High Idle Causes

by Heather Robson
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New car petrol engines image by Christopher Dodge from Fotolia.com

A high idle in your car can cause you to worry. When the engine seems to rev up on its own while you are at a standstill, you may wonder if it's a sign of trouble that needs the attention of a mechanic. The answer to that question depends on the cause of the high idle. Some causes are harmless, while others can indicate the need for a repair.

Faulty Fuses

Most modern car engines control the engine idle speed through a motor called an IAC (idle air control) motor. One of the leading causes of IAC motor malfunction is a blown or faulty fuse. A malfunctioning fuse in any of the electronically controlled components including the air bag system can disrupt the function of the IAC motor and lead to a higher idle speed than usual.

Computer Malfunctions

Computers control a modern car's engine and transmission, its brake system and its airbag system. If the fuses are fine, the computer system itself in one of these components might be malfunctioning. If that's the case, that might be the source of the high idle speed in your engine.

In some cars, the powertrain control computer can misinterpret the information it receives about the idle speed, leading to higher idling speeds. Cold weather can trigger this problem, and it usually corrects itself once the engine is warmed up.

Leaking Vacuum

The IAC motor works with a vacuum port to control the engine's idle speed. By opening and closing a valve to the vacuum port at set intervals, the IAC motor sets the idle speed. If the seal on the vacuum port is leaking, that will interfere with the overall function of the IAC and may lead to high idle speeds. Often it is the hoses connected to the vacuum system that become damaged, leading to an imperfect vacuum.

Malfunctioning Throttle

The throttle system in the vehicle might not be working properly, which can cause engine stalling or high idling. Common causes of a throttle malfunction are a buildup of dirt and contaminants in the engine's secondary air intake or a cracked air intake tube.

IAC Motor Malfunction

The IAC motor itself may not be working properly. A short circuit in the motor can trigger high idle problems. Sometimes IAC motors develop high resistance in the circuitry, which slows the reactions of the IAC and can lead to idling problems.

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