Idle Speed Control - Don't stand idle?
August 2011

The automotive fuel injection system has gone through many changes since it was first introduced many years ago. One of the areas that has transformed from a basic mechanically operated heat controlled to a fully electric motorised control operation is the engine idle speed monitoring and operating system.

The common types found on many of the vehicles driven on our roads may be categorised as:

  • AAV-Auxiliary Air Valve - Coolant or electrically heated
  • IAC- Idle Air Control - ECU Controlled
  • Idle Actuator - physically opens throttle
  • Stepper Motor- ECU Controlled
  • Stepper motor combined with AAV
  • Electronic Throttle Body System (not covered in this article)
In this article we are going to concentrate on the "ECU controlled" idle system so we can better understand the variable operating principles of these systems.

IAC – Idle Air Control

This is a widely used system in the industry and its characteristics are common on most vehicles. These include:
  • It has a spring return to base position when in a NOT activated position, or when disconnected.
  • It is generally duty cycle pulsed by the ECU
  • They are subject to carbon build up and causing the valve to stick in various positions
  • Vehicle must have the correct unit fitted to operate correctly, many look similar.
There are common faults that occur in IAC's, including:
  • Idle speed high or low due to IAC valve sticking or internal electrical fault
  • High idle speed generally causes engine fluctuations at idle. (Hunting)

IDLE Actuator System

This system is ECU controlled and physically operates the throttle valve – even though it was used on a few earlier models it is not an extremely common unit. Common characteristics include:
  • Base idle adjustment is critical for correct operation
  • Correct (TPS) throttle position sensor adjustment is also critical
  • Any physical binding / sticking will affect RPM
Step it up!

Typical Stepper Motor Operation

This is a commonly used system found in vehicles from all corners of the world.

An example of this ECU controlled system is the common GM type stepper motor. The ECU operates this unit in small steps starting from zero where the pintle has extended and completely closed the air by pass. The ECU will then operate the stepper motor up to 255 steps, fully retracting the pintle. The desired stepper motor position for "idle" is based on:
  • Coolant temperature
  • Actual RPM
  • Engine Load
  • Battery voltage
A typical VX Commodore 6 Cyl ECU stepper motor hot idle reading is approximately 25 steps. If the steps are excessive this would indicate a dirty throttle body or Typical earlier type IAC without internal electronics Typical mid 2000 later IAC with internal electronics Idle Activator similar causing the ECU to retract the pintle to compensate and allow more air. Note: Disconnecting the harness plug will lock the pintle into that position until reconnected.

Stepper Motor Assembly integrated with AAV

Many stepper motor assemblies incorporate 4 winding phases that are controlled by the ECU and a coolant temperature controlled wax pellet type AAV for total idle air control. e.g. N16 Nissan Pulsar and a variety of Mitsubishi vehicles.

The ECU generally operates 2 windings in sequence to alter air quantity. The mechanical AAV allows for air by pass when the engine coolant is cool and reduces this air by-pass as the engine heats up.

Common faults with this system

  • Stepper motor windings open / short circuit.
  • Corrosion of the alloy housing may allow coolant to enter the motor windings. This can cause extreme current flow in the circuit and damage the ECU (IAC circuit).
General Precautions

It is important when dealing with all these systems to understand that there are external factors affecting their operation. e.g. Vacuum leaks.

The base idle settings are also important to ensuring a smooth deceleration to idle operation. Vehicle stalling is common when the base idle is incorrectly set. In many cases a manufacturer's procedure (e.g. bridging electrical connections) must be performed to ensure the correct positioning of the IAC motor / pintle, prior to any adjustments being carried out at the throttle body.