System Diagnosis - Fuel Metering System
DIAGNOSISSome failures of this system will result in an "Engine Cranks But Will Not Run" symptom. If this condition exists, see Computers and Control Systems / System Diagnosis / Chart A-3. Testing and Inspection
Chart A-3 will determine if the problem is caused by the ignition system, PCM, or fuel pump circuit. If it is determined to be a fuel problem, Chart A-7 should be used. This includes the fuel injectors, pressure regulator, fuel pump, and fuel pump relay. The fuel system wiring schematic is covered in Chart A-5. If a malfunction occurs in the fuel metering system, it usually results in either a rich or lean HO2S signal. This condition is indicated by the HO2S voltage, causing the PCM to change the fuel calculation (injector pulse width) based on the HO2S reading. Changes made to the fuel calculation will be indicated by a change in the long term fuel trim values which can be monitored with a scan tool. Ideal long term fuel trim values are around 128; for a lean HO2S signal, the PCM will add fuel, resulting in a fuel trim value above 128. Some variations in fuel trim values are normal because all engines are not exactly the same. If evaporative emission canister purge is "ON," the fuel trim may be as low as 90. If the fuel trim values are greater than 138, see DTC P0171 for items which can cause a lean HO2S signal.