my heat used to work perfect, on cold mornings i would start my truck before i left crank the heat and by the time i was ready to leave it was nice and warm in there. recently (and now that its even colder) the heat isnt working to well. it seems to only be hot when im on the highway doing 75+ mph. any ideas?

 

lets start with the basics, how is the coolant level?

 

Low coolant level, defective thermostat, plugged heater core.

 

x3 coolant level if mine gets even a little low no more heater.

 

x4. blow out the coil, both directions!

 

Is the blower spinning as fast as always?
Does it sound out-of-balance? (Vibrating)
Is the air blowing as hard out of the registers?
Is the tailpipe sooty black?

 

have you used any liquid leak stoppers in the radiator? thatll plug everything up and block the heat. if not, check the coolent. if theres not coolent, youll just get cold air. but i had the EXACT same thing happen to me, with the 75+ mph thing...thats the only time the heat worked, and my problem was caused by a liquid aluminum i used in the radiator. it took quite some time for it all to drain out of the system and then of course my leak came back. im just gonna deal with filling the coolent a million times rather than lose my heat lol.
so if its not a clogged up system, or low coolent level- it could be the heater core gone bad...dont know much about that though