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

• Open Loop F/A vs. Coolant Temp vs. MAP: This table is used to determine the commanded AFR when in open loop mode. It divides the Stoich AFR value. AFR is determined by 14.7/table value. Example: 14.7/1.30 = 11.3 AFR
• Open Loop F/A vs. Coolant Temp vs. MAP (Alcohol): Flexible Fuel Capable calibrations only. This table is used to determine the commanded AFR when in open loop mode. It divides the Stoich AFR value. If you make changes to the non alcohol table AFR, you must make the exact changes on this table as well. If not, then you will wind up with a difference between the two for your open loop AFR. AFR is determined by 14.7/table value. Example: 14.7/1.30 = 11.3 AFR
  
• Open Loop EQ Ratio Gas (Gear): This table is used to determine the commanded open loop equivalence ratio when running on gasoline and the transmission is in gear.
• Open Loop EQ Ratio Gas (P/N): This table is used to determine the commanded open loop equivalence ratio when running on gasoline and the transmission is in park or neutral.
• Open Loop EQ Ratio Alcohol (Gear): This table is used to determine the commanded open loop equivalence ratio when running on alcohol and the transmission is in gear.
• Open Loop EQ Ratio Alcohol (P/N): This table is used to determine the commanded open loop equivalence ratio when running on alcohol and the transmission is in park or neutral.
  
• Airflow Gain: This table is used to multiply the commanded open loop equivalence ratio based on airflow parameters.
• IVT Gain: This table is used to multiply the commanded open loop equivalence ratio based on intake valve temperature parameters.
• Injector Temp Gain: This table is used to multiply the commanded open loop equivalence ratio based on fuel injector temperature parameters.
• AIR Enrich: This table is used to multiply the Equivalence Ratio when in open loop mode when the AIR pump is on.
• AIR Gain: This table is used to multiply the Equivalence Ratio when in open loop mode when the AIR pump is on.
• AIR Airflow Gain: This table is used to multiply the Equivalence Ratio when in open loop mode when the AIR pump is on.
• Misfire EQ Ratio: Equivalence Ratio when in Cat protection mode due to misfire.
• EQ Ratio Minimum vs. ECT: This table is used to determine the minimum Equivalence Ratio when in open loop mode.
• TCS EQ Ratio Minimum: Minimum EQ Ratio when TCS is active.
• ETC Reduced Power EQ Ratio: EQ Ratio when ETC Reduced Power Mode is active.
• Closed Loop Blend Time: EQ Ratio will be blended from openloop to 1.0(stoich) over this amount of camshaft reference pulses.
• EQ Ratio Min: This table is used to define the lowest allowed commanded open loop equivalence ratio.

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

• Closed Loop Enable ECT vs. IAT: This table sets the coolant temperature required to enable Closed Loop in relation to inlet air temperature (IAT).
• Closed Loop Enable Coolant Temp vs. Startup Coolant Temp: This table sets the coolant temperature required to enable Closed Loop in relation to startup coolant temperature.
• Closed Loop Enable Delay: This table sets the delay (secs) after startup and before closed loop will be enabled.
• Closed Loop ECT: This is the Engine Coolant Temperature (ECT) above which closed loop will be enabled.
• O2 Readiness ECT: This is the Engine Coolant Temperature (ECT) above which the O2 readiness tests will be enabled. On some vehicles once the O2 Readiness tests complete successfully the PCM will enter Open loop STFT mode ie. enabling the STFT's even thought he closed loop enable temperature has not been reached.
• O2 Readiness ECT vs. Startup Coolant Temp: Below this coolant temp the O2 readiness tests will not run.
  
• Closed Loop Mode vs. Airflow: This table defines the Airflow mode the Closed Loop routines use to operate. It controls selection of various gains and delays used to interpret the O2 sensor signals.

  These tables define the Rich/Lean switching voltages for the O2 sensor in relation to Airflow Mode.

