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)'s Keep Alive Memory (KAM) to compensate for variability 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 a "biased" rich or lean fuel system. The correction is stored in the fuel trim tables. The fuel trim has two 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 relies on the fuel trim tables and Short Term refers to the desired air/fuel ratio parameter "LAMBSE". LAMBSE is calculated by the PCM from 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 will correct the rich condition by moving 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 allows Short Term Fuel Trim to return to a value near 0%. Input from the Engine Coolant Temperature (ECT) or 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. Once activated, the Fuel System Monitor looks for the fuel trim tables to reach the adaptive clip (adaptive limit) and LAMBSE to exceed a calibrated limit. The Fuel System Monitor will store the appropriate Diagnostic Trouble Code (DTC) when a fault is detected as described below.

1. The heated oxygen sensor (HO2S) detects the presence of oxygen in the exhaust and provides the PCM with feedback indicating air/fuel ratio.
2. A correction factor is added to the fuel injector pulsewidth 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 Diagnostic Trouble Code (DTC) as follows:

The DTCs associated with the monitor detecting a lean shift in fuel system operation are DTCs P0171 and P0174.

The DTCs associated with the monitor detecting a rich shift in fuel system operation are DTCs P0172 and P0175.

4. The MIL is activated after a fault is detected on two consecutive drive cycles.

Typical Fuel System Monitor Entry Conditions:
^ RPM range between Idle and 4,000 rpm.
^ Air Mass Rang greater than 0.75 lb/min.
^ Purge duty cycle of 0%.

Typical Fuel Monitor Malfunction Thresholds:
^ Lean Malfunction: LTFT > 25%, STFT > 5%.
^ Rich Malfunction: LTFT < 25%, STFT < 10%

Fuel System Monitor: