Corvette V8-5.7L VIN G (1997)
Engine Control Module: Description and Operation
Fuel Trim
SHORT TERM FUEL TRIM DESCRIPTION
The short term fuel trim is a PCM erasable memory register. The neutral value for the short term fuel trim is 0%. Any deviation from 0% indicates the
short term fuel trim is changing the injector pulse width. The amount of pulse width change depends on how far the short term fuel trim value is from
0%. The short term fuel trim is rich when the scan tool indicates -99%. The short fuel trim is lean when the scan tool indicates 99%. The short term
fuel trim changes the pulse width by varying the Closed Loop factor of the base pulse width equation. As the PCM monitors the oxygen sensors input,
it is constantly varying the short term fuel trim value. The value is updated very quickly, therefore, the short term fuel trim only corrects for short term
mixture trends. The correction of long term mixture trends is the function of long term fuel trim.
When the PCM determines that the Short Term Fuel Trim is out of the operating range, the following DTCs will set:
^
DTC P0171 Bank 1 Too Lean.
^
DTC P0172 Bank 1 Too Rich.
^
DTC P0174 Bank 2 Too Lean.
^
DTC P0175 Bank 2 Too Rich.
LONG TERM FUEL TRIM DESCRIPTION
The long term fuel trim is a matrix of cells arranged by RPM and 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%, 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%. If the mixture is still
not correct (as judged by the HO2S), the short term fuel trim will continue to have a large deviation from the ideal 0%. 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, the following DTCs will set:
^
DTC P0171 Bank 1 Too Lean.
^
DTC P0172 Bank 1 Too Rich.
^
DTC P0174 Bank 2 Too Lean.
^
DTC P0175 Bank 2 Too Rich.
Under the conditions of power enrichment, the PCM sets the short term fuel trim to 0% and freezes it there 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 commanded richness of power enrichment.
