GM: Electra/Park Avenue/Ninety-Eight 1990-1993

Idle Speed Control (ISC)



Incorrect diagnosis and/or misunderstanding of the idle speed control systems used on efi engines may lead to unnecessary replacement of the iac valve. Engine idle speed is controlled by the ecm which changes the idle speed by moving the iac valve. The ecm adjusts idle speed in response to fluctuations in engine load (A/C, power steering, electrical loads, etc.) to maintain acceptable idle quality and proper exhaust emission performance.

The following is provided to assist the technician to better understand the system and correctly respond to the following customer concerns:

  1. Rough idle/low idle speed
  3. High idle speed/warm-up idle speed; no kickdown

Rough Idle/Low Idle Speed

The ecm will respond to increases in engine load, which would cause a drop in idle speed, by moving the iac valve to maintain proper idle speed. After the induced load is removed the ecm will return the idle speed to the proper level.

During A/C compressor operation (max, bi-level, norm or defrost mode) the ecm will increase idle speed in response to an A/C-on signal, thereby compensating for any drop in idle speed due to compressor load.

During periods of especially heavy loads (A/C-on plus parking maneuvers) significant effects on idle quality may be experienced. These effects are more pronounced on 4-cylinder engines. Abnormally low idle, rough idle and idle shake may occur if the ecm does not receive the proper signals from the monitored systems.

High Idle Speed/Warm-Up Idle Speed(No Kickdown)

Engine idle speeds as high as 2100 rpm may be experienced during cold starts to quickly raise the catalytic converter to operating temperature for proper exhaust emissions performance. The idle speed attained after a cold start is ecm controlled and will not drop for 45 seconds regardless of driver attempts to kickdown.

It is important to recognize the efi engines have no accelerator pump or choke. Idle speed during warm-up is entirely ecm controlled and cannot be changed by accelerator kickdown or pumping.

Abnormally low idle speeds are usually caused by an ecm system-controlled or monitored irregularity, while the most common cause for abnormally high idle speed is an induction (intake air) leak. The idle air control valve may occasionally lose its memory function, and it has an ecm programmed method of relearning the correct idle position. This reset, when required, will occur the next time the car exceeds 35 mph. At this time the ecm seats the pintle of the iac valve in the throttle body to determine a reference point. Then it backs out a fixed distance to maintain proper idle speed.