System Diagnosis - Fuel Metering System

Fuel Supply System:

DIAGNOSTIC PROCEDURES
Always begin diagnosis with the On-Board Diagnostic (OBD) System Check before proceeding to any other diagnostics. This will reduce diagnosis time and prevent unnecessary replacement of parts. The "On-Board Diagnostic (OBD) System Check" will give direction to further diagnostics, such as "Engine Cranks But Won't Run" and "Fuel System Diagnosis," including diagnosis of injectors, pressure regulator, fuel pump, fuel pump relay, and oil pressure switch. Refer to Computers and Control Systems / System Diagnosis. Testing and Inspection

FUEL TRIM
If a problem occurs in the fuel metering system, it usually results in either a rich or lean exhaust and causes the control module to change the fuel calculation (injector pulse width). The change made to the fuel calculation is indicated by a change in the short term and long term fuel trim values, which can be monitored by a Tech 1 scan tool. Average short term and long term values will be around 128, but vary slightly from engine to engine.

If both the short term and long term fuel trim values are fixed well above 128, refer to DTC 44 for items which can cause a lean system.

A momentary rich condition (system in control) will appear on Tech 1 scan tool as:

- Short term fuel trim value less than 128 (reducing fuel).
- Long term fuel trim value around 128.

A long term rich condition (system in control) will appear on Tech 1 scan tool as:

- Short term fuel trim value around 128.
- Long term fuel trim value less than 128 (reduced fuel).

A long term rich condition (system out of control) will appear on Tech 1 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 short term and long term 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 Diagnosis by Symptom for additional items to check for that particular Symptom. Testing and Inspection