use Lambda in practice to see how well it correlates to the real world. A little experience here goes a long way in building confidence as to the efficacy of this parameter!

It is possible to use Lambda as an aid when tuning an engine provided that the engine is in good running order.

Using Lambda alone however, it is not enough to diagnose particular emission related problem.  Having 4 or 5 Gas Analyser at your disposal is an invaluable tool for engine diagnostics.

Here are some general guide lines.

Lambda - Low - <1.0 Lambda - High - >1.0 Lambda - High - >1.0 Lambda = 1.0

CO = High CO = Low CO = Low CO = Low

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CO2 = Low CO2 = Low CO2 = Low CO2 = HighHC = High HC = Low HC = High HC = LowO2 = Low O2 = High O2 = High O2 = Low

Rich Mixture Exhaust Leak Lean Mixture Tuned

 

Typical Emission Values With and Without Catalytic Converter ( good system - guide lines only )

  CO CO2 HC O2 Lambda AFR

With Catalyst

0,5 % or less14,5 % or

more50 ppm or

less0,5 % or less 0,97 - 1,03

14,3:1 to 15,1:1

Without Catalyst

1,5 % or less13 % or

more250 ppm or

less0,5 % - 2 % 0,90 - 1,10

13,2:1 to 16,2:1

 

Typical Emission Values Measured Before and After the Catalytic Converter ( good system - guide lines only )

  CO CO2 HC O2 Lambda AFR

Before Catalyst

0,6 % 14,7 % 100 ppm 0,7 % 1,0 14,7

After Catalyst

0,1 % 15,2% 15 ppm 0,1 % 1,0 14,7

 

  The effect of various ‘octane’ fuel mixes on Lambda:

Various mixes of gasoline contain differing ratios of short and long hydrocarbon chains, resulting in a variation of octane rated fuels. This has a small effect on the ratio of hydrogen to carbon in the fuel, but these variations have a trivial effect on the lambda calculation! So before you blame your Gas Analyser for the Lambda = "rubbish", make sure you actually know  what fuel ( or mixture of fuels !) is the engine running on! An difficult task to achieve, so if everything else looks normal, then the improbable must be the truth! Trust your equipment!

The effect of Oxygenated fuels on Lambda:

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Oxygenated fuels release oxygen contained a very small amount of oxygen in the fuel, which is released as the fuel is burned. The total O2 equivalence in typical oxygenated fuel is on the order of 0.1% O2, so this effect is small.

The effect of NOx on Lambda:

NOx has a relatively immaterial effect on the Lambda calculation, as 1,000 ppm NOx is only equivalent to 0.05% Oxygen utilization. A 4-gas analyzer is adequate for Lambda calculation - but at least 4 gases must be measured. At idle NOx is typically close to 0 ppm so it can be ignored. At fast Idle and light load, this gas analyser replaces the NOx value with an "automatic replacement equation", so as close as possible results are achieved even with 4 gas analyser! Using 5 Gas Analyser however is the ultimate way to go.

Sample Dilution and Air Injection Effects on Lambda:

As a side note, it is important to understand the effect that sampling air leaks or outright Air Injection may have on Lambda calculation. The percentage of extra air in the exhaust gases will result in the same percentage error in the Lambda calculation. I.E, a 5% air leak will not only dilute (lower) the CO, HC, CO2 and NOx gas readings by 5%, but will increase the Oxygen reading by about 1.00% (5% of 20.9%) and will result in the calculated Lambda being 5% leaner than it should. That means that a perfect Lambda of 1.000 will be reported as 1.050 if there is 5% Air Leak or Injection.

This is a significant error, and can occur relatively easily. It should be noted that air leaks or injection will always bias the lambda calculation toward the lean side so they should be dealt with and corrected before any lambda calculations using measured gases are attempted. Air injection should be disabled for Lambda to be calculated correctly.

Engine Misfire & the effect of Combustion Efficiency on Lambda:

Because the Lambda calculation determines the Balance between Oxygen and Combustible Gases by comparing all the oxygen available to the combustibles bearing gases it is relatively insensitive to the degree to which the combustibles have been oxidised. Thus, an engine misfire has absolutely no effect on the Lambda calculation!

Pre and Post Catalytic Converter gases:

Because the Lambda calculation determines the Balance between Oxygen and Combustible Gases by comparing all the oxygen available to the combustibles bearing gases, it is relatively insensitive to the degree to which the combustibles have been oxidised. Thus, the gas stream

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before a catalytic converter should calculate the same Lambda value as the gases after a catalytic converter.

In essence, because ALL of the gases are used in the Lambda calculation, the gas mix in the intake manifold, half-way through the combustion process, before a catalytic converter, or at the tailpipe should ALL yield the same Lambda result. The intake manifold will contain Oxygen, HC, and no CO, CO2, or NOx. They will, however be in balance. The tailpipe should contain low levels of Oxygen and HC and CO (the sources of combustion), but high levels of CO2 and Water Vapour. They will be at the same balance as the intake manifold gases. Nothing is lost or gained, just converted! It really does not matter where the gases are measured, or how efficient the combustion process is!

Lean   Mixture

The following lists some of the possible combinations of exhaust gas values and the most likely causes.

 

CO CO2 HC O2 A/F Ratio too Lean: possible problems, conditions or causes

Low Low High High

Lean fuel mixture, Ignition misfire, Vacuum leaks / air leaks (between air flow sensor and the

throttle body), Bad EGR valve or vacuum hoses misrouted, Carburator settings incorrect, Fuel injector/s bad, O2 sensor bad or failing, ECM malfunctioning, Float level too low

Low High Low LowGood combustion efficiency and catalytic converter working

properly!

 

Minor and Major Engine systems faults

The following lists some of the possible combinations of exhaust gas values and the most likely causes.

 

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CO CO2 HC O2A/F Ratio : possible problems, conditions or

causes

Low toModerate Low Low toModerate Low

Major Engine Faults:

Compression Low, Camshaft lobe lift insufficient

Low toModerate Low Low toModerate Low

Minor Engine Faults:

Ignition Timing over advanced Spark Plug/s wire/s grounded or open ECM trying to compensate for a vacuum

leak

Low High Low HighInjector misfire, catalytic converter operating correctly

High Low High LowThermostat or coolant temperature sensor faulty - "cold running engine"

Low High Low LowThermostat or coolant temperature sensor faulty - "hot running engine"

Low Low Low High Exhaust leak after the catalytic converter

High High High HighCombination of rich mixture and vacuum leak, Injector misfire, Cat. converter not working.

Low High Low LowGood combustion efficiency and catalytic converter working properly!