E lectronic fuel injection systemscome in several versions. Throt-

tle Body Injection (TBI) was used inthe 1980s on many vehicles as an in-termediate step from electronic car-buretion to multiport fuel injection.TBI uses one or two fuel injectorsmounted in a throttle body to fuelthe engine.

Multiport fuel injection (MFI),which is used on almost all late-model engines, has a separate fuelinjector for each cylinder. The injec-

tors are usually mounted in the in-take manifold and spray fuel intothe intake ports, but on some newdirect injection applications, the in-jectors spray directly into the com-bustion chamber. These includesome new GM, Mazda and VW en-gines. Direct injection requiresmuch higher injection pressures be-cause the injector has to overcome

the pressure inside the cylinderduring the compression stroke. Butthe advantages include finer fuel at-omization, cleaner combustion(lower emissions), better fuel econ-omy and better performance. Be-cause of this, more and more newengines will be going to direct injec-tion.

First-generation MFI systems fireall the injectors simultaneously oreach bank separately, but most ofthe newer MFI systems are sequen-tial fuel injection (SFI) systems thatfire each injector separately just asthe intake valve is about to open.The advantage with the latter ap-proach is that the air/fuel mixturecan be adjusted almost instantly(from one cylinder firing to thenext), and even adjusted for indi-vidual cylinders.

GM’S CENTRAL POINT INJECTIONOne notable variation is GeneralMotor’s Central Point Injection(CPI) system.

Here, a centrally-located Maxi in-jector routes fuel to mechanicalpoppet valve injectors at each cylin-der. The system was used on GM’s4.3L V6 Vortec engines from 1992through 1995. The Maxi injectorroutes fuel into six nylon fuel linesthat have poppet-style spray noz-zles on the end. When the pressurein the lines reaches (43 psi), thepoppet valves open and fuel spraysinto the intake ports. All the noz-zles spray simultaneously when theMaxi injector opens (three times percrankshaft revolution).

GM redesigned the CPI system in1996 and called it Central Sequen-tial Fuel Injection (CSFI). The sin-gle Maxi injector was replaced withsix or eight electronic injectors, buteach injector still routes fuelthrough a plastic line to a mechani-

cal poppet valve. The fuel injectors in most fuel in-

jection systems are electronic andhave a solenoid valve at the top toopen the nozzle. The PowertrainControl Module (PCM) determinesthe on-time (pulse width modula-tion) of each injector to regulate fueldelivery. The PCM uses varioussensor inputs (airflow, temperature,oxygen, throttle position, enginespeed and load) to control fuel de-livery. The injectors typically re-ceive voltage when the ignition ison, and open when the injectordriver circuit in the PCM groundsthe circuit.

COMMON PROBLEMSDirty injectors are a common prob-lem. Injector nozzles can becomeclogged with fuel varnish overtime, causing a loss of engine per-formance and misfiring. Contribut-ing factors include short tripdriving and using gasoline thatdoes not contain adequate levels ofdetergent to keep injectors clean.Symptoms include rough idle (leanmisfire), hesitation or stumblewhen accelerating, and reducedperformance.

Fuel injectors can also leak fuel,causing an increase in fuel con-sumption and emissions. An injec-tor failure will result in a deadcylinder and power loss. This willusually turn on the Check EngineLight in 1996 and newer vehicleswith OBD II, and set a misfire code.

Fuel pressure is critical with fuelinjected engines. Operating pres-sures may range from 35 to 85 PSIdepending on the application, butthe specifications typically allow avery limited range in pressure (sayfrom 45 to 55 psi). If fuel pressure isoutside specifications, it may causehard starting, driveability or other

60 COUNTERMAN OCTOBER 2007 www.counterman.com

Fuel SystemThere will be three questions on the ASE P2 test that deal with fuel system, parts and products.

Automatic Transmission

Braking System

Cooling System

Drivetrain

Electrical System

Emission Control System

Engine Parts

Gaskets and Seals

Exhaust System

Fuel System

HVAC

Ignition System

Manual Transmission/Transaxle

Steering & Suspension

62 COUNTERMAN OCTOBER 2007 www.counterman.com

performance issues. That’s why fuelpressure checks are a must when di-agnosing fuel-related problems.

FUEL PUMPSMost fuel injection systems use anelectric pump mounted inside thefuel tank. Pump designs also varyand include single- or double-vane,roller vane, turbine or gerotor-stylepumps. Most have a one-way checkvalve to maintain pressure in the fuelsystem when the engine is shut off.

Fuel pumps can fail for a variety ofreasons: old age, loss of voltage,ground at the power relay, wiringconnections and pump motor or bear-ing damage. Running the fuel tankempty may damage the pump be-cause it relies on fuel for lubrication.

Note: Many fuel pumps are re-placed unnecessarily because ofmisdiagnosis. A no-fuel conditionmight indicate a bad pump, butcould also be a plugged fuel filter,bad electrical connection, bad fuelpump relay or leaky fuel pressureregulator. New fuel pump benchtesters are now available for testingcustomers’ old fuel pumps, andshould be used to reduce fuel pumpwarranty returns.

Replacement fuel pumps musthave the same pressure rating as theoriginal. The pump is usually partof the fuel sending unit and may be

replaced separatelyor as a complete as-sembly. The fuelinlet strainer sockshould also be re-placed when thepump is changed.

FUEL PRESSUREREGULATORWhen fuel reachesthe engine, it enters afuel rail and goes tothe injectors. A fuel pressure regula-tor on the fuel rail maintains a certainoperating pressure. Inside is a spring-loaded diaphragm attached to asource of intake vacuum. As engineload (vacuum) changes, pressure isadjusted as needed to maintainproper fuel delivery. Excess fuel isrouted back to the fuel tank througha return line. Many newer vehicleshave returnless systems that do nothave a regulator on the engine fuelrail. The regulator is in the fuel tankwith the pump.

Problems with the fuel pressureregulator can cause hard starting,idle roughness, hesitation or stum-ble when accelerating, and engineperformance problems. Fuel leak-ing into the regulator vacuum sup-ply hose indicates the vacuumdiaphragm inside the regulator hasfailed, and the regulator needs to be

replaced.Airflow into the engine is regu-

lated by a throttle body attached tothe intake manifold. Air first flowsthrough an air filter, then throughthe throttle body before passingthrough the manifold and into theengine. The PCM must monitor theamount of air entering the engine,so many fuel injected systems havea vane or mass airflow sensor aheadof the throttle body. A dirty or de-fective airflow sensor is a commoncause of fuel-related driveabilityproblems.

Other engines use a speed-den-sity system that estimates airflowbased on throttle position, RPM,temperature and engine load. Thereis no airflow sensor on a speed-den-sity system, so bad inputs from theTPS sensor or MAP sensor cancause problems on these engines. t

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