Ok guys, I bought a 92 Z28 that on the drive home the vent was constantly open no matter what I did with the control. It did have a new heater core just installed but I also found the vacuum line to the hot water control valve was disconnected too. I'm not able to drive the vehicle at the moment but just wanted to hear some opinions of what I might be up against. Anyone have a similar issue?

 

When I installed my heater core, I didn't pay attention and installed it in such a manner that the door/flapper was pinned open and couldn't move. I only had the drfrost setting. I just recently remedied this when installing a new heater core.

Hope this helps.

 

It does help and sounds exactly like what I have going on. It unfortunately sounds like I'm going to be taking the heater core out now.

 

You will definitely need to get access to the heater core. If you are lucky, you could just "wiggle" the heater core and manipulate the flapper door back into place. But you will have to be on your back for awhile (gigity), getting all the panels out of the way.

Good luck.