GM > Engine > Fuel Control > Open & Closed


Open and Closed Loop
The VCM fueling generally operates in one of three modes: 1) Open loop, 2) Open loop STFT or 3) Closed Loop. Open loop is used when the O2 sensors are not warmed up, failed, or when an enrichment mode (such as power enrichment) is active. Open loop STFT is used on some PCM's when the O2 sensors are showing signs of operation (O2 Readiness), however the closed loop enable ECT has not been reached. In this mode the STFT's may function. The final mode is Closed Loop and in this mode the full closed loop fuel controller is operational.

This section allows controlling of the various base Equivalent Ratio's and AFR's when the VCM is running in openloop. It also allows control over the entry of the various modes described above.


Open Loop


Closed Loop
These parameters influence the VCM’s Closed Loop Fuelling behavior.


Long Term Fuel Trims

LTFT Enable


LTFT Boundaries


General


LTFT Idle Cells Thresholds


Fuel Trims LS1 V8
0 - 15 defined by the LTFT RPM and MAP boundaries
16 - Idle PN (AC on)
17 - Idle PN (AC off)
18 - Idle InGear (AC on)
19 - Idle InGear (AC off)
20 - Idle (EVAP closed)
21 - Coastdown (EVAP closed)
22 - Non-Idle (EVAP Closed)

Usually EVAP closes at WOT hence cell #22 is mostly used at WOT, but you will see it used during warmup (before evap purge begins). At WOT you will also see cell #15 if the EVAP purge opens (ie. vacuum drops enough for the purge to begin).


Fuel Trims LS2/LS7/L76, Cobalt SS 2.0L s/c and other later model vehicles that use RPM/MAP boundaries
The LS2/7 and others have more cells but follow a similar pattern to the LS1:

EVAP Open
0 - 15 defined by the LTFT RPM and MAP boundaries
16 - Idle PN (AC on)
17 - Idle PN (AC off)
18 - Idle InGear (AC on)
19 - Idle InGear (AC off)
20 - Decel

EVAP Closed
21 - 36 defined by the LTFT RPM and MAP boundaries
37 - Idle
38 - Decel


Fuel Trims later LS3 and other later model vehicles that use Airflow Mode boundaries

EVAP Open
0 - 5 defined by the LTFT Airflow Mode Purge On boundaries
6 - Idle
7 - Decel

EVAP Closed
8 - 13 defined by the LTFT Airflow Mode Purge Off boundaries
14 - Idle
15 - Decel


Fuel Trims V6 vehicles
0 = Idle
1 = Decel
2 = Light Load / Engine Not Running
3 = High Load
4 = Med Load