A high idle can be caused from a number of issues, ranging from a faulty fuse to something a bit more complicated like a leaking vacuum hose.
How this system works:
The technical definition of idle speed is the rotational speed an engine runs on when it is uncoupled from the drivetrain and the throttle is not being depressed. It is measured in revolutions per minute (rpm). A passenger car will typically have an idle speed between 600-1000 rpm.
The idle control valve, also called an idle air control motor controls and maintains the engine idle speed.
If a number of vehicle accessories are running, for example, air conditioning, this will push up the idle speed, as the engine needs to generate enough power to run these accessories. This engine behaviour though, is normal.
Common reasons for the engine idling speed to be high:
- Faulty Fuse: In the majority of vehicles, engine idle speed is controlled via an idle control valve (ICV). A faulty or blown fuse in other electronically controlled systems can cause the IAC motor to malfunction, leading to a higher than normal engine idle speed.
- Faulty Idle Control Valve (ICV): The job of the idle control valve is to maintain and control a consistent idle speed. The ICV will help counter any extra loads put on the engine such as lights, entertainment system and air conditioning. This part will often malfunction due to excessive carbon buildup in the intake manifold.
- Vacuum Leak: On a fuel injected engine a vacuum leak will cause a high engine idle. The engine’s oxygen sensor will detect the extra oxygen entering the engine via the vacuum leak which will alert the engine’s computer that the exhaust is running lean, which will prompt the computer to pump in additional fuel, resulting in an abnormally high idle.
- Malfunctioning Throttle: A malfunctioning throttle system can cause both a high idle as well as an engine stall. A dirty air intake or cracked air intake tube is often the culprit.
- Computer Malfunction: A computer malfunction can cause a high engine idle, especially the powertrain control computer. If it misinterprets the information it is receiving about the current idle speed, a high idle speed may be the result.
How important is this service?
In most cases a high engine idle is something that should be looked at as soon as possible. If the weather is cold, see if the issue corrects itself as the engine warms up, if not a professional should look at the vehicle.