System Diagnosis - Fuel Metering System

Some failures of this system will result in an engine cranks but won't run symptom. If this condition exists, see CHART A-3. This chart will determine if the problem is caused by the ignition system, Powertrain Control Module (PCM), or fuel pump circuit. If its determined to be a fuel problem, CHART A-7 will be used. This includes the fuel injectors, fuel pressure regulator, fuel pump, and fuel pump relay. The fuel system wiring schematic diagram is covered in Diagnostic Trouble Code (DTC) 54.

If a problem occurs in the fuel metering system, it usually results in either a rich or lean exhaust condition. This condition is sensed by the Heated Oxygen Sensor (HO2S) and causes the PCM to change the fuel calculation (injector pulse width). The change made to the fuel calculation will be 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 will be around 128, but vary slightly from engine to engine.

NOTICE: 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 DTC 44 for items which can cause a lean system.

^ 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 both fuel trim values are fixed well below 128, refer to DTC 45 for items which can cause the system to run rich. If a driveability symptom exists, refer to the particular symptom in Diagnosis by Symptom for additional items to check.