2002 air vent changes under strain

2002 4.0 Explorer when under strain (hills, strong take off, etc) air stops blowing from vents. Comes back when no longer under strain. Probably vacuum leak but dont know where to start looking. Any help would be appreciated.

Clarification: The strain is not major like it sounds. Small hills and acceleration to pass causes this problem also. Not normal I know but where do I start.

Actually, it is basically designed to do that. The airflow defaults to the defroster (for safety reasons) and vacuum is used to position the controls elsewhere. When under extreme loads, the available vacuum decreases dramatically and therefore the controls default back to the defroster. As soon as the vacuum levels return, the controls will also return to where they were previously set. Make sense?

Thanks. I understand what your saying but I'm not sure if it makes sense for it to do that. I appreciate your response, just glad I dont live in mountains, would never have air blowing on me. Just got another 02 that I'm gonna see if it is the same way or not.

The other 02 is not that way. So still need help.

Most people never notice it because it normally only does it under an EXTREME load when your manifold vacuum drops really low. My 01 Limited with a 5.0 does it when it hits passing gear. Actually, over the years, I have noticed V8s are more susceptable than smaller engines. Apparently, they have a greater vacuum fluctuation than V6s or 4 cylinders. In fact, I can't recall my Miata ever doing it but my V6 Bronco and Tauras always have but like I said, not as noticably as V8s.

BTW...you are right about the mountains. When I lived in Utah and Alaska it was very noticable, especially when pulling a large trailer or in a motorhome at high elevations (and the motorhome had a V10).

there is a check valve in the vac. line to the hvac control head to prevent this problem - either you have a vac. leak in the dash just to the left of the glove box at the double connector or the check valve is leaking back