Fuel Metering

The fuel pump relay is mounted in the underhood electrical center located in the engine compartment. This includes the fuel injector poppet assembly, fuel pressure regulator, fuel pump and fuel pump relay. The fuel system wiring schematic diagram is covered in Fuel Pump Circuit Diagnosis.

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 HO2S. This condition causes the VCM 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 by 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 measure around 128. The averages may vary slightly from engine to engine.

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 the following items:
- 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 the following items:
- 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 the following items:
- 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, see DTC P0131 for items which can cause a lean system. Refer to DTC DTC P0131 HO2S Circuit Low Voltage Bank 1 Sensor 1.
^ A momentary rich HO2S signal (system is in control) will appear on the scan tool as the following items:
- 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 the following items:
- 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 the following items:
- 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, see DTC P0132 for items which can cause the system to run rich. Refer to DTC P0132 HO2S Circuit High Voltage Bank 1 Sensor 1.

If a driveability symptom exists, refer to the particular symptom in Symptoms, for additional items to check.