GM > Engine > Idle > Idle Airflow

Idle
The PCM has a number of parameters that influence the idle behavior of the engine. These parameters control the desired idle RPM and the behavior of the Idle Air Control (IAC) motor or Electronic Throttle Control (ETC) during idle conditions. The IAC is controlled via a desired airflow value that has many contributing factors, the PCM then translates this "Idle Desired Airflow" into a number of steps for the IAC or throttle % for ETC fitted vehicles.

Base Running Airflow

• Idle Air Flow vs. Coolant Temp vs. (Gear & P/N): This table is used to determine the base airflow (in g/sec) for the IAC/ETC control vs Engine coolant temperature. This is used as a base setting (or startpoint) for the adaptive idle routines and also the base IAC position when the vehicle is moving.
• Final Idle Airflow Minimum vs. RPM vs. Gear: This is the minimum allowed idle airflow after all idle calculations.
• Idle Airflow Mult vs. Baro: Idle Airflow multiplier based on barometric pressure.
• Idle Airflow Mult vs. ECT: Idle Airflow multiplier based on ECT.
• Airflow Final Minimum: This table is used to determine the minimum base airflow (in g/sec) for the ETC control versus RPM and Gear. The idle airflow will be limited to values higher than this number. Increasing these values can result in "hanging" RPM.

AC Airflow

• Idle AC Airflow Ramp In Rate: The idle airflow ramp in step size after the AC clutch releases.
• Idle AC Airflow Ramp Out Rate: The idle airflow ramp out step size after the AC clutch is applied.
• Idle Trans Airflow Ramp In Rate: The idle airflow ramp in step size on a Park to Drive transition.
• Idle Trans Airflow Ramp Out Rate: The idle airflow ramp out step size on a Park to Drive transition.

Cooling Fan Airflow

• Fan 1: This value is the extra airflow added to the base running airflow to compensate for extra electrical load when a single cooling fan is active.
• Fan 1 & 2: This value is the extra airflow added to the base running airflow to compensate for extra electrical load when both cooling fans are active.

DFCO Airflow

• DFCO Entry Hold Time: Time delay before decaying DFCO airflow.
• DFCO Entry Ramp Rate: When DFCO is entered airflow is ramped to the target Entry DFCO value at this rate.
• DFCO Exit Ramp Rate: When DFCO is exited airflow is ramped to the target DFCO Exit value at this rate.

Throttle Follower
The throttle follower parameters are used to adjust decay and delay rates for the IAC motor during throttle closure to slow the return to idle during a sudden throttle closure.

• Throttle Follower Airflow Decay vs. Speed (In Neutral): The decay value is the rate at which the IAC motor closes when the throttle is closed and the selector is in Park or Neutral. These values are subtracted from the current IAC Airflow value every 12.5 ms, hence higher numbers mean faster decay.
• Throttle Follower Delay Time vs. Speed (In Neutral): This is the delay before the decay value begins to be subtracted in P/N.
• Throttle Follower Airflow Decay vs. Speed (In Gear): The decay value is the rate at which the IAC motor closes when the throttle is closed and the selector is in gear. These values are subtracted from the current IAC Airflow value every 12.5 ms, hence higher numbers mean faster decay. The 4th Gear row is used for gears including and above 4th gear for M6 transmissions.
• Throttle Follower Delay Time vs. Speed (In Gear): This is the delay before the decay value begins to be subtracted in gear.

  
  

• TPS Tracker Step Size %: This is the rate at which the throttle follower tracks the current TPS value.
• Throttle Follower Airflow vs. TPS: This is the throttle follower airflow value used as the throttle is opened. It is the value that is reduced to zero by the decay parameters if the throttle is closed. It is versus TPS %.
• Throttle Follower Airflow Mult vs. RPM: This table is used to enable the throttle follower in relation to RPM.

Throttle Cracker
The throttle cracker parameters are used to open the IAC motor as vehicle operating conditions change.

• Throttle Cracker Airflow vs. RPM vs. MPH: This table is used to open the IAC motor as RPM and MPH vary.
• Enable Speed: MPH threshold to disable the adaptive idle and enable the throttle cracker.
• Disable Speed: MPH threshold to enable the adaptive idle and disable the throttle cracker.

Adaptive Idle Airflow Limits
During the adaptive idle process the PCM is constantly updating a short term idle trim (STIT) and a number of long term idle trims (LTIT). The goal of the LTIT's is to make the STIT equal zero. The LTITs have limits to their adjustment capability and it is important to tune areas such as Base Running Airflow and various other compensation values such that the long term trims remain within their bounds. It is very useful to log the LTITs and the STIT using the scanner to ensure that the values once stabilized are within the limits below.

