what causes motor to reve up and down when first start? on 1990 Honda Prelude

No engine light on. But when first start car in mornings , the car reves up and down on its own until it warms up , then it runs fine. What can be sum possibilities for it doing that.

Asked by for the 1990 Honda Prelude
Your fast idle thermo valve needs replacing. This is the part that causes the car to rev at just a slightly higher rpm upon starting. The purpose of the slightly higher rpm rev is to warm the car up in colder temperatures. After the car reaches a certain temperature, your idle air control valve takes over.
Bottom line -
If idle bounces immediately after starting, replace fast idle thermo valve.
If idle bounces after driving a while but not when first starting, replace idle air control valve.
1 more answer
Idle control valve needs to be replaced.