I have an issue with my 2001 ES300 overheating. It started one day when my radiator started leaking very badly (I didn't notice if the car overheating caused the radiator to blow or if the leaking radiator caused the car to start overheating). I changed the radiator, upper radiator hose, and thermostat and the car is still overheating. I took it by a mechanic and they cleared out any air locks in the system and ensured it was topped off with fluid. As far as I can tell, the water pump is fine (no water coming from the weep hole).

The car overheats when I drive it for about 10 minutes. What I have noticed is if I leave the heater on, as long as it is blowing out hot air, the car will not overheat. However, after driving it a bit, the hot air will turn cold and the vehicle will overheat in about 2 minutes. I have checked the oil and the exhaust and do not see any evidence of water. The cooling fans do work however, they do not come on automatically when the air conditioner is turned on which I find odd. I have left the car idling for over 30 minutes several times without the car overheating at all but once I drive it, it will overheat

I have never heard of a car doing this before. I took it back my mechanic and told me they were not sure but they think it is a blown head gasket or a cracked head. Has anyone seen this occur before? Any help would be much appreciated.

 

The heat turning cool sounds like coolant is not flowing at least to the heater core.