What isn't mixing? Over heating could be anything from a blown head gasket to a cracked cylinder wall to a blocked water jacket. Check for oil in coolant....does the oil look like chocolate milk? Unexplained loss of coolant? Does it try to blow the water out of the radiator when you first start it? Take the radiator cap off while it's cold. Insert a thermostat or watch your gauge...does the water start to flow by the specified temp? If you aren't loosing coolant and everything checks out that I mentioned... try doing a cooling system flush. Hope this helps.
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