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HOMESERVICE MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 235 (ENGINE CONTROLS - THEORY & OPERATION - 3.4L)FUEL SYSTEMSFUEL CONTROLFUEL TRIM
1999 GMC Forward Control 5.7 R, Automatic

Fuel Trim

1999 GMC Forward Control 5.7 R, AutomaticSECTION Fuel Trim
WARNING: This page is about a different car, the 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, 2003 Chevrolet Monte Carlo, and 2003 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.

  1. Short Term Fuel Trim

    The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes "fine tune" the engine fueling. The ideal fuel trim values are around 0 percent. A positive fuel trim value indicates that the PCM is adding fuel to compensate for a lean condition. A negative fuel trim value indicates that the PCM is reducing the amount of fuel to compensate for a rich condition. When the PCM determines that the short term fuel trim is out of the operating range, DTC P0171: BANK 1 TOO LEAN or DTC P0172: BANK 1 TOO RICH, will set. To diagnose, go to SELF-DIAGNOSTICS - 3.4L ALERO, GRAND AM, IMPALA & MONTE CARLO article.

  2. Long Term Fuel Trim

    The long term fuel trim is a matrix of cells arranged by RPM and manifold absolute pressure (MAP). Each cell of the long term fuel trim is a register like the short term fuel trim. 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 0 percent, the PCM will change the long term fuel trim value. Once the long term fuel trim value is changed, it should force the short term fuel trim back toward 0 percent. If the mixture is still not correct, the short term fuel trim will continue to have a large deviation from the ideal 0 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 its control and still cannot correct the condition, the short term fuel trim would also go to its 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 their 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, DTC P0171: BANK 1 TOO LEAN or DTC P0172: BANK 1 TOO RICH, will set. To diagnose, see SELF-DIAGNOSTICS - 3.4L ALERO, GRAND AM, IMPALA & MONTE CARLO article.

    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.

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