Learned Fuel Control

The front oxygen sensor sends signals on exhaust gas oxygen content to the PCM to control a 14.7 to 1 air/fuel ratio under normal driving conditions. The PCM can make fuel corrections from a nominal 128 (0% correction) short/long term fuel trim value to a maximum window of 101 to 147 (-21% to 15%).

If the PCM determines a rich condition exists (02 voltage above 450 mv), it will decrease injector pulse width (reducing fuel) to maintain a 14.7 to 1 air/fuel ratio. The short term fuel trim value will be between 101 and 128 indicating a rich condition. If the PCM determines a lean condition exists (O2 voltage below 450 mv), it will increase injector pulse width (adding fuel) maintaining a correct air/fuel mixture. The short term fuel trim value will be between 128 and 147 indicating a lean condition. When certain conditions have been met to begin learning fuel control (ECT closed loop, etc.) the PCM will use the short term fuel trim cell to update one of the four long term cells (idle, decel, cruise, accel) being used.

During every drive cycle (trip) the PCM will use and update the idle and cruise long term cells and will keep track of how long the control system is operating in these cells. After approximately two minutes of being in each of these cells the fuel control system will enable purge (purge On) and the purge long term cells will be used. If the average of the idle and cruise fuel cells is 147 and the short term fuel trim cell is currently over 128, a lean fuel system DTC will set. This test is performed before the purge cells are used.