System Diagnosis - Fuel Metering System

FUEL SYSTEM DIAGNOSIS STARTING POINT
System Check
Always start with the Powertrain On-Board Diagnostic System Check. This will reduce diagnosis time and prevent unnecessary parts replacement.

Component Checks
Diagnosis of the fuel metering system is covered in two tables:

Table A-3 Engine Cranks But Does Not Run
and
Table A-7 Fuel System Diagnosis.

These diagnostic tables cover the diagnosis of the injectors, pressure regulator, and the fuel pump.

FUEL SYSTEM MALFUNCTION INDICATORS
A malfunction in the fuel metering system usually results in either a rich or lean Heated Oxygen Sensor (HO2S) signal. This signal causes the Powertrain Control Module (PCM) to change the fuel calculation (injector pulse width). The change made to the fuel calculation is indicated by a change in the short and long term fuel trim values which can be monitored with a scan tool. A momentary change to the fuel calculation is indicated by the short term fuel trim value, while a prolonged change is indicated by the long term fuel trim value. Average fuel trim values are around 128, but vary slightly from engine to engine.

SCAN TOOL DIAGNOSIS

IMPORTANT:
^ When using a scan tool to observe fuel trim values, remember that if the system is in control, no action is required unless a driveability symptom is present. Listed below are examples of lean and rich HO2S signals with the system in control and out of control.

^ A momentary lean HO2S signal (system is in control) will appear on the scan tool as:
- Short term fuel trim value above 128 (adding fuel).
- Long term fuel trim value around 128.

^ A prolonged lean HO2S signal (system is in control) will appear on the scan tool as:
- Short term fuel trim value around 128.
- Long term fuel trim value above 128 (added fuel).

^ A prolonged lean HO2S signal (system is out of control) will appear on the scan tool as:
- Short term fuel trim value well above 128 (adding fuel).
- Long term fuel trim value well above 128 (added fuel).

If both fuel trim values are fixed well above 128, refer to Diagnostic Trouble Code (DTC) P0131, P0151, P0171 or P0174 for items which can cause a lean HO2S signal.

^ A momentary rich HO2S signal (system is in control) will appear on the scan tool as:
- Short term fuel trim value less than 128 (reducing fuel).
- Long term fuel trim value around 128.

^ A prolonged rich HO2S signal (system is in control) will appear on the scan tool as:
- Short term fuel trim value around 128.
- Long term fuel trim value less than 128 (reduced fuel).

^ A prolonged rich HO2S signal (system is out of control) will appear on the scan tool as:
- Short term fuel trim value much less than 128 (reducing fuel).
- Long term fuel trim value much less than 128 (reduced fuel).

If the fuel trim values are fixed well below 128, refer to DTC P0132, P0152, P0152, P0172 or P0175 for items which can cause the fuel control system to run rich.

DRIVEABILITY SYMPTOMS
If a driveability symptom exists, refer to the particular symptom in Diagnosis by Symptom for additional items to check.