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 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 short term fuel trim. Both are described in greater detail in this article under POWERTRAIN CONTROL SOFTWARE , Fuel Trim. Long term fuel trim relies on the fuel trim tables. Short term fuel trim refers to the desired air to fuel ratio parameter called LAMBSE. The LAMBSE is calculated by the PCM from the heated oxygen sensor (HO2S) inputs and helps maintain a 14.7:1 air to 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 airflow (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 (LONGFT1) corrections. As components continue to change beyond normal limits, or if a concern occurs, the LONGFT1 reaches a calibrated rich or lean limit and the adaptive fuel strategy is no longer allowed to compensate for additional fuel system changes. The LONGFT1 correction at their limits, in conjunction with a calibrated deviation in short term fuel trim (SHRTFT1), indicates a rich or lean fuel system concern. The fuel system monitor stores the appropriate DTC when a concern is detected as described below.
- The HO2S detects the presence of oxygen in the exhaust and provides the PCM with the feedback indicating air to fuel ratio.
- A correction factor is added to the fuel injector pulse width calculation or MAF calculation, according to the long and short term fuel trims as needed to compensate for variations in the fuel system.
- When deviation in the parameter LAMBSE increases, air to 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:
- P0171 - monitor detecting a lean shift in fuel system operation
- P0172 - monitor detecting a rich shift in fuel system operation
- The malfunction indicator lamp (MIL) is activated after a concern is detected on two consecutive drive cycles.
Typical fuel system monitor entry conditions:
- engine coolant temperature is between 68°C to 110°C (155°F to 230°F)
- engine load is greater than 30%
- intake air temperature is between -40°C to 65°C (-40°F to 150°F)
- fuel level is greater than 15%
- purge duty cycle of 0%
Typical fuel monitor thresholds:
- lean condition: LONGFT1 is greater than 28% and SHRTFT1 is greater than 2%
- rich condition: LONGFT1 is less than 24% and SHRTFT1 is less than -2%