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Q:

My engine heats up in idle, cools when back on the road. on 2007 Pontiac Grand Prix

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There has been a very slow leak on coolant, I'm thinking it's the thermostat. Any thoughts??
(1) Answer
Could be the thermostat or water pump. It could also be an issue with the head gasket (check to see if your coolant is dirty looking).

You should have your car looked at by a professional.