Bad idle air control valve

Bad idle air control valve symptoms

Idle speed goes up and down or high idle

First off, this is a fairly common problem for all makes and models. Sometimes the problem is caused by a gummed up or sticking idle air control valve (IAC) or

GM style IAC

Ford Style IAC

a dirty mass airflow sensor (MAF). But it can also be caused by a vacuum leak.

How an idle air control valve works

If you’re familiar with older carbureted cars you remember that you turned a screw to set the idle speed. The screw turned against a cam and that cracked the throttle plate open slightly to allow air in. The more air you let in, the more gas got sucked out of the carburetor bowl and thus a higher idle speed. Well, in a fuel injected engine, the throttle plate is NEVER cracked open at idle. Instead, the car makers install a idle air control valve that bypasses air from the front of the closed throttle plate to the intake manifold. The computer controls how far the valve opens and closes and it’s all based on outside air temperature, engine temperature, and the kind of readings the computer is getting from the oxygen sensors.

The computer relies on data from the mass airflow sensor (MAF) to tell it how much air is coming into the engine. Based on that data, it computes how to regulate the open time on the fuel injectors. If you have a vacuum leak or a leak in the air duct AFTER the MAF sensor, more air gets into the engine than the computer anticipated. How does the computer know that? The oxygen sensor sees a lean exhaust stream. The computer figures it miscalculated the fuel, so it adds fuel to compensate for the extra air. That raises the idle speed. The computer then looks at the higher idle speed and takes a second look at the air and engine temperature and then looks at the fact that your foot is off the gas. It concludes that the idle speed is way too high for the conditions and reduces the amount of gas. Great. Now the idle speed slows and the computer gets another lean reading from the oxygen sensors and the process starts all over again. Like I said in the beginning, this “hunting idle” problem can be caused by a sticking IAC or MAF or a vacuum leak. You don’t have a lot to lose by cleaning the IAC with throttle body spray cleaner or the MAF with MAF cleaner (NEVER brake cleaner). But if that doesn’t work, and you can’t find a vacuum leak, replace the IAC.

For more information on idle air control valves, read this post

© 2012 Rick Muscoplat

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