It is important to understand that the current STIT is always reset to zero when not at idle and when idle conditions return the STIT begins to learn again from its remembered position when last at idle. The STIT has a number of different memories depending on the current PN/Gear/ACon/ACoff state but none of these values can exceed the limit set. Hence the further the STIT memory is out from the actual required STIT, it has to a quick catch-up every time the engine returns to idle which is not desirable and can result in non-ideal return to idle behavior.

It is desirable to keep LTIT within the learning limits to ensure the STIT does no have to move far under the various PN/Gear/ACon/ACoff state changes to ensure smooth and consistent idle transitions. It is also important to realize that the LTIT values are slow moving and take time to adjust to any base of compensation airflow calibration changes. Again use the scanner to make sure the LTITs have reached steady state in the region changed before making any tuning judgments.

• Max - In Gear/AC Off: LTIT maximum limit when in Gear and AC off.
• Min - In Gear/AC Off: LTIT minimum limit when in Gear and AC off.
• Filt - In Gear/AC Off: LTIT filter coefficient defines how fast the LTIT responds to correct the current STIT when in gear and AC off.
• Max - PN/AC Off: LTIT maximum limit when in Park/Neutral and AC off.
• Min - PN/AC Off: LTIT minimum limit when in Park/Neutral and AC off.
• Filt - PN/AC Off: LTIT filter coefficient defines how fast the LTIT responds to correct the current STIT when in Park/Neutral and AC off.
• AC Offset Min - In Gear: AC offset airflow minimum limit, AC on and in gear.
• AC Offset Max - In Gear: AC offset airflow maximum limit, AC on and in gear.
• AC Offset Min - PN: AC offset airflow minimum limit, AC on and in park/neutral.
• AC Offset Max - PN: AC offset airflow maximum limit, AC on and in park/neutral.
• AC Offset Filter: Filter coefficient defines how fast the AC offset responds to correct the current STIT when AC on.
• ST Adapt Airflow Max: Current STIT maximum memory value. STIT will move higher than this to maintain idle speed however, remembered idle value will not exceed this value (eg. if the STIT was 3.44 and the maximum memory value was 2.0, then upon returning to idle in this state the remember value would be 2.0 and the STIT would have to make a quick learn to the previous value of 3.44). This applies to the STIT learned during any of the various PN/Gear/ACon/ACoff states.

Startup
The PCM commands extra airflow during engine startup to ensure proper starting. In simple terms the startup airflow adds to the base airflow and decays away to zero as the engine begins running and the adaptive routines start enabling and controlling idle. The startup airflow consists of two components that are added together, Friction Airflow and Startup Airflow.

• Off Idle Startup Scalar: Amount to reduce startup airflow if an off-idle transition is made during startup airflow decay. This value multiplies the current startup airflow ie. a value of 0.5 would halve the current startup airflow if an off-idle transition occurs during the decay. A value of 1.0 would have no effect, and value of 0.0 would instantly decay the startup airflow to zero if any off-idle transition occurs.
• Underspeed Time: Transition into adaptive idle (PID) control if no startup flare control has occurred within this time after startup.
• Friction Airflow Decay: Decay value for startup airflow decay. This value is subtracted from the initial value.
• Friction Airflow Initial: Initial value for Friction Airflow, decay starts from this value.
• Friction Airflow Delay: Delay before Friction Airflow starts to decay.
• Startup Airflow Decay: Decay value for startup airflow decay. This value is subtracted from the initial value.
• Startup Airflow Initial: Initial value for Friction Airflow, decay starts from this value.
• Startup Airflow Delay: Delay before Startup Airflow starts to decay.
• Startup Airflow: Used to control the ETC opening during engine cranking.
• Idle Startup Airflow vs. ERT vs. ECT: Startup idle airflow value used to control the ETC opening during startup.

General

• IAC Position vs. Effective Area: This table converts the desired IAC/ETC effective area (in square millimeters) to IAC steps for IAC stepper motor control.
• IAC Park Position Airflow vs. IAT: This table is used to determine the Airflow to control the IAC/ETC when the IAC motor is in the park position. (ie. when the ignition is in the off position).
• IAC Reset Park Position: This value is used to set the IAC motor position after a PCM reset.
• Choke Flow Time: Time to delay before using choke flow model for effective area estimation (requires MAF is working).
• Desired IAC Area Max: Maximum desired effective area in square millimeters.
• ETC Area Scalar: Constant used to convert from Desired Effective Area (mm2) to Desired ETC percent. Used for ETC fitted vehicles.
• Desired Airflow Max: Maximum allowed Desired Idle Airflow.
• Idle Effective Area Max: Maximum allowed idle Desired Effective Area (mm2).
• Idle Effective Area Min: Minimum allowed idle Desired Effective Area (mm2).
• Idle Effective Area Max - Fail: Maximum allowed idle Desired Effective Area (mm2) during failure conditions.
• Idle Effective Area Min - Fail: Minimum allowed idle Desired Effective Area (mm2) during failure conditions.