I have this problem as well. I have looked at other posts and they all seem to suggest there is a blockage in the heater core.
Remove the hoses to the heater core. Then use your garden hose back flush the heater core.
I will attempt it this weekend myself. I will let you know how it went. Right now it is a balmy 10 degrees outside.
Thursday, December 6th, 2007 AT 6:01 AM