Transcript Chapter 31

Chapter 31
Fuel Injection System
Diagnosis and Service
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FI Diagnosis
• FI systems are integrated into engine control
systems
• Self-test modes may be available
• Performance complaint may be caused by
something unrelated to the fuel system
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Preliminary Checks
•
•
•
•
•
•
Battery condition
Proper starting and charging system operation
Condition of all fuses
Wiring harness routing and condition
Vacuum hose routing and condition
MIL is working correctly
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Preliminary Checks
• Air induction and exhaust systems for leaks
• Emission control device operation and
condition
• Cooling system condition
• Ignition system operation
• Engine mechanical condition
• Gasoline quality
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Service Precautions
• Relieve fuel pressure before disconnecting
components
• Never turn the ignition on with fuel system
components disconnected
• Use the proper test equipment
• Turn off the ignition switch before connecting
test equipment
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Service Precautions (Cont.)
• Disconnect both battery cables before arc
welding an computer-controlled vehicle
• Never disconnect the battery while the
engine is running
• Avoid static electricity charges when
handling electronic components
• Isolate or disable high-voltage systems
before testing or servicing the fuel system
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Basic EFI System Checks
• FI problems usually result in no-start or poor
driveability problems
• Check MIL first
• No MIL or no DTC does not mean the system is
operating normally
• Fuel trim should be observed regardless of if a
DTC is set
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Fuel Trim
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Fuel Trim Following O2 Output
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Adaptive Fuel Control
• Short-Term Fuel Trim (STFT)
– The computer makes small corrections based
on oxygen sensor information
– Is shown on a scan tool in percent of increase
or decrease
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Adaptive Fuel Control
• Long-Term Fuel Trim (LTFT)
– The computer makes corrections if a
condition continues to exist
– Is shown on a scan tool in percent of increase
or decrease
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Fuel Trim
• The fuel trim monitor looks at the sum of the
STFT and LTFT
• If the PCM detects an extreme change in fuel
strategy, a DTC will set
• A fuel system DTC does not mean the fuel
system is at fault
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Fuel Trim (Cont.)
• STFT adjustments are temporary and are
not stored in PCM memory
• LTFT adjustments are stored and influence
base timing and injector pulse width
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Fuel Trim (Cont.)
• STFT responds to O2S information
• Once a certain engine temperature is reached,
the PCM starts updating LTFT
• This is based on engine speed and STFT
• If STFT shifts 3% and stays there, the PCM
adjusts LTFT to bring STFT close to 0%
correction
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Fuel Trim (Cont.)
• Since V-type motors have two sets of O2S, a
pair of STFT and LTFT numbers will show on
scan tool
• Both can vary in correction from bank to
bank
• Other systems can cause fuel trim to be out
of range:
– Air induction, PCV, ECT, ignition, exhaust, and
O2 or A/F sensors
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Oxygen Sensor Diagnosis
• Signal should vary between 0 and 1 volt
• Activity is best checked with a scope
• Voltage should switch quickly - A and C
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Knowledge Check
• How does oxygen sensor data affect fuel trim?
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A/F Sensors
• Voltage toggling does not occur
• Scan tools not designed to read A/F sensors
will display voltage similar to an HO2S
– Sensor voltage stays around 0.66 volts
– If lean, voltage increases to about 0.8 volts
• Scan tools designed for A/F sensors will show
a range of 2.4 to 4.0 volts
– Voltage increases when lean and drops when rich
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Air Induction System Checks
• All air entering the engine must be
accounted for, especially on MAF equipped
engines
• Perform a careful inspection of all hoses
• Vacuum leaks will not affect speed-density
systems as much as MAF system
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Vacuum Leak
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Check for Leaks
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Airflow Sensors
• MAF sensors measure intake air before the
throttle plate
• Air bypassing the MAF will cause a lean
condition
• A hot wire MAF can become dirty or
contaminated, also causing a lean condition
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Manifold Absolute Pressure (MAP)
Sensors
• Increased manifold pressure (lower vacuum)
causes increased injector pulse width
• Reason for increased manifold pressure open throttle, vacuum leak, restricted
exhaust, does not matter
• Vacuum leaks with MAP sensors decreases
idle air control counts
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MAP Data
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Throttle Body
• Designed to allow a certain amount of air to
pass at a particular amount of throttle
opening
• Deposits reduce air flow
• Some throttle bodies have a special coating
and should not be cleaned with chemicals
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Knowledge Check
• Technician A says a vacuum leak can make the
engine run lean. Technician B says a vacuum
leak has no effect on a MAF equipped engine.
