1990 Honda Prelude Q&A
1990 Honda Prelude Question: what causes motor to reve up and down when first start?
Idle control valve needs to be replaced. - JTParts
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. - Visitor