Rick's Free Auto Repair Advice

Rough idle, high idle

Rough idle, high idle

Both a rough idle, high idle can be caused by a vacuum leak or carbon buildup in the throttle body.

Diagnose and fix a car has rough idle condition

Any time you see a rough idle  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 rough idle

Always start your rough idle or 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 or throttle body

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 clean the electronic throttle body on a late model car, remove it from the intake manifold. See this article on how to clean an electronic throttle body

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

Once vacuum leaks are fixed, here are the remaining possibilities

If there are no vacuum leaks a rough idle can be caused by:

Worn spark plugs
Worn spark plug wires
Worn boots on COP ignition coils
Clogged fuel injector(s)
Incorrect fuel pressure
Improper valve adjustment
Worn camshaft, piston rings

images of failed ignition coils

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat



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