Hey all, new member here just wondering if someone can enlighten me about heating problems Im having....the problem is no heat, 91 model F-150fullsize PU, 302 V8, Ive had her for 3 years now, and its never heated right, the blower motor/fan works correctely and the vent/duct lever works right, it just blows barley warm air after 5 mins or so it blows cooler air, I changed the thermostat last spring (Stant) 190 and it didnt fix my problem, is there a valve (electric or vacum) that cuts off the hot water to the heater coil, I dont think its the heater coil I dont have any leaks and the hoses are hot with holding pressure, Im tired of freezing my rear off please help!