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

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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.
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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.
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