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would reach the top of Mount Everest.On May 29, 1953, Sir Edmund Hillaryand Tenzing Norgay had reached the

highest point on earth, some 29,029 ft.above sea level. At this elevation thereis very little oxygen in the atmosphere,

A

flash of sunlight reflected off the ice hammers

head as it swung forward, breaking through the

age-old ice. Normally this would have made a

loud noise, but the only noise that Edmund heardwas his own heart racing. The lack of oxygen had every muscle

in his body aching in agony as he took his next step up the

mountain. He thought to himself,Just a few more steps and Ill

be standing on top of the world. He had dreamed of the day he

29J uly 2008

FUEL INJECTIONDIAGNOSIS:

ITS ALL ABOUT THE AIRBY BERNIE THOMPSONAn engine cant run without fuel and air.But how much fueland how much air

are needed to make it run efficiently?

If one quantity can be established,

the fuel injection system has a better

chance of figuring out how much of

the other should be required.

Photoillustration:HaroldA.

Perry;p

hotos:WieckMedia&JupiterImages

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and this climb had been accomplishedwithout the aid of bottled oxygen. Theamount of oxygen thats contained in

the atmosphere is important becauseits this oxygen thats taken in by ourbodies and converted chemically togive us the energy we need to do work.A similar release of energy is what pow-ers the internal combustion engine.

In order for this combustion energy

to be released, a chemical reaction musttake place between oxygen and the hy-drocarbons in the fuel. This chemical

reaction is based on the weight mass ofthe two elementsoxygen and hydro-carbonsthat react together. In thespark ignition internal combustion en-gine, this weight ratio can change from11:1 under a power demand to 17:1 in alean cruise condition. At both weight ra-

tio extremes, the tailpipe emissions lev-els will rise considerably.

Many years ago tailpipe emissions lev-

els were not regulated. As the concernabout air quality around large citiesgrew, government regulations were im-posed on vehicle manufacturers. For themanufacturers to meet these emissionsregulations, a new technology emerged.This technology employs a method ofweighing precisely the air entering theengine and then delivering the correctweight of hydrocarbons or fuel for anengines running condition. This tech-nology is referred to asfuel injection.

Fuel injection can be either mechani-cal or electronic, or a combination of

both. This discussion will center on elec-tronic fuel injection for the spark igni-tion internal combustion engine. Thereare two basic methods of fuel injectioncurrently used in vehiclesspeed densi-ty and airflow. I ts important to knowwhich system youre working on. For ex-ample, an exhaust gas recirculation(EGR) valve stuck open on a speed den-sity system produces a lower vacuumreading, which would normally indicatethat the engine was under load. Underthis condition additional fuel would be

added, which would overfuel the en-gine, so the fuel trim correction wouldgo negative and take fuel away. (Moreon fuel trim later.) On an airflow systemwith the same EGR valve problem, theairflow would be read correctly, so nofuel trim correction would be needed.

With the speed density method, anindirect calculation of air weight ismade by measuring the intake pressurechanges using the manifold absolutepressure (MAP) sensor. This sensordoes not directly measure the intakemanifold pressure; instead, it measures

the displacement of a diaphragm thatsdeflected by intake manifold pressure.This intake pressure change is convert-ed by the MAP sensor to an outputmeasurement of pressure in kilopascals(kPa). The change in intake manifoldpressure can be used to calculate theload placed on an engine. The MAPsensor accomplishes this by monitoringthe intake pressure; as the throttle bladeis opened, it allows more air to enter the

30 J uly 2008

FUEL INJECTION DIAGNOSIS

Fig. 1Screencaptures:BernieThomp

son

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engine and, thus, changes the pressurefrom a negative state (vacuum) to onethats slightly under atmospheric pres-

sure at wide-open throttle.The MAP sensor is desirable to usebecause the absolute engine workingpressure (vacuum) at idle and light loadis unchanged by elevation. At sea level,the barometric pressure is 101 kPa, anda good engine idle pressure is 27 kPa.Therefore, the engine vacuum is 101kPa 27 kPa 74 kPa, or 29.9 in./Hg 10 in./Hg 19.9 in./Hg. At 5500 ft.of elevation, the barometric pressure is84 kPa and a good engine idle pressureis 27 kPa. Therefore, the engine vacu-um is 84 kPa 27 kPa 57 kPa, or

