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

 

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hillbilly72, 2.0L 4 Cylinder SOHC, Zanesville, OH, November 10, 2010, 05:21
 Rookie

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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  • Answer #1

    JTParts November 10, 2010, 10:42
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     Enthusiast

    Idle control valve needs to be replaced.

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  • Medium-star Best Answer
    Visitor, January 03, 2011, 23:41

    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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