I have a '78 F150 Explorer with 400 and factory A/C.
My heater won't blow hot air, it will blow lukewarm/room temperature at best. The fan is working fine. I have hot water coming into the firewall and return hose gets hot too. Is there a vacuum operated door or something that I should suspect? Heater has worked great for the past 33 years.
Maybe the heater core is partially blocked, maybe only a few tubes free to allow flow thus only getting warm and not hot? What about the levers operating the doors are they free and working? just guessing here.
I can think of a few things it could be.
1) You can reverse the heater hoses believe it or not, I didn't at first. You may need to switch them.
2) There is a cable running from your temperature slide/switch on the dash to a vent. This can come unhooked and allow very little warm air into the cab.
3) Surrounding the heater core is some weather stripping, it breaks down over time and allows air to flow around instead of through the heater core.