Your heater core might be plugged. Disconnect the hoses that goto the heater core & try to blow air through the heater core. Visually look in the heater control valve & make sure when the ar turns that it opens & closes. You might have an air bubble in the cooling system. Make sure the cooling system is full & with the heater valve in the open position, run the engine until the t-stat opens then top off the cooling system
