Transcript Fuel System Service

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
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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
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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
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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
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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
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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
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Fuel Injection Diagnosis and
Service
• Defects in other engine systems can be
mistaken for fuel injection problems
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–
–
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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
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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
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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
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Injector Problems
• Fuel injectors can leaking, dirty, shorted, or
open
– Use a stethoscope to listen to opening and
closing of injector as engine operates
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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
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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
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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
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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
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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
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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
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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
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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
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Inlet Air Temperature Sensor
• Air change
temperature
(ACT) sensor
measures
temperature of
incoming air
– Removed and
tested in hot
water as with
ECT sensor
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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
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© 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
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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
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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
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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
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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
© 2012 Delmar, Cengage Learning