I had same problem. Replaced thermostat and heater core. Here's all it was. An ink pen.
Take the glove box door off to see the blend door cable. Make sure it appears to be working. Then remove the heater core cover and blend door cable.( I had to loosen up the dash to get the cover out.) Now look inside the plenum and manually move the blend control door from hot to cold. If it doesn't go completely to the left closing off the plenum then you may have a pen or something out of view on the floor of the plenum. Take a wire or hook shape tool and rake it along the bottom next to the door and listen for anything loose.I was able to squeeze my hand in enough to remove it. Check door again and it should go completely closed.
Friday, January 29th, 2010 AT 12:44 PM