There are many reasons a car idles fast, but one common reason is a vacuum leak. Older muscle cars have only two or three vacuum hoses, making finding the leak easy. Newer vehicles have complex vacuum systems to reduce emissions, so leaks are more difficult to find. Popular Mechanics suggests spraying carburetor cleaner on a suspected leak and listening for the engine speed to increase.
An overheating engine increases the speed at which a vehicle idles. The overheating condition often leads to more serious consequences, such as a blown head gasket or a warped head. About.com recommends preventing the potential damage and stopping the fast idle by repairing the cooling system.
Ignition timing affects the idle speed. A timing light quickly reveals problems with vehicle timing and allows the user to adjust it to the manufacturer's recommended specifications.
On vehicles with a powertrain control module, issues with the on-board computer may affect idle. Older vehicles experience similar problems due to problems with the distributor cap, rotor, plugs or wires. About.com suggests testing the control module with a scan tool and replacing defective ignition parts.
A failing alternator may also affect the engine idle. In this case, when the alternator is replaced the speed returns to normal.
A fuel pressure regulator operating incorrectly also increases the engine's idle speed. This is not a do-it-yourself job, so drivers should have a mechanic check the pressure using a fuel pressure gauge and replace if necessary.