Fuel System Diagnosis

FUEL METERING SYSTEM
Always start with A Powertrain On Board Diagnostic (OBD) System Check. This will reduce diagnosis time and prevent unnecessary parts replacement.

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 will determine if the problem is caused by the ignition system, the PCM, or the fuel pump electrical circuit. Symptom Related Diagnostic Procedures

Refer to Fuel Pump Electrical Circuit Diagnosis for the fuel system wiring schematic. Component Tests and General Diagnostics

If there is a fuel delivery problem, refer to Fuel System Pressure Test. diagnoses the fuel injectors, the fuel pressure regulator, and the fuel pump. If a malfunction occurs in the fuel metering system, it usually results in either a rich HO2S signal or lean HO2S signal. This condition is indicated by the HO2S voltage, causing the PCM to change the fuel calculation (fuel injector pulse width) based on the HO2S reading. Changes made to the fuel calculation will be indicated by a change in the Long Term FT values which can be monitored with a scan tool. Ideal Long Term FT values are around 0 percent. For a lean HO2S signal, the PCM will add fuel, resulting in a fuel trim value above 0 percent. For a rich HO2S signal the PCM will take away fuel, resulting in a fuel trim value below 0 percent. Some variations in fuel trim values are normal due to power train design. If the fuel trim values are excessively more or less then 0 percent Refer to DTC P0171 Fuel Trim System Lean or DTC P0172 Fuel Trim System Rich. Fuel System Pressure Test P0171 P0172