There's a lot of causes of overheating. Typically if a head gasket is leaking combustion gases will pool under the thermostat and prevent it from opening. Thermostats have to be hit with hot liquid to open. Hot air won't do it. When the thermostat doesn't open, coolant circulation is restricted and you may not get hot coolant into the heater core. That results in warm or cool air from the heater instead of hot air.
If the overheating only occurs at low speeds, suspect a radiator fan that isn't turning on. When it only occurs at higher speeds, look for corroding or crumbling cooling fins on the radiator.
There's a real quick chemical test that can identify a leaking head gasket. That involves drawing air from the radiator, while the engine is running, through a glass cylinder with two chambers partially-filled with a special dark blue liquid. If combustion gases are present, that liquid will turn bright yellow.
Monday, January 25th, 2016 AT 12:47 PM