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Q: Severe coolant loss

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Severe coolant loss from coolant reservoir. No apparent leaks or liquid anywhere near vehicle. Engine overheats at idle. Highway speeds vehicle is at normal operating temperature. Milky white substance in oil fill tube on engine

My car has 200000 miles.
My car has an automatic transmission.

A "milky white substance" is an indication that coolant and engine oil are mixing. In all likelihood, you are losing coolant but not seeing it leak anywhere because it is leaking into the oil pan, mixing with - and thinning - the oil to create that milky substance. This can be caused by a head gasket gone bad, or possible a cracked cylinder head. This will cause the engine to overheat as well. Prolonged driving with the coolant and oil mixing will damage the engine internally. The engine may be running cooler at highway speeds due to the increased air flow though the engine compartment. To reduce the risk of damaging the engine beyond repair, it should not be driven and this coolant leak should be inspected by a certified technician, such as one from YourMechanic, as soon as possible.

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