Cadillac Repair: heat doesnt come through vents.. change temp *digital* and it comes through with a/c???, vacuum problem, vacuum line

Question
well i got a 1998 cadillac deville. All digital dash of course. So the question of the day is... heat comes through defrost and floor.. won't come from dash vents. But! if you turn down the temp so the a/c turns on it will automatically start blowing from the vents. I'm in the process of selling this car and i showed it to somebody and he told me it was a heater core and he didn't want it because of it. I know its not a heater core because the heat infact works it just doesn't come out the vents.. seems like a vac line to me maybe you can give me some points of interest.. I'm new to caddys and im trying to get out of this car as cheap as possible! lol thx!!!

Answer
Hello,

it is a vacuum problem. It could be a vacuum pump under the hood, broken vacuum line, vent door vacuum or in the heater/A/C control.

To diagnose, if you used a hand held vacuum pump and eliminate the items listed, your problem would be taken care of. For some goof to say the problem is a heater core, it surely shows his ignorrance in these repair areas.


Rob