Fuel System Monitor

DESCRIPTION
The fuel system monitor is a self-test strategy within the Powertrain Control Module (PCM) that monitors the adaptive fuel table. The fuel control system uses the adaptive fuel table to compensate for normal variability of the fuel system components caused by wear or aging. During normal vehicle operation, if the fuel system appears "biased" lean or rich, the adaptive fuel table will shift the fuel delivery calculations to remove the bias.

Hardware
The fuel control system uses the Heated Oxygen Sensors (HO2S). Other EEC-V components that provide input to the adaptive fuel table are the Intake Air Temperature (IAT), Engine Control Temperature (ECT), and Mass Air Flow (MAF) sensors. The HO2S measures the amount of oxygen in the exhaust and as a result, fuel will be adjusted rich/lean. The IAT and ECT sensors measure ambient and engine coolant temperature. The MAF sensor measures the mass flow rate of incoming air.

Operation






As fuel control and air metering components age or vary from nominal value, the adaptive fuel strategy learns corrections while in closed loop. These corrections are stored in a table that is a function of engine speed and load. It is called Long Term Fuel Trim (LONGFT). LONGFT is used throughout the engine operating strategy. While 0% is an ideal value, -35% to 35% is acceptable, depending on vehicle application. The table resides in Keep Alive Memory (KAM) and is used to correct fuel delivery while in open or closed loop. As the components continue to change and the table learns the corrections, some values in the table may reach the calibrated clip or limit. This means that the adaptive fuel table can no longer compensate for additional changes in the fuel system. Further changes in the fuel system will cause deviation in the closed loop fuel parameter, called Short Term Fuel Trim (SHRTFT). SHRTFT is a fuel correction value used by the PCM during closed-loop operation to adjust the air/fuel ration to meet current conditions. When the HO2S input indicates the that the air/fuel ratio is acceptable, the SHRTFT value is 0%. As the HO2S input indicates that conditions become rich or lean, acceptable SHRTFT values may range from -35% to 35%, depending on vehicle application. As this trim gets larger and larger, fuel/air control suffers and emissions may increase and a DTC is stored.

Entry conditions for the fuel system test are that engine rpm is within calibrated limit, air mass into the engine is within a calibrated limit, engine is at operating temperature and closed loop. Idle and decelerations are excluded from fuel system testing.

DTCs and MIL Operation
When a malfunction has been present for two drive cycles, the DTC is stored in the PCM and the MIL is turned On. The MIL is turned Off after three consecutive drive cycles without the same malfunction being detected provided no other DTCs are stored that would independently turn the MIL On. The DTC will be erased from memory after 40 warm-up cycles without the malfunction being detected after the MIL is turned Off. Another method of erasing the DTC is initiating a Powertrain Control Module Reset.