CH31 fuel injection system diagnosis and service STUDENT VER

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Chapter 31

Fuel Injection System Diagnosis and Service

• Use the proper test equipment.

• Turn off the ignition switch before connecting test equipment

Service Precautions (cont’d)

• 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

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

Fuel Trim (cont’d)

• STFT adjustments are temporary and are not stored in PCM memory

• LTFT adjustments are stored and influence base timing and injector pulse width

Fuel Trim (cont’d)

• 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

Fuel Trim (cont’d)

• 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

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

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

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

• A hot wire MAF can become dirty or

contaminated, also causing a lean condition

Volume Airflow (VAF) Sensors

• Check the flap for binding, sticking, or

scraping

• Can be checked with a DMM or scope

• Should have a smooth voltage change as flap is moved

Karman-Vortex Sensors

• Can cause surging, hesitation, stalling, and increased emissions

• Check signal frequency with a scope

• Digital signal should cycle between 0 and 5 volts

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

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

Fuel System Checks

• Check fuel delivery

– Low pressure can cause a lean condition or start

no-– High pressure can cause a rich condition

• Check fuel pressure and how long pressure holds with the key off

• Inspect regulator for leaks

• Resistance Test

– Uses an ohmmeter to check condition of

injector coils.

Injector Tests (cont’d)

Conventional Injector Waveform

Injector Tests (cont’d)

• 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

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

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.

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

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.

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

Cold Start Injectors

• Should be checked with other injectors

• Clean when other injectors are cleaned

• The injector can be checked with an

ohmmeter