You are dealing with a vacuum leak. The air flow is directed by vent doors that open and close. The are actuated by vacuum. If there is a leak, that is exactly what happens. Check under the dash first and then under the hood for dry rotted, broken, cracked. Vacuum hoses. If you are lucky, you will hear a hissing sound with the engine running.
Let me know what you find.
Thursday, August 5th, 2010 AT 11:45 PM