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 Diagnosis and repair is expensive 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 Heated sensors 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