When you first put your car in park, it will idle at a consistent rotational speed so that all of its vital components can run without shutting down. This is to prevent engine stalling, warm up emission control components as quickly as possible, and compensate for gasoline that doesn't atomize efficiently in cold intake manifolds.
The ideal idling speed is generally between 600 and 1000 RPM, though this can vary depending on the engine size and type of vehicle. If your car is idling at much lower than this, it may be a sign of serious engine trouble that should be taken care of as soon as possible.
High Idle - If your engine idles high when it is warm, this could be an indication that the vehicle is using too much fuel. This is especially common in cars with 2 liter and smaller engines, but it can happen with larger vehicles as well.
Rough Idle - If your vehicle is idling rough, this can be caused by many different issues. Often, this is due to the failure of certain electrical components or faulty ignition systems.
Another cause of a rough idle is an issue with the idle air control valve. This valve controls how much air enters the engine and can cause it to idle erratically if it's not working properly. This can cause the engine to stall or overheat and can also damage the bearings in the idle air control valve, which can shorten the lifespan of the idle air control valve.