1990 Honda Prelude Q&A

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

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. -
Answer 1
Idle control valve needs to be replaced. -
Answer 2
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. -