I have been trying to track a lack of heat in my 1997 F250HD with the 7.5 L gas engine. I have replaced the thermostat and get "normal" heat readings. Both hoses to the heater core are hot. I looked at the end of the cable attached the heater dial and the external components move. There is a vacuum actuated valve in the engine compartment. It activates when I start the vehicle.

Any ideas? Thanks. And I did search the forum, but might not have used the right key words.

 

is the heater core clogged?...I believe on those trucks the heater core is located behind the glove box....also make sure your antifreeze levels are good. My 97 f350 diesel was running a little hot and I was getting no heat....it turned out to be about 3 gallons low on antifreeze.... now the truck is cool and the heat is working again...where the antifreeze went is my bigger problem..hahah