• O2 Sensor B1 R/L vs. Airflow Mode: Table for Bank 1 (Driver Side)
• O2 Sensor B2 R/L vs. Airflow Mode: Table for Bank 2 (Passenger Side)

  
  These two tables affect the % fuel needed for proper closed loop response and start the STFT process

• Closed Loop Proportional Base vs. Airflow Mode: This table returns the base proportional % fuel change. Proportional base rate table is the primary amount of fuel needed to drive the closed loop fuel control into oscillation. Proportional fuel acts like an on/off switch to keep the fuel moving around the current O2 Rich/Lean vs. Mode table set point. The values on the Proportional Table add or subtract to the base fuel rate depending on the previous fuel condition (i.e. if rich then switch lean, if lean the switch rich). The amount of fuel to add or subtract increases with the airflow mode and should be based on injector size and % fuel switching needed.
• Closed Loop Proportional Gain vs. O2 Error: This table returns a multiplier value for the increase/decrease of the base rate table. A multiplier value of 1.000 will have no effect on the proportional base rate. If the difference between the current O2 reading and its desired value from table O2 Rich/Lean vs Mode (the current Fast O2 error) is large the VCM will need to change the proportional fuel a lot. If the error is small, it should change it only a little to continue oscillation without undershoot or overshoot. Undershooting will cause the Closed loop fuel to become sluggish or miss the current O2 Rich/Lean vs. Mode table set point. Overshooting with cause the engine to vary excessively in RPM.

  
  The TPS and MPH values determine the following
  - Alternate fuel trim gain behavior during idle
  - Idle AFR when in open loop mode
  
  Both conditions must be met for idle behavior to be selected.

• Fuel Idle TPS Threshold: Minimum Throttle Position Percent Required
• Fuel Idle Speed Threshold: Minimum Speed Required

  
  These two tables affect the STFT time needed for proper closed loop response

• Closed Loop Integrator Delay vs. Airflow Mode: This table returns the base integrator delay.
  The integrator is a function of the Slow Filtered O2 Error over time, The integrator handles persistent rich or lean fuel conditions based on the slow O2 error and Rich/Lean vs Airflow mode. The Base Delay vs Airflow Mode table is the minimum time in milliseconds an overly rich or lean condition needs to exist before the VCM is allowed to make a fuel correction.
• Closed Loop Integrator Delay Mult vs. O2 Error: This table returns a multiplier value for the increase/decrease of the base integrator time delay. A multiplier value of 1.000 has no effect on the base delay. If the slow o2 error is extremely large the integrator will need to act more quickly. If the error is small the integrator will need less control to keep the closed loop fuel switching closer to the current O2 Rich/Lean vs. Mode table set point.

Long Term Fuel Trims

LTFT Enable

• LTFT Min ECT: LTFT learn is disabled below this engine coolant temperature.
• LTFT Max ECT: LTFT learn is disabled above this engine coolant temperature.
• Long Term Fuel Trim Min MAP vs. Baro: MAP must be above this value to enable LTFT learning.

LTFT Boundaries

• Long Term Fuel Trim RPM Boundaries: This table defines the RPM boundaries for the LTFT cells.
• Long Term Fuel Trim MAP Boundaries: This table define the MAP boundaries for the LTFT cells.

General

• Closed Loop Mode vs. Airflow: This table defines the airflow mode the Closed Loop routines use to operate. It controls selection of various gains and delays used to interpret the O2 sensor signals.

LTFT Idle Cells Thresholds

• LTFT Idle Cells Enable TPS: LTFT idle cells will be used below this throttle position.
• LTFT Idle Cells Disable TPS: LTFT idle cells will be disabled above this throttle position.
• LTFT Idle Cells Enable VSS: LTFT idle cells will be used below this vehicle speed.
• LTFT Idle Cells Disable VSS: LTFT idle cells will be disabled above this vehicle speed.

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