Re: pleeeease help !!!!!!
There are two door in the heater box---a mode door and a blend door. The mode door controls WHERE the air goes--defrost, dash vents, floor. The blend door controls how much heat gets added to the air. They're operated by vacuum motors. When you accelerate, intake manifold drops, so if one of the vacuum lines or motors has a leak, they'll default to their spring operated position. Engine vacuum is what pulls against the spring.
So check for a broken vacuum line, a bad vacuum switch, or bad vacuum motor. I use a hand-held vacuum pump to pull a vacuum and then see if the line holds it.