System Diagnosis - Fuel Metering System

DIAGNOSIS OF FUEL METERING SYSTEM
Always start with the "On-Board Diagnostic (OBD) System Check" in Computers and Control Systems / System Diagnosis / Flow of Diagnosis. Testing and Inspection
This will reduce diagnosis time and prevent unnecessary parts replacement. Diagnosis of the fuel metering system is covered in Chart A-3, "Engine Cranks But Will Not Run," and Chart A-7, "Fuel System Diagnosis." These charts cover the diagnosis of the injectors, pressure regulator, and the fuel pump. These Charts are located at Computers and Control Systems / System Diagnosis. Testing and Inspection

If a malfunction occurs in the fuel metering system, it usually results in either a rich or lean exhaust condition. This condition is sensed by the 02S, and causes the PCM to change the fuel calculation (injector pulse width). The change made to the fuel calculation is indicated by a change in the short term fuel trim 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 are around 128, but vary slightly from engine to engine.

NOTE: 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 conditions with the system in control and out of control.

- A momentary lean condition (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 condition (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 condition (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, see DTC 42 or 44 for items which can cause a lean system. Testing and Inspection

- A momentary rich condition (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 condition (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 condition (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, see DTC 43 or 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. Testing and Inspection