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Fuel System Monitor
The fuel system monitor allows the PCM to evaluate how close the fuel control system can regulate the fuel mixture in attempting to achieve an optimum air/fuel ratio in the combustion chamber throughout varying engine rpm and load ranges. The monitor executes continuously under normal engine operating and driving conditions providing there are no DTCs present and the EVAP system is not purging fuel vapors into the engine.
The fuel control system uses fuel trim tables stored in the PCMs memory to calculate and compensate for the variability that occurs in fuel system components due to normal wear and aging. Fuel trim tables are based on engine RPM and engine load. During closed-loop fuel control, the fuel trim strategy learns the corrections needed to correct for a rich or lean fuel system. The correction is stored in the fuel trim tables. The fuel trim tables have upper and lower limits for modification of the fuel injection base pulse width needed to maintain a 14.7:1 air/fuel ratio during closed-loop operation.
If the oxygen sensor(s) indicate the engine is running rich, the PCM will correct the rich condition by reducing the fuel injection pulse width. In a similar manner if the oxygen sensor(s) indicate the engine is running lean, the PCM will correct the lean condition by increasing the fuel injection pulse width. The extent to which the base pulse width can be modified is limited. When the PCM calculates it will require pulse width modification outside the upper or lower limits of adjustment to achieve a 14.7:1 air/fuel ratio, a pending DTC is stored. If the lean or rich condition recurs during the next 3 warm-up cycles the PCM stores a DTC and illuminates the check engine light.
