I also have this problem, I own an 82 150 and not only do I have problems with the heater not blowing hot enough, I think I have a major draft comming frome somewhere.
And my temp gauge never works either, but my odometer never does either.
I had my fan go out and now its directly wired to the battery and I noticed that my heater core doesnt stay warm for more than 20 seconds after I turn the key off.
Monday, January 22nd, 2007 AT 10:23 AM