A car may idle up and down due to "Improper fuel mixture". (The fuel in the car may consist of an un-even balance of air or gas). However, there are a few other ways a car can idle up and down.
The car is designed this way for cold starts. It should idle up to 3000 rpm then makes it's way down to around 1500 and finally down to 750 as it warms up. DO NOT drive when it is cold or If you have to do not take it over 3 grand until warmed up or you risk blowing coolant seals.
do you have a/c switched on when this is happening a/c makes engine idle up and down as it cycles on and off
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.
More than likely it's your Idle Control Valve.
Idle Air Control Valve....if this breaks you car wont idle properly....the rpm will go up then down all the time when idling
When you stop or idle after the car warms up the engine will idle sparatically up and down. Then one day the car will crank but not turn over.
I believe its up, which ever one makes it idle faster
A vacuum leak could cause that. The year, make and engine info would help us help you.
the car is garbage the car is garbage
My mechanic (who I trust) replaced my O2 sensor and exhaust manifold and the problem went away.
Might want to check your idler sensor.
When you crank it? What are you driving a Model T Ford?