Coolant leak
2 Answers
2 options... 1--blown head gasket 2--freeze plug leaking -if the coolant wasn't changed like it should have, it becomes acidic and actually eats away at the gasket. My ford truck had a coolant leak at the very back of the engine on the passenger side
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-also, check the heater control valve, also in that same area. They're plastic and fail
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