Commonly called an idle air control (IAC) valve or idle speed control (ISC) valve, this device is used on many cable-operated throttle bodies to regulate the engine idle speed. This process is completed by regulating airflow through a bypass circuit around the throttle plate to increase or decrease idle speed.
Increasing the volume of air that flows through the bypass circuit around the throttle plate increases idle speed. Reducing the bypass airflow decreases idle speed. The engine control module controls and monitors this process and can command the fuel mixture to compensate for the airflow.
When we refer to the engine idle speed, this is the speed in the engine’s revolutions when the driver’s foot is not activating the throttle. While the throttle plate is closed, the engine’s main intake passage is closed off, so a bypass circuit is necessary to keep from choking the engine.
When the engine’s idle speed is above or below the predetermined range in the computer’s program, the computer commands the valve to increase or decrease the bypass airflow. The computer may also use additional sensor inputs from the coolant sensor, brake switch, and vehicle speed sensor to regulate idle speed according to various operating conditions.
Idle speed may also be increased when the A/C compressor is engaged. The alternator is charging above a specific voltage, and the automatic transmission is in gear to prevent the engine from lugging down.
Common Symptoms Of A Bad Or Failing Idle Air Control Valve (IAC)
When the idle air control valve (IAC) fails, it can cause all sorts of issues; and, in some cases, may even render it undrivable. Usually, a bad or failing idle air control valve (IAC); will produce a few symptoms that can alert the driver of a potential issue:
Irregular Idle Speed
One of the most common symptoms associated with a problematic idle air control valve (IAC) is irregular idle speed. Because the (IAC); is programmed to regulate and maintain the engine idle speed at a constant rate. If the valve fails or has any issues, it can cause the idle speed to be thrown off. This may result in an unusually high or low idle speed, or in some cases, a surging idle speed that repeatedly climbs and falls.
Check Engine Light Illuminated
Another potential issue with the idle air control valve (IAC) is; an illuminated Check Engine Light. If the engine control module detects an issue with the idle air control valve or signal, it will set off the Check Engine Light to notify the driver that there’s an issue.
Check Engine Light
A wide variety of issues can also set off the Check Engine Light, so having the computer scanned for trouble codes is highly recommended.
Rough Engine Idle
A normal healthy idle air control valve (IAC) will provide smooth idling in your vehicle. But if the valve goes bad for any reason, the idling will go from smooth to rough. A rough idle will result in intense vibrations forming whenever your vehicle is stopped with the engine running. Since less air will be going into the engine during its idle state, the car will react by shaking profusely.
Another more serious symptom of an issue with the (IAC) is engine stalling. If the idle air control valve (IAC) fails, it may leave the vehicle without a source of air to maintain a proper idle. This may result in the engine stalling while operating; and, in some cases, may result in an engine that will not idle at all; and stalls as soon as it is started.
Stalling Under Load
Sometimes the engine stalling will happen on its own, while other times, increasing the load on the engine will cause it to stall. For example, if you turn on your heater or air conditioner, when you have a bad idle air control valve (IAC), your engine will probably stall immediately afterwards. Consequently, the steering wheel might also feel like it’s being dragged to one side too.
Using a multimeter to check IAC valve resistance specification
- Step 1: Gain access to the IAC valve. Consult the vehicle service manual for the location of the IAC valve on your vehicle.
- Step 2: Disconnect the IAC valve. Locate the IAC valve electrical connector and unplug the IAC valve.
- Step 3: Remove the IAC valve from the vehicle. Use the procedure detailed in the vehicle service manual to remove the IAC valve.
- Step 4: Inspect the IAC valve. Inspect the valve and the mounting location for carbon build-up, rust or dirt. Inspect the IAC valve pintle and mounting location for damage. Rules these items out before condemning the IAC valve.
- Step 5: Check the resistance of the IAC valve. Use the specification listed in the vehicle service manual for the IAC valve and follow the guideline on testing the valve using a digital multimeter against the electrical terminal pins on the IAC valve electrical connector. If the reading falls within specification, the valve should be electronically sound, and the fault lies somewhere else. If the reading is not within the specification, replace the unit with a new one.
Note: The new IAC valve may or may not come with a new seal. Remember to replace the seal any time a sealed part is removed from the engine to avoid a vacuum leak or a coolant leak where coolant runs through the IAC valve body.
The idle air control valve can be found on most vehicles and plays an important role in managing engine idle speed. It does this by allowing air to bypass the throttle body and into the engine when idling.
As you can imagine, a bad or failing idle air control valve will cause the engine to act erratically by affecting the idle speed and frequently causing the engine to cut out entirely.
A professional technician equipped with a professional-grade vehicle scan tool may plug into the vehicle and command the IAC valve through the scanner to check operation. If you have followed all testing methods and are still stumped, consider hiring a professional technician to take a look. The technicians we have to offer here are happy to make house calls.