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**

__Target Idle Speed vs. Coolant Temp vs. (Drive & Park, A/C On & Off):__This table controls the desired idle RPM vs Engine Coolant Temperature. There are a number of settings depending if the transmission is an A4 or M6 (in gear or PN) and also is the AC is on or off. When the vehicle speed is below the Throttle Cracker speed and Throttle Cracker and Throttle Follower airflows are zero, the PCM will enable adaptive, feedback-based (PID) idle routines to control IAC/ETC and achieve the desired idle RPM.__Target Idle Speed vs. Coolant Temp:__This table controls the desired idle RPM vs Engine Coolant Temperature. .__Target Idle Speed (AC Off):__The desired idle RPM with AC off.__Target Idle Speed (AC On):__The desired idle RPM with AC on.__Target Idle Speed:__The desired idle RPM in relation to coolant temp.__Target Idle Speed(EPR active):__The desired idle RPM when the exhaust pressure regulator is active in relation to coolant temp.__Idle RPM (warm,not moving):__Idle speed when vehicle is not moving and engine is warm.__Idle RPM (driveaway):__Idle speed for driveaway.__Idle RPM AC Adder:__This value is added to the base idle RPM when the AC is on.__Idle RPM PN-Gear Adder:__This value is added to the base idle RPM when a PN to In Gear selector transition is made.__Idle RPM PN-Gear Ramp In:__This value controls the rate at which the PN-Gear Adder ramps in.__Idle RPM PN-Gear Ramp Out:__This value controls the rate at which the PN-Gear Adder ramps out.__Idle RPM Max:__Maximum allowed idle speed.__High Idle:__Master Enable/Disable high idle control.__High Idle RPM:__Desired high idle speed.__High Idle Disable RPM:__If RPM exceeds this value, high idle will be disabled.

**Adaptive Idle RPM**

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

__Max ECT:__Engine coolant temperature maximum for adaptive idle updates to occur.__Min ECT:__Engine coolant temperature minimum for adaptive idle updates to occur.__Update RPM Error Max:__RPM Error (Desired RPM - Actual RPM) maximum for adaptive idle updates to occur.__Update RPM Error Time:__RPM error must be below Update RPM Error Max for this time before updates are allowed.

**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.

__RPM - In Gear:__Below this RPM at the current desired idle RPM in gear, stall saver strategy will be implemented.__RPM - PN:__Below this RPM at the current desired idle RPM in Park/Neutral, stall saver strategy will be implemented.

**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.

__Startup PID Delay:__Delay the activation of the PID controller this long after engine is running.__Startup P Delay:__Delay before Proportional control is activated once idle conditions are reached.__Startup I Delay:__Delay before Integral control is activated once idle conditions are reached.__Startup D Delay:__Delay before Derivative control is activated once idle conditions are reached.__Active Spark Delay:__Delay before Idle Spark control is activated once idle conditions are reached.

**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.

__Enable RPM Error:__Proportional Idle correction will be calculated and updated if RPM Error is higher than this (positive or negative).__Idle Proportional Airflow vs. RPM Error:__Proportional idle airflow adjustment. When Error RPM is negative(RPM high) then values are subtracted from idle airflow, otherwise values are added.__Airflow High/In Gear/AC Off:__Proportional adjustment when RPM Error is positive (RPM is greater than Desired Idle RPM), transmission is in gear and AC is off.__Airflow Low/In Gear/AC Off:__Proportional adjustment when RPM Error is negative (RPM is less than Desired Idle RPM), transmission is in gear and AC is off.__Airflow High/In Gear/AC On:__Proportional adjustment when RPM Error is positive (RPM is greater than Desired Idle RPM), transmission is in gear and AC is on.__Airflow Low/In Gear/AC On:__Proportional adjustment when RPM Error is negative (RPM is less than Desired Idle RPM), transmission is in gear and AC is on.__Airflow High/PN:__Proportional adjustment when RPM Error is positive (RPM is greater than Desired Idle RPM), transmission is in Park/Neutral.__Airflow Low/PN:__Proportional adjustment when RPM Error is negative (RPM is less than Desired Idle RPM), transmission is in Park/Neutral.__Airflow:__Proportional adjustment in relation to RPM Error. Increasing these numbers makes the idle react more quickly but can also lead to instability very easily. Decreasing these numbers makes the idle react more slowly and increases stability.

**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.

__Enable RPM Error:__Integral Idle correction will be calculated and updated if RPM Error is higher than this (positive or negative).__Idle Integral Airflow vs. RPM Error:__Integral idle airflow adjustment. When Error RPM is negative(RPM high) then values are subtracted from idle airflow, otherwise values are added.__Airflow High/In Gear:__Integral adjustment when RPM Error is positive (RPM is greater than Desired Idle RPM), transmission is in gear.__Airflow Low/In Gear:__Integral adjustment when RPM Error is negative (RPM is less than Desired Idle RPM), transmission is in gear.__Airflow High/PN:__Integral adjustment when RPM Error is positive (RPM is greater than Desired Idle RPM), transmission is in Park/Neutral.__Airflow Low/PN:__Integral adjustment when RPM Error is negative (RPM is less than Desired Idle RPM), transmission is in Park/Neutral.__Airflow:__Integral adjustment in relation to RPM Error. Increasing these numbers makes the idle react more quickly but can also lead to instability very easily. Decreasing these numbers makes the idle react more slowly and increases stability.

**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.

__Fast RPM Filter:__The filter value used to calculate the fast filtered RPM signal. Values closer to 1.0 mean the filtered RPM signal reacts faster to changes in the actual RPM.__Slow RPM Filter:__The filter value used to calculate the slow filtered RPM signal. Values closer to 0.0 mean the filtered RPM signal reacts slower to changes in the actual RPM.__Airflow RPM Low:__Derivative correction airflow if RPM is less than Desired Idle RPM.__Airflow RPM High:__Derivative correction airflow if RPM is greater than Desired Idle RPM.