Date post: | 23-Dec-2015 |
Category: |
Documents |
Upload: | blanche-west |
View: | 222 times |
Download: | 1 times |
© 2012 Delmar, Cengage Learning
Fuel System Service
Chapter 41
© 2012 Delmar, Cengage Learning
Objectives• Service fuel delivery system components• Diagnose rich and lean air-fuel mixtures and
their causes• Service carburetors and fuel injection systems
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
Injector Problems• Fuel injectors can leaking, dirty, shorted, or
open– Use a stethoscope to listen to opening and
closing of injector as engine operates
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
Inlet Air Temperature Sensor• Air change
temperature (ACT) sensor measures temperature of incoming air– Removed and
tested in hot water as with ECT sensor
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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
© 2012 Delmar, Cengage Learning
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