Idle Speed.

[Trancendental]

when a car idles it usually does it around 500 - 1200 RPM depending on its age and how its running.

Could someone explain to me was is meant by the term "Fast Idle"?

I know that its not the same as "normal" idle.

 

[Chas Uno]

The term 'fast idle' used to be used a lot with carburetored cars - it's the idle speed set for when the choke is pulled out.

The choke is pulled out when the engine is cold (usually, or if the vacuum diaphragm fails on old Unos ). A cold engine won't run very happily at normal idle speed, so needs to warm up first at a higher idle. The 'fast' idle raises the tickover speed until the choke can be pushed in, at which point the idle speed drops to the normal setting.

It's normally a simple mechanical device that operates both the choke flap and the throttle butterfly at the same time. Look at where the choke cable connects onto a carburetor and it should be fairly obvious.

Fuel injection cars have a similar system, but it's all controlled by the ECU which detects engine temperature and other such variables. It is less pronounced because the ECU can adjust the mixture and tickover more precisely than a choke on a carburetor, so the 'fast' idle tends to be only a couple of hundred revs higher than normal. A carburetor car can easily fast idle 500 revs higher.

Hope this helps!

 

[Trancendental]