Long Term Fuel Trim

The long term fuel trim is a matrix of cells arranged by RPM and Manifold Absolute Pressure (MAP). As the engine operating conditions change, the PCM will switch from cell to cell to determine what long term fuel trim factor to use in the base pulse width equation.

While in any given cell, the PCM also monitors the short term fuel trim. If the short term fuel trim is far enough from zero percent, the PCM will change the long term fuel trim value. Once the long term fuel trim value is changed, the long term fuel trim should force the short term fuel trim back toward zero percent. If the mixture is still not correct, the short term fuel trim will continue to have a large deviation from the ideal zero percent. In this case, the long term fuel trim value will continue to change until the short term fuel trim becomes balanced. Both the short term fuel trim and long term fuel trim have limits which vary by calibration. If the mixture is off enough so that long term fuel trim reaches the limit of long term fuel trim control and still cannot correct the condition, the short term fuel trim would also go to the short term fuel trim limit of control in the same direction. If the mixture is still not corrected by both short term fuel trim and long term fuel trim at short term fuel trim and long term fuel trim extreme values, a fuel trim Diagnostic Trouble Code (DTC) will likely result. When the PCM determines that the long term fuel trim is out of the operating range, the following DTCs will set:

• DTC P0171: FUEL TRIM SYSTEM LEAN
• DTC P0172: FUEL TRIM SYSTEM RICH

Under the conditions of power enrichment, the PCM sets the short term fuel trim to zero percent until power enrichment is no longer in effect. This is done so the Closed Loop factor and the long term fuel trim will not try to correct for the power enrichment condition.