A car that is cold has a fast idle to quickly bring it to optimal operating conditions. This is a part of the design of the engine. Older vehicles with carbureted engines have a choke to increase the idle speed to as high as 1200 rpm, or revolutions per minute, until the engine warms, according to About.com.
When the vehicle is running a large number of accessories, including air conditioning, the idle speed automatically increases to provide the additional power to operate these units. Turning off the air conditioning or other device sends a signal to the onboard computer to decrease idle speed.
If the carbureted vehicle continues to idle too high after warming up, it may have a defective power circuit or accelerator pump. Replacing the accelerator pump or carburetor usually brings the idle speed to normal.
Ignition problems sometimes lead to a fast idle. Defective ignition parts, including spark plug wires, distributor cap or rotor require replacement. If the timing is off, it should be set to factory specifications using a timing light.
The idle speed is sometimes an indicator of mechanical trouble. A worn engine, causing bad compression, increases the idle speed. Computerized engine control systems develop issues that require repair. These problems generally need professional assistance to return the idle speed to normal.