Rick's Free Auto Repair Advice

High idle, rough idle

High idle, rough idle — causes and fixes

Diagnose and fix high idle or rough idle

Any time you see a high idle, rough idle high, or hunting idle (idle speeds up and slows down), suspect a vacuum leak. Extra air leans out the exhaust mixture, which is detected by the oxygen sensor and reported to the powertrain control module (PCM).

To correct the lean condition, the PCM adds fuel. As the engine speeds up, the PCM notices that you really haven’t pushed down on the gas (watches the throttle position sensor to determine this) and it starts to scale back on the gas. That’s why you get a “hunting” idle that goes up and down—the computer is shadow boxing the problem. The problem can also be caused by a sticky idle air control valve.

Find a vacuum leak to fix a high idle

Always start your high idle diagnostic procedures by checking for vacuum leaks or air duct leaks (the plastic/rubber duct that runs from the air filter box to the throttle body). Check all rubber and plastic hoses first. Then, try spraying carb cleaner around any vacuum ports, intake manifold gasket and fuel injector O-rings. Listen for a change in engine speed. Unfortunately, many manufacturers have experienced problems with bad intake manifold gaskets.

Next, check and clean the idle air control valve

Most older fuel injected engines supply air at idle with the use of either

Hona idle air control valve

Honda idle air control valve

an idle air control valve, while newer engines simply open the electronic throttle body. If your engine has an idle air control valve, it may be carboned up or the electronics may have failed. Remove the valve and check for both.

To see how an idle air control valve works, read this post

To learn how to clean an idle air control valve, read this post

Clean an electronic throttle body

If your engine uses an electronic throttle body, carbon buildup in the throttle body throat can prevent the throttle plate from closing far enough. That larger than expected opening allow too much air into the engine, which causes the computer to add more air. See this post for instructions on how to clean an electronic throttle body.

If you don’t plan on doing this repair yourself and you’re fairly certain you might have a bad intake manifold gasket, it might pay for you to take it to a pro and have them perform a smoke test on it. They pressurize the engine with smoke, and watch to see where it comes out.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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