What cause my truck to only over heat when heater is not on
it always run about afourth of the wait when I turn off the heat now it runs three fourth almost in the red I turn heat on it goes to just half
its a slant 6 cylonder im not losing antifreeze and its not in the oil
The heater core produces heat but also is functioning as a small radiator. That is helping to cool the engine. Based on your description, it sounds like you have a plugged radiator or there isn't enough coolant circulating. Try to flush the radiator and engine block to see if that helps.