Too much coolant (over 50/50) will actually cause overheating. An engine with the prpoer amount of coolant ratio and the correct cap will be good to about 256'F as the coolant and pressure increase the boiling point over the 212'F boiling point of water. A 180'F thermostat could help as long as your engine gets to the correct operating temperature and maintains it during all 4 seasons.

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