If the upper radiator hose is hot, coolant is circulating. There's two fans for the radiator. Be sure both are turning on. Look at the cooling fins between the tubes in the radiator. If they crumble when you scratch them, they are corroded away and the radiator isn't able to give up its heat to the air. That will cause more overheating at highway speed.
If it does not overheat on the highway, insufficient air flow is the problem at lower speeds. Look for a bug collection in front of the AC condenser or anything else that is blocking air flow through the radiator or allowing air to bypass it.
If the upper radiator hose is not too hot to hold onto for very long, suspect a leaking head gasket. Combustion gas can form a pocket under the thermostat which will let it close. Thermostats open in response to hot liquid, not hot air. Your mechanic can perform a test that draws air from the radiator through a glass cylinder with two chambers partially filled with a dark blue liquid. If that liquid turns bright yellow, combustion gases are leaking in.
Saturday, July 16th, 2011 AT 10:06 AM