24.9 in./Hg 10 in./Hg 14.9 in./Hg.An equation is needed to calculate

the airflow into the engine, and certainvalues must be known: the size of theengine in liters; the intake manifold ab-solute pressure, as determined by theMAP sensor; and the revolutions perminute (rpm), because in a four-strokeengine, only one stroke produces in-coming air. The rpm is determined bythe crankshaft position sensor. This willbecome a factor since the air mass iswhat were trying to measure. The air

temperature will also become a factor

because a change in temperature willcause a change in the density of the air.This is read by the intake air tempera-

ture (IAT) sensor. At an air temperatureof40F, the air weighs 1.51 grams perliter (g/L); at an air temperature of

104F, the air weighs 1.12 g/La 35%difference. The speed density base airequation is made with only three sen-sors and is as follows:

RPM x Litersx MAP x IAT Air Density =Mass Air in g/sec60 2

With the airflow method, a calcula-tion of air weight can be made that is

an indirect measurement or a direct

measurement of the air entering theintake manifold, depending on whichtype of sensor is used. This airflow is

measured with a device called a massairflow (MAF) sensor. There are sever-al styles of these devices, the most pop-ular of which is the heated-elementtype. I ts based on the hot-wireanemometer weather forecasters usefor measuring wind velocity. A wire orelement is electrically heated to a settemperature above the temperature ofthe inlet air. As the throttle blade isopened, the velocity of air increases,which transfers the heat from the ele-ment into the air. An electronic circuitis designed to keep the element at a set

temperature so that as its temperaturedecreases, the current flow across it in-creases. By monitoring the current, theairflow will be known. The PCM con-verts this signal into air weight, whichis read in g/sec.

In either of these methodsspeeddensity or airflowthe air is the un-known quantity; therefore, the airflowing into the engine is what must bedetermined. Fuel injection is based onairflow, not fuel flow. The fuel deliveryweight is a known factor.

One example can be seen by using a25-lbs./hr. fuel injector. This number isbased on the engines brake-specificfuel consumption (BSFC), which indi-cates the engines fuel consumption ef-ficiency. The BSFC is usually mea-sured in pounds of fuel used per hourfor each unit of horsepower. This rela-tionship means that horsepower multi-plied by BSFC equals pounds of fuelconsumed per hour.

A 25-lbs./hr. fuel injectors fuel deliv-ery is based on a constant fuel pressureand volume equal to 255cc/min., or

.00425cc/millisecond. One cubic cen-timeter is equal to .162 gram of gaso-line. This fuel weight is a known quan-tity that will be delivered by the fuelinjector to the engine.

Now that both the air weight enter-ing the engine and the fuel weight be-ing delivered can be determined, anequation can be derived that will set aprecise air/fuel ratio for the engine.The equation is used to determine the

32 J uly 2008

FUEL INJECTION DIAGNOSIS

Fuel injection isbased on airflow,

not fuel flow. The

air flowing into

the engine is what

must be determined.

Fig. 2

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air mass weight contained in eachcylinder so the fuel weight can be de-livered properly. I f there were any

problems, such as the sensors misread-ing or incorrect fuel delivery, the baseair equation would need to be changedso that the correct air/fuel weightwould be maintained. This is donewith a multiplier to the base air equa-tion thats called fuel trim (see Fig. 1on page 30). The purpose of fuel trimis to monitor the ratio of air to fuelweight and to keep it at a predeter-mined target value.

