Fuel System Service

1. Fuel System Service Chapter 41

2. Objectives • Service fuel delivery system components • Diagnose rich and lean air-fuel mixtures and their causes • Service carburetors and fuel injection systems

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. Injector Problems • Fuel injectors can leaking, dirty, shorted, or open • Use a stethoscope to listen to opening and closing of injector as engine operates

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. Inlet Air Temperature Sensor • Air change temperature (ACT) sensor measures temperature of incoming air • Removed and tested in hot water as with ECT sensor

23. 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

25. 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

26. 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

27. 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

28. 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

29. 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