Who is correct?
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Fuel System Checks
• Check fuel delivery
– Low pressure can
cause a lean
condition or no-start
– High pressure can
cause a rich
condition
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Fuel System Checks (Cont.)
• Check fuel pressure
and how long
pressure holds with
the key off
• Inspect regulator for
leaks
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Injector Tests
• At Rest Test
– Determines causes of system not holding
pressure
• Power Test
– Uses a voltmeter or Noid light to determine
if the injector is receiving power and is
being pulsed to ground
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Fuel Injector Control Test
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Injector Tests (Cont.)
• Resistance Test
– Uses an ohmmeter
to check condition of
injector coils
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Injector Tests (Cont.)
• Balance Test
– Helps isolate a clogged or dirty injector
• Sound Test
– Uses a stethoscope to listen for clicking of injector
• Flow Test
– Checks injectors after removing them from the
engine
• Oscilloscope Tests
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Injector Balance Test
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Conventional Injector Waveform
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Peak and Hold Injector Waveform
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Pulse Modulated Injector
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Injector Tests (Cont.)
• For all injector types, there should be a
clean, sudden drop in voltage when turned
on
• Check voltage spike levels
• High coil resistance or a shorted coil will
reduce inductive kick when turned off
• Current ramping can be used to compare
current flow and voltage
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Injector Cleaning
• Never soak an injector in cleaning solvent
• Never use a wire brush, pipe cleaner,
toothpick, or other tool to clean opening
• Injector cleaning runs a chemical through the
injector to remove deposits
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Fuel Rail, Injector,
and Regulator Service
• When removing and reinstalling components,
lightly lubricate O-rings with engine oil or
transmission fluid
• Cap injector openings in the intake to prevent
the entry of dirt or debris
• Cap all fuel rail and injector openings
• Do not use compressed air to flush or clean the
fuel rail or injectors
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Special GDI Checks
• Use an ohmmeter to check for opens or shorts
in windings
• Typical fuel volume tests cannot be done due
to high fuel pressures
• Scan tool will display fuel pressure
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Knowledge Check
• Technician A says a restricted fuel injector can
cause a lean condition. Technician B says an
injector balance test may locate a restricted
injector. Who is correct?
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duplicated, or posted to a publicly accessible website, in whole or in part.
Electronic Throttle Controls
•
•
•
•
•
No idle speed adjustment necessary
Idle speed can be monitored with a scan tool
PCM must know base throttle position
Follow learn/relearn procedures exactly
Does not need to relearn after battery
disconnect
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ETC Data
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Idle Speed Checks – Non ETC
• Check linkage and vacuum lines
• Many different inputs will affect IAC:
– Lower than normal TP signal
– Higher than normal TP signal
– Incorrect ECT signal
– Stuck closed A/C switch
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IAC System Tests
• Check IAC counts with a scan tool:
– If counts are low or zero, look for a vacuum leak
– If counts are higher than normal, look for a dirty
or restricted throttle body
– Look for count change while performing a wiggle
test
– Counts should be steady
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Carbon Buildup
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Checking the Extension of an IAC
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Servicing the IAC Motor
• Some vehicles allow for IAC control via a scan
tool
• If the IAC does not respond, further testing is
required
• Carbon deposits in IAC passage can cause
erratic idle speeds and stalling
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