Diagnose a high idle problem

What causes a high idle, fast idle?

Here are the most common causes of a high idle, surging idle or fast idle:

An air leak can cause a high idle — An air leak is unmetered air and causes a huge problem for the engine computer. Most late model vehicles use a mass airflow sensor (MAF) to

A mass airflow sensor detects the amount and density of air coming into the engine

measure how much air enters the engine so it can determine the proper air/fuel mixture. You’ll find the MAF sensor located right after the air filter box.

The delicate wires in the MAF sensor can get dirty and provide false readings to the computer. Read this post to see how to clean a dirty MAF sensor

If the air duct leading from the air filter box to the intake manifold develops a leak, that’ll allow more air into the engine than the computer was expecting.

The extra air will cause the oxygen sensor to detect a lean condition. How? Well, the computer expected a certain amount of air and had the injectors deliver the proper amount of fuel. The unmetered air screwed up the air fuel mixture by having too little fuel.

To compensate for the lean condition, the computer adds more fuel. That extra fuel, combined with the extra air results in a high idle. However, if the idle goes too high, the computer will cut back on the fuel and thus lower the idle speed. This is called a “hunting idle or surging idle” and is almost always the result of an air leak.

Vacuum leak — A vacuum line that’s cracked or hasn’t been reinstalled can also cause a high idle or surging idle. Vacuum leaks can also develop in the gasket between the upper intake plenum, upper intake manifold, or vacuum brake booster

Carbon buildup in throttle body or idle air control valve can cause a high idle

If your engine is equipped with fuel injection and a mechanical

Idle air control valve or idle air bypass valve

throttle body, it will also have an idle air bypass valve. See this post to understand how they work. If carbon builds up in the throttle body, that can cause a hunting surging idle or a constantly high idle.

However, if you have a late model vehicle with an electronic throttle body, carbon buildup occurs directly under the throttle plate that restricts airflow at idle speeds. To compensate for the low idle, the computer commands the electronic throttle body to open the throttle plate a bit to allow more air in. The fix is to clean the electronic throttle body. Read this post to see how that’s done.

©, 2017 Rick Muscoplat