no heat when trucks warm, cant see it has anything to do with head issues as the heats pulled from the heater core. I cant recall the name of it but there a box with a large flapper in it depending on the position it controlls the temp of the air ciming out. I'd bet 100 your little flapper is failing to move like it should possible causes are a malf in the switch on the ac contol pannel or the valve itseld. you tube power strok help. he's got a vid that describes the box im refering to and how its functions.