Fuel System Monitor

FUEL SYSTEM MONITOR

Fuel System Monitor:

The fuel system monitor is an on board strategy designed to monitor the fuel trim system. The fuel control system uses fuel trim tables stored in the powertrain control module (PCM) keep alive memory (KAM) to compensate for variability in fuel system components due to normal wear and aging. The 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 a biased rich or lean fuel system. The correction is stored in the fuel trim tables. The fuel trim has 2 means of adapting: long term fuel trim and a short term fuel trim. Both are described in greater detail under Powertrain Control Software, Fuel Trim. Long term fuel trim relies on the fuel trim tables and short term fuel trim refers to the desired air/fuel ratio parameter called LAMBSE. LAMBSE is calculated by the PCM from the heated oxygen sensor (HO2S) inputs and helps maintain a 14.7:1 air/fuel ratio during closed-loop operation. Short term fuel trim and long term fuel trim work together. If the HO2S indicates the engine is running rich, the PCM corrects the rich condition by moving the short term fuel trim in the negative range (less fuel to correct for a rich combustion). If after a certain amount of time the short term fuel trim is still compensating for a rich condition, the PCM learns this and moves the long term fuel trim into the negative range to compensate and allow the short term fuel trim to return to a value near 0%. Input from the cylinder head temperature (CHT), intake air temperature (IAT), and mass air flow (MAF) sensors is required to activate the fuel trim system, which in turn activates the fuel system monitor. As the fuel system components age or otherwise change over the life of the vehicle, the adaptive fuel strategy learns deviations from stoichiometry while running in the closed loop. These learned corrections are stored in the KAM as long term fuel trim (LONGFT) corrections. As components continue to change beyond normal limits, or if a malfunction occurs, the LONGFT reaches a calibrated rich or lean limit and the adaptive fuel strategy is no longer allowed to compensate for additional fuel system changes. LONGFT correction at their limits, in conjunction with a calibrated deviation in short term fuel trim (SHRTFT), indicate a rich or lean fuel system malfunction. The fuel system monitor stores the appropriate DTC when a fault is detected as described.

1. The HO2S detects the presence of oxygen in the exhaust and provides the PCM with the feedback indicating air/fuel ratio.
2. A correction factor is added to the fuel injector pulse width calculation and/or mass air flow calculation, according to the long and short term fuel trims as needed to compensate for variations in the fuel system.
3. When deviation in the parameter LAMBSE increases, air/fuel control suffers and emissions increase. When LAMBSE exceeds a calibrated limit and the fuel trim table has clipped, the fuel system monitor sets a DTC as follows:

The DTC associated with the monitor detecting a lean shift in fuel system operation is P0171.

The DTC associated with the monitor detecting a rich shift in fuel system operation is P0172.

4. The malfunction indicator lamp (MIL) is activated after a fault is detected on 2 consecutive drive cycles.

Typical Fuel System Monitor Entry Conditions:
- RPM range greater than idle.
- Air mass range greater than 0.75 lb/min.
- Purge duty cycle of 0%.

Typical Fuel Monitor Malfunction Thresholds:
- Lean malfunction: LONGFT greater than 25%, SHRTFT greater than 5%.
- Rich malfunction: LONGFT less than -25%, SHRTFT less than -10%.