Electric radiator fan that doesn't turn on will cause overheating at slow speeds but not on the highway where there is plenty of air flow. Leaking head gasket can cause an air bubble in the cooling system. That could cause the thermostat to not open and allow hot coolant to flow to the radiator. Thermostats only open in the presence of hot coolant, not hot air. Your mechanic has a tool for testing at the radiator cap for a head gasket leak. Volkswagens have had some problems with the flimsy plastic water pump impeller breaking free and failing to circulate the coolant. The clue is there will also be cold air coming out of the heater.
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Sunday, March 20th, 2011 AT 8:34 PM