The Engine Management System processes information regarding idle speed and sends instructions (in the form of voltage signals) to manipulate the Idle Air Control (IAC) valve. The goal of the EMS along with the IAC is to provide the lowest idle speed without causing a stalling condition. The lower the idle speed the better the fuel economy. Maintaining the lowest possible idle speed for acceptable engine operation is one method of maintaining fuel economy. There are mechanical, electrical or fuel related problems that may hinder low-speed engine operation and cause the idle air control system to appear faulty. Some applications use a second air passage and control device to provide additional air to compensate for the additional load created when the A/C compressor is running.
There are two types of IAC motors that are found on current production vehicles; the pintle and shutter types. The pintle-type IAC motor has a dual winding and an armature that drives the pintle. As the IAC motor is activated the pintle is driven in or out to adjust the throttle plates. The shutter type IAC motor (or speed stabilizer) continually adjusts an opening in the bypass passage around the throttle plate by rotating its open window in accordance with the dwell signal provided by the EMS.
Idle air control systems have the ability to compensate for minor vacuum leaks. IAC counts (position information) that are below normal with elevated RPM values are an indication of vacuum leaks. In addition to vacuum leaks poor idle quality and speed may be attributed to retarded spark advance or restricted fuel delivery.
The following symptoms can be caused by a faulty IAC due to loose connections, bad grounds, high resistance in the circuit, or opens in the circuit: