GM > Engine > Idle > Idle Airflow

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

AC Airflow

Cooling Fan Airflow

DFCO Airflow

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

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.

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.