Why does my 1993 GMC Van heater blow cold air?

Answer 1

under the hood on the left center near the inside firewall is a hose that has a small open and close valve on it. It kind of looks like a pin that slides into the side of the hose. Clean it and spray a little wd40 on it. The switch somtimes gets road junk built up on it. Mine works great now. you may have to open the motor compartment to get to it from the inside of the van. Not sure, my '89 van does the same thing. I am going to look at the cable from the heat control knob to the heat box, maybe the door isn't opening up. If it is opening like it should, then the heater core my be plugged up. This would require removing and cleaning or replacing the heater core. Also a cooling system flush now is good. Any help on how to remove the heater core?