System Diagnosis - Fuel Metering System

DIAGNOSTIC PROCEDURES
Some failures of this system will result in an engine cranks but will not run symptom. If this condition exists, refer to Engine Cranks but Does Not Run.

This table will determines if the problem is caused by the ignition system, the PCM, or the fuel pump circuit. If a fuel problem exists, use the Fuel System Pressure Test. This includes the fuel injectors, the fuel pressure regulator, the fuel pump, and the fuel pump relay. The fuel system wiring schematic diagram is covered under Engine Cranks But Will Not Run (Fuel Pump Circuit). Testing and Inspection

FUEL TRIM
If a problem occurs in the fuel metering system, it usually results in either a rich or lean 02S signal. This 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 and long term fuel trim values which a scan tool can monitor. The short term fuel trim value indicates a momentary change to the calculation. The long term fuel trim value indicates a prolonged change. Average fuel trim values will measure around 128, but the average fuel trim value will vary slightly from engine to engine.

NOTE: When using a scan tool to observe the 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 02S signal (system is in control) will appear on the scan tool as the following items:

- A short term fuel trim value above 128 (adding fuel)
- A long term fuel trim value around 128

- A prolonged lean 02S signal (system is in control) will appear on the scan tool as the following items:

- A short term fuel trim value around 128
- A long term fuel trim value above 128 (added fuel)

- A prolonged lean 02S signal (system is out of control) will appear on the scan tool as the following items:

- A short term fuel trim value well above 128 (adding fuel)
- A 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) P0172 Fuel Trim System Rich for items which can cause a lean system.

- A momentary rich 02S signal (system is in control) will appear on the scan tool as the following items:

- A short term fuel trim value less than 128 (reducing fuel)
- A long term fuel trim value around 128

- A prolonged rich O2S signal (system is in control) will appear on the scan tool as the following items:

- A short term fuel trim value around 128
- A long term fuel trim value less than 128 (reduced fuel)

- A prolonged rich O2S signal (system is out of control) will appear on the scan tool as the following items:

- A short term fuel trim value much less than 128 (reducing fuel)
- Long term fuel trim value much less than 128 (reduced fuel)

Whenever both fuel trim values are fixed well below 128, refer to DTC P0171 Fuel Trim System Lean for items which can cause the system to run rich.

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