Close
Q:

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

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
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.
(2) Answers
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.