Fuel System Monitoring

System description

The determination of how much fuel to deliver to the engine is the calculation of injector pulse width. Basic injection duration (injector pulse width) is determined by:
- Engine load (MAF sensor)
- Engine Speed (SGT)
- Short fuel trim correction factor
- Long fuel trim correction factor
- Fuel trim is monitored to meet the OBD-II requirements

1. Short fuel trim (FT)
Short FT is a temporary correction to fuel delivery which changes with every cycle of the front Heated Oxygen Sensor. Under normal conditions, it fluctuates rapidly around its ideal value of 0 % correction and is set to 0 % during open loop operation.

Short FT is a parameter of the OBD-II serial data stream and can be displayed by a scan tool. The normal range of short FT is ± 25 % but under normal operating conditions it rarely goes beyond ± 10 %. Short FT responds to changes in front HO2S output. If basic injection duration results in a lean air/fuel ratio, Short FT responds with positive corrections (+1 % to +25 %) to add fuel and enrich the mixture. If basic injection is too rich, Short FT responds with negative corrections (-1 % to -25 %) to subtract fuel and lean the mixture.

2. Long fuel trim
Long FT is a data parameter on the OBD-II data streams. It is a more permanent correction to fuel delivery because it is part of the basic injection duration calculation. Long FT changes slowly, in response to Short FT. It's normal range is ± 25 %, positive values indicating rich correction and negative values indicating lean correction.

Unlike Short FT which affects injection duration calculation in closed loop only, the Long FT correction factor affects the basic injection duration calculation in open and closed loop. Because Long FT is stored in a non-volatile RAM and is not erased when the ignition is switched OFF the fuel system is able to correct for variances in engine and fuel conditions even during warm-up and wide open throttle conditions.

Monitoring process
The ECM monitors the averages of long term and short term fuel trim. If these fuel trim values reach and stay at their maximum limits (±25 %) for a defined period of time, a diagnostic trouble code is set.

MIL illumination
When the fuel trim value exceeds the failure thresholds, then the MIL is only turned ON if the same fault is detected during a second drive cycle.