An oxygen sensor is located in theexhaust system to continually measurethe air/fuel ratio. Its set up in a feed-

back loop so it can report the air/fuelratio to the microprocessor, which will

use this information to adjust the fueltrim multiplier to keep the air/fuel ra-tio at the target value. This method ofcontrol is referred to as a closed-looplimit-cycle control system. One exam-ple of this type of control system is anoven. When the temperature is set to,say, 350F, the electrical elementcomes on to heat the oven. The oven

stays on until it reaches a temperatureof 355F, then shuts off. This tempera-ture is sensed by a sensor in the oven.The oven then cools down until itreaches 345F. At this point the heat-ing element turns on, heating the ovento 355F again. This cycle continues,to keep the oven close to the targettemperature of 350F.

This type of control system canmaintain an average value very close to

the command input. On an internalcombustion engine, this system worksin much the same way. The fuel trimworks like the ovens heating element,driving the system rich or lean. Theoxygen sensor works like the heat sen-

sor in the oven, only it reports the

air/fuel changes. The oxygen sensor re-porting limits are set between .1 and .8volt. The oxygen sensor in this range isstoichiometric. For this sensor to berich it must be above .8 volt; to be leanit must be below .1 volt.

A vehicles fuel control system under

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FUEL INJECTION DIAGNOSIS

Fig. 4

Fig. 3The purpose of fuel

trim is to monitor

the ratio of air to

fuel weight and tokeep it at a pre-

determined value.

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most conditions will cycle the oxygensensor in this .1- to .8-volt range. This isusually accomplished with Short Term

Fuel Trim (STFT). Since STFT drivesthe oxygen sensor, if the oxygen sensorresponse is slow, the STFT peak-to-peak value will increase. If the STFTvalue exceeds 8% peak-to-peak, the O2sensor will have to be replaced (Fig. 2on page 32). The cycling oxygen sensorwill maintain the air/fuel ratio at 14.66lbs. of dry air to 1 lb. of gasoline. This isreferred to as stoichiometry, which isthe ideal mixture of air and fuel that,when ignited, will completely burn allof the hydrocarbons and leave only car-bon dioxide and water.

In a running engine, the air/fuelmixture will never completely burn,due in part to unvaporized fuel and hy-

drocarbons packing into the piston ringlands and the valve pocket areas. Thisair/fuel ratio is desirable for the catalyt-ic converter to work correctly, therebylowering the levels of tailpipe emis-sions.

Now that we have an understandingof the fuel injection fuel control system,

lets put it to work in repairing vehicles.Since the fuel injection system is allabout the air, it will be necessary to cal-culate the volumetric efficiency (VE) ofthe engine (see Mark Warrens June2003 Driveability Corner for a conciseexplanation of VE). A Toyota 4Runnerwith a 3.0L engine was brought in be-cause of low power. The Check Enginelight came on and the driver com-plained of low power. The following di-

agnostic trouble codes (DTCs) werepulled: P0171 (system too lean), P0325(knock sensor 1 circuit) and P0330(knock sensor 2 circuit).

To find a diagnostic direction quicklyits necessary to calculate the VE. This

can be done by collecting the parame-

ter identifications (PIDs) that will beneeded while test driving the vehicle.Once you have the information, justrun the VE calculation to see whetherthe air going into the engine is correct.In this example (Fig. 3 on page 34), the

scan tool automatically calculated the

36 J uly 2008

FUEL INJECTION DIAGNOSIS

Fig. 6

Fig. 5A vehicles fuel

control system uses

Short Term Fuel

Trim to cycle the

oxygen sensor in a.1- to .8-volt range.

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volumetric efficiency of the enginewhile the vehicle was being driven. Theyellow trace is the MAF sensor signal

that the scan tool reads as grams persecond (g/sec) and the red trace is theVE reading, or theoretical airflow.

When these two measurements of theair flowing into the engine are com-pared, its easy to see whether a prob-

lem exists. In this case, the actual air-flow reported from the MAF sensor(yellow trace) is much lower than the

VE calculation (red trace). This lowMAF reading shows that a problem ispresent in the airflow to the engine.

