I did a search and didnt come up with too much info on this subject so...

I'm having problems on my 90 IROC getting the defroster going. The air comes out of the vents no matter where I put the selector switch. I know on 4th gens, the default setting when there is a vacuum problem is the defrost/floor vents, is it the same on 3rd gens? I have checked the vacuum at the check valve underneath the map sensor and it seems like it's pulling 18-20 Hg at idle in all directions proving that something isn't leaking. I've changed the HVAC controls to a GTA one I had laying around and it doesn't change anything, it is stuck on the vents. Any ideas?

 

feel the heater core lines cold...
put the HVAC selector to cold...
run the engine to operating temp...
feel the heater core lines again...
if it's warm, you have a leak in your HVAC line

hell, mine came unhooked and burnt off on the header...

 

Its a vacume leak. I had the same problem on my GF's 89 RS The line going inside the car had dryrotted and crack leaving the air coming out the top 24/7

 

Yeah but there is 18" Hg of vacuum coming off the check valve going into the hardline by the heater box. The default position (unless Im mistaken) when there is a vacuum leak is the window vent and mine is stuck on the front vents. Also when I pull the vacuum lines from the HVAC controls on the back there is vacuum on both sources. What vacuum line are you referring to on your GF's car being cracked?

 

Ok I'll try this.

 

Tried it, doesnt seem to be a problem.

Ive kinda worked it down to the vacuum diaphrams under the dash for the vents...does anyone know how to test these to see if they've "collapsed" ?

 

Not sure how the venting system works on Camaros, but my car has a "door" that controls where the air is directed when the selector is moved. It is tucked underneath the dash next to the heater core if I am not mistaken.

My "door" is broken somehow and I have been too lazy to figure out what is causing it, but since my entire front trim plate is off, I can use my hand to manually direct where the air is supposed to go.

Anyway, what I am getting at is this: see if that "door" is stuck in one position and if it is, fix it.

Hope that helps.