The engine idle is in many ways the pulse of your vehicle and if it is running high, it could show a serious issue with the car. A high idle can be produced from a number of issues, starting from a faulty fuse to something a bit more complicated like a leaking vacuum hose.

How this system functions:

The technical definition of idle speed is the rotational speed an engine operates on when it is uncoupled from the drivetrain and the throttle is not being pressed. It is measured in revolutions per minute (rpm). A passenger vehicle will typically have an idle speed between 600-1000 rpm.

The idle control valve, also called an idle air control motor handles and maintains the engine idle speed.

If a number of the cars accessories are running, for example, air conditioning, this will push up the idle speed, as the engine has to generate enough power to run these accessories.

Common reasons why engine idles at high speed

Faulty Fuse: In the majority of cars, engine idle speed is controlled through an idle control valve (ICV). A faulty or blown fuse in other electronically controlled systems can make 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 keep and control a consistent idle speed. The ICV will help counter any extra loads put on the engine such as lights, stereo and air conditioning. This component will often malfunction because of excessive carbon buildup in the intake manifold.
Vacuum Leak: On a fuel injected engine a vacuum leak will produce a high engine idle. The engines oxygen sensor will note the extra oxygen entering the engine through 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, leading to an abnormally high idle.
Malfunctioning Throttle: A malfunctioning throttle system can generate both a high idle as well as an engine stall. A dirty air intake or cracked air intake tube is often the cause.
Computer Malfunction: A computer malfunction can produce 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 end point.
MORE AT:http://www.automobileglobe.com/what-makes-an-engine-idle-at-high-speed/