A low airflow reading could be asso-ciated with many problems, such as arestricted exhaust or intake, an air leakbetween MAF sensor and throttle, anincorrect MAF sensor calibration, in-correct camshaft timing, engine me-chanical faults, etc. To identify whatthe problem is, its necessary to checkthe fuel trim. When doing this, youmust check the trim values over arange of engine load and rpm. In thisfuel trim test (Fig. 4), total fuel trimreadings are taken. Total fuel trim isLong Term Fuel Trim added to Short

Term Fuel Trim. When checking thefuel trim chart, look at the way the fueltrims change over the load of the en-gine. In this example at idle, the fueltrim is taking away29% from thebase air equation. As the load and rpmchange, the fuel trim starts to add+23% to the base air equation. As theload steadily increases, the fuel trimstarts to add up to +49% to the base airequation. This indicates that the MAFsensor is dirty. The MAF sensor uses aheated element to measure the incom-

ing air to the engine. When this ele-ment becomes dirty it overreads theincoming airflow at idle, so the fueltrim has to modify the base air equa-tion to compensate.

At hot unloaded idle, the MAF sen-sor reading in g/sec should be veryclose to the liter size of the engine, soon this 3.0L Toyota, at hot idle theMAF sensor should read about 3 to 3.2g/sec. This is a good way to seewhether the MAF sensor is readingcorrectly at idle. If the MAF sensorreading in g/sec is higher or lower than

the liter size of the engine at idle,check the fuel trim. I f the fuel trim isgood (10%), then the MAF sensor isreading the airflow correctly.

If the fuel trim is greater than this,its an indication of a problem. As theengine load changes, the dirty MAF el-ement cannot give up its heat to the airflowing over it, thus it underreads theairflow. The fuel trim has to correct thisairflow reading from the MAF sensor.

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FUEL INJECTION DIAGNOSIS

Circle #21

Circle #22

Circle #23

Circle #24

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It does this by multiplying the base airequation by the trim value needed.

Another example of a MAF sensor

reading incorrectly is if the MAF sen-sors Wheatstone bridge is out ofrange, the actual g/sec reading wouldalso be out of range. Since the MAFsensor reading sets the fuel deliveryweight, the fuel trim would correct theairflow. This would create out-of-rangefuel trims as well. However, theres adifference in the way the fuel trimsload on the chart; rather than goingfrom a negative to a positive value, thefuel trims stay linear. In other words,they stay very close to the same per-centages from the bottom of the chart

to the top. In this case, the MAF sen-sor would need to be replaced.

In another example, if the enginehas a fuel delivery problem, the MAFsensor reading would be correct butthe fuel trims would read out-of-range.Whether the fuel trims are positive or

negative tells you which direction togo. When theyre negative (taking away

fuel), theres too much fuel getting tothe engine. When theyre positive(adding fuel), theres not enough fuelgetting to the engine. If the engine hasa misfire with low fuel trim values, theproblem could be the ignition systemor engine mechanical. I f the engine

has a misfire with high fuel trim values,look at a possible problem with the in-jectors.

Now back to our 3.0L Toyotas MAFsensor problem. The sensor was re-moved and cleaned, repairing not onlythe P0171, but the P0325 and P0330(Figs. 5 and 6 on page 36). Recheckingthe work youve done is important, as itverifies that the repair has correctedthe problem. This entire diagnosis wasmade while on a test drive.

So the next time you go for a testdrive, take your scan tool with you. Itmay save you hours of diagnostic timelater. Also, remember the lesson ofMount Everest: In an internal combus-

tion engine, just as in our bodies, theamount of available air determines theamount of work that can be done.

39J uly 2008

This article can be found online at

www.motormagazine.com.

Circle #25

Fuel trim corrects

the MAF sensorreading by multi-

plying the base air

equation by the

trim value needed.