Hi guys, I'm looking for some advice with my heating problem. The problem is the heat that comes out of the vents is luke warm....but if I switch from floor heat to the upper vents it momentairly gets very warm (then after 10 seconds goes back to being luke-warm). I just had a local shop flush the system and back flush the heat core...this didn't seem to help it at all. Any ideas?

 

I think the door that opens and closes to allows the heat in the box,from the core, is vacumme operated i'd look under dash for broken or loose vacumme hose and listen for hissing sound while it's running !!! Good luck!