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 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

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 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

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