GM > Engine > Idle > Idle RPM


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.


Idle RPM


Adaptive Idle RPM
The adaptive idle parameters are only updated under warm engine, steady idle conditions. These conditions are described by the parameters below.


Stall Saver
These tables indicate to the PCM when an impending stall condition is detected. The PCM has the ability to set the derivative RPM idle control system into action at a fast rate and disengage the AC clutch if a potential stall condition is detected.


PID Delays
These timers indicate the delay before the various Proportional, Integral and Derivative idle control systems are activated after engine startup and after idle conditions have been met.


Proportional
The proportional idle correction routines provide an increase or decrease to the Idle Airflow that is directly proportional (as the name suggests) to the RPM Error (Desired Idle RPM - Actual RPM). Various tables are provided to increase decrease the speed rate of correction for various conditions described below.


Integral
The integral idle correction routines provide a slow moving increase or decrease to the Idle Airflow that is proportional to the integrated RPM Error (Desired Idle RPM - Actual RPM). Various tables are provided to increase decrease the speed rate of correction for various conditions described below. Note, the integral adjustment makes slow and small changes to the Desired Idle Airflow to make final fine idle adjustments once the derivative and proportional correction has got the idle within their effective working regions. Do not make the integral tables fast moving and unstable idle conditions may result as the proportional and integral corrections get into a race condition.


Derivative
The derivative idle correction routines provide a fast moving increase or decrease to the Idle Airflow that is related to the derivative of the RPM Error. Various tables are provided to increase decrease the speed rate of correction for various conditions described below. The derivative control is designed to make fast corrections to the Idle Airflow when large deviations from the Desired Idle RPM over a short time occur. The derivative control also works in conjunction with the stall saver function to attempt to prevent engine stalling. The derivative airflow uses a fast and slow filtered RPM signal and then calculates the ratio of the two values (the derivative or rate of change of the RPM). This ratio is used as the axis for the lookup tables. The filter values are configurable to make the time response of the system faster or slower.