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Q: Why does my coolant overflow tank overflow when my 2005 Chevrolet Malibu heats up.

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What could be the problem. I have changed the thermostat and tested the ect sensor.

My car has 196000 miles.
My car has an automatic transmission.

Hello. Your coolant overflow tank level should change when the engine warms up, but it should not overflow. If it is overflowing, but the engine is not overheating then the first thing I would check would be the cooling system pressure cap. If it is not holding pressure then this will happen. If the pressure cap is fine then the coolant temperature needs to be checked with an infrared gauge to see if the engine is actually having an overheating issue or an issue with the coolant not circulating. If you need to have this checked out, consider YourMechanic, as a certified mechanic can come to you to [diagnose why the vehicle is losing coolant] https://www.yourmechanic.com/services/coolant-is-leaking-inspection

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