Reasons Why Your Car's Idle Speed Might Be Too High

If your car seems to be revved up to a higher-than-normal RPM while idling, it's not necessarily a problem. If this problem happens while the engine is cold, it may be part of the engine's design. Some cars, especially older cars with carburetors are designed to run at 1200 rpm or so until they are warmed up. And in modern cars, if you are running several accessories, such as the air conditioner or heater, the engine's onboard computer may be telling it to run at a higher RPM to provide the necessary power.

But if the accelerated idle persists even after the engine is fully warmed up, it probably indicates a genuine problem. 

Troubleshooting Fast Idle Problems

The first step is to determine if there are any diagnostic trouble codes stored in the PCM (powertrain control module). If there are, this will give you a good starting point for troubleshooting. Some auto-parts chain stores will read your codes for free—all you have to do is ask. When you find these codes, you will be able then to follow them to possible causes, or consult with a mechanic for further interpretation.If

If the PCM offers no hints, the best place to begin looking for problems is with the Idle Air Control Valve/Bypass Air Control (IACV/BAC). You can try cleaning it and see if that improves your idle speed. A throttle body cleaning is likely to cure the high idle speed as well.

Possible Causes of High Idle Speed

There are many possibilities when your engine is idling too fast. Here are some common ones to help guide you towards the root of the problem. 

• In vehicles that have carburetors, a bad accelerator pump or power circuit may be to blame.
• The engine may be overheating, and the cooling system might need repair.
• The fuel pressure regulator may be operating at too low of a pressure and might need to be replaced.
• The Ignition timing might be in need of adjusting.
• Ignition problems stemming from the distributor cap, rotor, ignition wires, or spark plugs can cause a high idling issue—each of these components should be examined. 
• If the computerized engine control system is faulty, high idling can be one symptom. 
• A vacuum leak in any of the hoses can be the culprit—inspect them all for leaks. 
• A bad idle-speed control unit can cause the problem; it may need to be replaced. 
• A faulty alternator can fast idle issues; if so, replace it. 

For the do-it-yourself mechanic, Many high-idle problems can be identified and remedied with some patient troubleshooting. Other solutions, though, are best performed by a service shop.

One tip is to make sure you are checking the engine idle with the air conditioning and defroster in the OFF setting. With some cars, the onboard computer automatically revs the idle speed slightly when accessories are running, and you won't get a true idle speed if they are at work.