Firebird V8-5.7L VIN G (2001)
Throttle Position Sensor: Description and Operation
The TP sensor attaches to the side of the throttle body opposite the throttle lever. It senses the throttle valve angle and relays that information to the
PCM. The PCM requires knowledge of throttle angle to generate the required injector control signals, or pulses.
The short term Fuel Trim (FT) is a PCM erasable memory register. The neutral value for the short term FT is 0 percent. Any deviation from 0 percent
indicates that the short term FT 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 percent. The short term FT is rich when the scan tool indicates a negative number. The short FT is lean when the scan tool indicates a number
greater than 0. The short term FT changes the injector pulse width by adding to or subtracting from the base pulse width equation. As the PCM monitors
the oxygen sensors input, the PCM constantly varies the short term FT value. The value is updated very quickly, therefore the short term FT only
corrects for short term mixture trends. The correction of the long term mixture trends is the function of long term FT.
When the PCM determines that the short term FT is out of the operating range, 1 of the following DTCs will set:
^
DTC P0171 FT System Lean Bank 1
^
DTC P0172 FT System Rich Bank 1
^
DTC P0174 FT System Lean Bank 2
^
DTC P0175 FT System Rich Bank 2
The long term Fuel Trim (FT) is a matrix of cells arranged by RPM and MAP. Each cell of the long term FT is a register like the short term FT. As the
engine operating conditions change, the PCM will switch from cell to cell in order to determine what long term FT factor to use in the base pulse width
equation.
While in any given cell, the PCM also monitors the short term FT. If the short term FT is far enough from 0 percent, the PCM will change the long term
FT value. Once the long term FT value is changed, the PCM should force the short term FT back toward 0 percent. If the mixture is still not correct, as
judged by the PCM, the short term FT will continue to have a large deviation from the ideal 0 percent. In this case, the long term FT value will continue
to change until the short term FT becomes balanced. Both the short term FT and the long term FT have limits which vary by calibration. If the mixture is
off enough so that long term FT reaches the limit of its control and still cannot correct the condition, the short term FT would also go to its limit of
control in the same direction. If the mixture is still not corrected by both short term FT and long term FT at their extreme values, a FT DTC will likely
result. When the PCM determines that the long term FT is out of the operating range, the following DTCs will set:
^
DTC P0171 FT System Lean Bank 1
^
DTC P0172 FT System Rich Bank 1
^
DTC P0174 FT System Lean Bank 2
^
DTC P0175 FT System Rich Bank 2
Under the conditions of power enrichment, the PCM sets the short term FT to 0 percent and freezes it there until the power enrichment is no longer in
effect. This is done so the Closed Loop factor and the long term FT will not try to correct for the commanded richness of power enrichment.
