The IAC duty cycle for each idle speed mode is continually learned and updated throughout the life of the vehicle. The learning is deliberately set to a slow rate for stability. If the PCM loses power or is otherwise reset, the learned values are lost and a set of default IAC flow rates are initially used to allow idling until the PCM can relearn the ideal values.
During the learning process, there could be dips and flares in the idle speed as the engine idle load is significantly changed (A/C compressor on/off, neutral/drive transitions, etc.). Even so, the idle duty cycles will eventually be learned to the same range of values as before the reset.
To avoid the potential for large dips and flares, there is a special quick learn routine available immediately after a PCM memory reset. However, if the truck is driven a short distance, the quick learn strategy is exited and the PCM reverts to the normal slow learn rate from the defaults.
Using the "Quick Learn" Idle Speed Routine
-Park on level ground; set the parking brake; chock the tires.
-Reset the PCM using a scan tool feature or by disconnecting the PCM from battery power for 5 minutes.
-Turn the headlights and accessories off.
-Set the HVAC blower fan to its lowest ON position (not OFF).
-Start the engine and allow it to idle until it is at operating temperature. (Do not drive it because there is a distance limit that will force the PCM to exit the quick learn routine during a short drive.)
-After the warmup, allow the engine to idle for 1 minute in each of its 4 load modes. The order does not matter.
- Park w/o A/C
- Park w/ A/C
- Drive w/o A/C
- Drive w/ A/C
-The method outlined above is how I do it. Proceed at your own risk.
-When idling in gear, apply the service brakes. Do not rely on the parking brake.
-Do not touch the throttle during the quick learn.
-Remember that the A/C compressor is allowed to run in 5 of the 8 HVAC mode selector positions. For simplicity, turn the mode selector to NORMAL A/C when A/C compressor load is desired and to OFF when it is not.
-A/C compressor operation disabled when low coil temperature may cause evaporator icing. In winter months in some areas, it may not be possible to run the 'A/C on' part of the quick learn procedure. For the best chance at a complete learn, try to run it when the ambient temperature is 40F or higher.
-Trucks with manual transmissions only have 2 load modes to learn:
- Neutral w/o A/C
- Neutral w/ A/C