Believe it or not, this isn't uncommon. First, let me explaine how air flow direction is done. In the HVAC system, there are vent doors that direct the air flow. They are actuated (moved) by engine vacuum and based on where you have them set. If there is a vacuum leak, they are unable to maintain the direction you want them to work in.
What is happening is this. When you climb a hill or accellerate, you open the throttle plate to the engine to give it more air and gas. As a result, the amount of vacuum in the intake decreases. If you have a leak + put the engine under a load, there is no longer enough vacuum to keep the vents stable. As a result, it blow from the defroster, floor. Where ever the vents feel like moving.
In this case, you are looking for a vacuum leak. This is not an expensive thing to fix by anymeans. However, that hard part is finding the leak. It will most likely be under the dash, so I would start there. Also, it could be under the hood. Check both places for damaged, cracked, dry rotted. Vacuum hoses and replace them when you find them. If you are lucky, you may hear a hissing sound where the leak is located with the engine running.
Let me know what you find.
I hope this helps, and let me know if you have other questions.
Sunday, July 18th, 2010 AT 9:40 PM