Fuel System Service

© 2012 Delmar, Cengage Learning

Fuel System Service

Chapter 41


Objectives• Service fuel delivery system components• Diagnose rich and lean air-fuel mixtures and

their causes• Service carburetors and fuel injection systems


Introduction• Fuel system

– Components• Fuel pumps• Filters• Tanks• Fuel injection systems and computer controls

– Diagnosis and repair is expensive

– Information in this chapter provides a basic understanding


Fuel Supply System Service• When necessary to remove fuel tank

– First pump fuel from the tank

• Bleed fuel system pressure – Before working

• Fuel gauge sending removal– Disconnect unit, float, and fuel lines beforehand

• Vapor recovery– Replace connected fuel hoses one at a time

• Hoses deteriorate – Must be inspected often


Fuel Filter Service• Fuel filters are replaced at specific intervals

– Located in fuel line, tank, and carburetor

• In-line fuel filter– If filter in fuel line becomes plugged fuel to the

rail will be reduced• Results in hard starting, lean running conditions,

or lack of power

– Place a shop towel or drain pan under fuel to catch spills

– If flared connection, hold nut with open-end wrench while tightening with a flare nut wrench


Throttle Linkage• Older car throttle: control by linkages and pivots

– Fuel injection systems: cable or drive-by-wire electronic throttle plates

• Throttle linkages have clips of various types– Fits around 90-degree bend on end of linkage

• Rotates to clip around linkage


Carburetor Problems and Service

• Carburetors – Found on vehicles older than 20 years

• Do visual inspection – Look for obvious problems

• Damaged or missing part, float level, accelerator pump, and linkages

• Cleaning a flooded engine– Depress accelerator to start carbureted vehicle

• Do not depress for fuel-injected vehicle

– Hold accelerator to the floor when engine floods


Exhaust Gas Analysis• Test exhaust using an infrared exhaust analyzer

– Modern exhaust gas analyzers test five gases• Hydrocarbons (HC)

• Carbon monoxide (CO) and carbon dioxide (CO2)

• Oxides of Nitrogen (NOX)

• Oxygen (O2)

• Exhaust analysis diagnoses – Incorrect air-fuel mixtures

– Engine and ignition system conditions

– Operation of emission system components


Fuel Injection Diagnosis and Service

• Defects in other engine systems can be mistaken for fuel injection problems– Emission controls

– Ignition system operation

– Engine compression and vacuum leaks

– Battery state of charge

• Visual check: locates obvious problems– Disconnected or damaged hose or wire

– Corroded electrical connections• Take apart and put back together


EFI Computer Self-Diagnostics• Late-model cars have self-diagnostic provisions

– Codes for many malfunctions the computer diagnoses

• Use a scan tool• High impedance voltmeter can also be used


Air-Fuel Mixture Problems• Unmetered air: results from leak in intake

manifold gasket– Results in lean air-fuel mixture when engine is

cold and system is in open loop

• Pressure testing– Test fuel pressure when pump is operating

• Installing pressure gauges– Install in series with line entering throttle body


Air-Fuel Mixture Problems (cont’d.)

• Testing a pressure regulator– Pressure regulator becomes defective due to

foreign material or ruined diaphragm

• Fuel pump volume– Better indicator of problems than pressure


Injector Problems• Fuel injectors can leaking, dirty, shorted, or

open– Use a stethoscope to listen to opening and

closing of injector as engine operates


Injector Problems (cont'd.)• Injector pulse width

– Check with digital multimeter, scan tool, or oscilloscope

• Testing fuel injector flow– Individual port fuel injectors: prone to plugging

from fuel deposits

• Bleed pressure before replacing injector– Do not soak injector in cleaning solvent

– Replace rubber O-rings


Injector Problems (cont'd.)• Residual fuel pressure

– System pressure should remain constant when the fuel pump is off

• Cleaning injectors– Injector cleaning fluid is a mixture of cleaner and

gasoline


Throttle Plate Service• Gum and carbon

– Accumulate around throttle plate• Causes surging, stalling, or rough or low idle

– Clean area with spray throttle plate cleaner and a brush

• Be sure cleaner is safe for oxygen sensors– If spray does not work: remove assembly and

soak in cleaner

– Cleaning may remove throttle plate coating


EFI Adjustments• Raising idle speed

– Allowing more air to pass the throttle plate• Older engines sometimes have a screw that

opens or closes a passageway• Some engines have an idle air control (IAC) motor

– Today, idle speed is controlled by the computer using an idle speed control (ISC) motor

• Engine flare: warm engine is restarted and engine idle increases for a short time and then decreases to normal


Sensor Service• Testing procedures

– Vary for the various fuel injection system sensors

• Before disconnecting a computer system component – Be certain the ignition key is off

• Use a scan tool, digital volt-ohmmeter, or test light– Follow manufacturer’s service manual

procedures


Throttle Position Sensor• Bad throttle position sensor

– Can cause idle speed change, stumble on acceleration, or engine stalling

• Sensor can tested – Use voltmeter with electrical wiring connected

• TPS – Can be tested with an ohmmeter with wiring

disconnected

– Watch for glitches at different throttle openings


Oxygen Sensor Service• Oxygen sensor

– Tested with engine running at operating temperature

– Voltmeter is connected to O2 sensor wire and grounded to perform test

• Heated sensors– Sensors with three or four wires

– Testing: when engine is running, O2 sensor voltage fluctuates rapidly from 0.2 volt to 0.8 volt

– O2 sensor becomes contaminated in many ways


Coolant Temperature Sensor• Coolant temperature sensor (ECT)

– Resistance varies with temperature changes

• Sensor is tested in hot water – Using a thermometer and ohmmeter– Technician will usually test it on the car using a

scan tool


Inlet Air Temperature Sensor• Air change

temperature (ACT) sensor measures temperature of incoming air– Removed and

tested in hot water as with ECT sensor


Map Sensor Diagnosis• Defective manifold absolute pressure (MAP)

sensor causes engine to run rich or lean– Engine off and key on: MAP sensors tell the

barometric pressure• Voltage specification varies with altitude and

weather

• Some MAP sensors produce voltage frequency signal– Tester changes frequency voltage to analog



Mass Airflow (MAF) Sensor Diagnosis

• Tested with voltmeter with min/max feature– Minimum voltage is zero: may be an open circuit

in sensor’s variable resistor

• Some vane-type MAF sensors manufacturer provides ohmmeter specifications– At some terminals thermistor might allow

temperature to affect resistance readings

• Run the engine and tap on the sensor when testing a hot wire or heated resistor MAF sensor


Testing Other Sensors• Other EFI sensors tested similar to sensors

previously discussed• Check resistance values

– Scan tool and computer self-diagnostic system

– Voltmeter or ohmmeter

• Refer to service manual for procedures


Computer Service• Computer rarely the cause of problems in fuel

system– Locate the problem in the system that caused

the computer to fail

– Replace faulty computer• Remanufactured computers are widely available• Older computers have replaceable PROM• Later-model computers have EE Flash PROMs


Mechanical Injection• Mechanical fuel injection systems run under

very high pressure– Spring-loaded valve requires at least 50 psi

– Spray in an even fuel pattern

– Check operation of injectors by removing each using two wrenches

– Each injector should flow an equal amount


Carbon Deposit Service• Carbon deposits cause drivability problems

– Results in rough idling when cold, loss of power, surging, and high emissions

• Carbon blaster – Uses crushed walnut shells blasted by

compressed air

• Accumulated carbon – Can drop off and be crushed against the cylinder

head

– Carbon removed using an additive

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Fuel System Service