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.