Ascender 2WD L6-4.2L (2003)
Air/Fuel Mixture: Description and Operation
Fuel Trim
The control module controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission
control. The control module monitors the HO2S signal voltage while in Closed Loop and regulates the fuel delivery by adjusting the pulse width of the
injectors based on this signal. The ideal fuel trim values are around 0 percent for both short and long term fuel trim. A positive fuel trim value indicates
the control module is adding fuel in order to compensate for a lean condition by increasing the pulse width. A negative fuel trim value indicates that the
control module is reducing the amount of fuel in order to compensate for a rich condition by decreasing the pulse width. A change made to the fuel
delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltage. These
changes fine tune the engine fueling. The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term
fuel trim. A scan tool can be used to monitor the short and long term fuel trim values. The long term fuel trim diagnostic is based on an average of
several of the long term speed load learn cells. The control module selects the cells based on the engine speed and engine load. If the control module
detects an excessively lean or rich condition, the control module will set a fuel trim diagnostic trouble code (DTC).
