1996 Honda Civic idling problem

3 dr 5 sp cx
Runs good, but just have driving for about 5 mins and the car get warm, the car idles up down, up down, up down and up down......keeps going like this.

What could this be?
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Devils Advocate wrote:

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Start by filling the coolant reservoir to the MAX level and check it each time the engine is cool. You'll probably find it will sip some to displace air in the system. Tap water is called Hondacide. Use Honda premix. It's foolproof, and you won't be using much.
'Curly'
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The coolant is full to the max and now not dropped an inch.

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