If it cools down at highway speed, that suggests there's not enough air flow at lower speeds, and the radiator fan might be only running at a lower speed when it turns on. If it overheats on the highway too, a leaking head gasket is a more likely cause. To test for that, your mechanic will draw air from the radiator through a glass cylinder with two chambers partially filled with a special dark blue liquid. If combustion gases are leaking into the cooling system, that liquid will turn bright yellow. That air can also collect under the thermostat and lead to overheating. Thermostats open in response to hot liquid. They do not open in the presence of hot air.
Monday, June 27th, 2011 AT 9:46 PM