There are several things that can cause high idle. An older carbureted engine may have a vacuum leak or sticking choke linkage. Newer computer controlled engines have an idle control device called an idle motor which could be faulty or receiving bad information from other sensors on the engine ( IE. temperature sensor ).
Cars often idle fast when you start them up. The automatic choke will make the car idles fast for a while then the idle speed will slow down, once the engine warms up.
Might want to check your idler sensor.
Over heating (running to hot), Fast idle (Incorrect idle adjustment), Incorrect timing, Carbon build up in combustion chamber,
Idle compenstorVehicles hve an idle copensator that speeds up the engine when the ac is turned on to ring the rpm back up to the proper idle speed, if yours is too fast then t needs to be adjsted.
Your car will idle high until the engine begins to warm. You could set the idle to be a bit lower, but that may cause the engine to stall out when it's cold out. The engine should begin to lower the RPMs when it is warmed up.
Check for leaks in vacuum hoses, check for a cracked or gunked up Idle Air Control valve, and check you throttle positioning sensor.
when u first start the car it will idle high, around 1.5 rpm to 2 rpm. when its warm up it should idle between 700-1000 rpm
!t can be the air flow censer it might be messed up or the rong one usually the rong one will make it idle high.
if the vehicle shifts and drives fine. the most probable cause for this is a high idle. most vehicles idle at around 650 to 700 rpms. when warmed up, check or clean the idle air control if idle is high. good-day !
need more info but just guessing here vacuum leak causing high idle and dieseling cause by high fuel air mix why it will not shut off
Have it checked out by a mechanic. The most likely thing is the car just needs a good tune up to replaced plugs and wires that are wearing out.
When you crank it? What are you driving a Model T Ford?