HVAC mannual override not working

[K-2]

Hi there every one, I m relatively very new to explorers so pls pardon my ignorance.

I have a 08 Explorer EB. the issue is with the air flow direction. all s working perfectly normal; i.e the cooling, heating, air flow etc. However when I accelerate fast this what happens:
(even when the air direction settings are manually selected to dash only) the air flow will first start coming from the floor/dash vent then will switch to the defrost/dash vent and then only the defrost vents and will continue till either I slow down the car speed and or apply brakes !!!

this behavior is independent of the temperature settings and or fan speed.

Is this a normal behavior or there is some thing wrong.
Its around 40C here and it really brings hell when it does that.

by the way if I keep driving slowly or at constant acceleration this doesn't happen.

pls doe excuse me for not meeting the table manners.

 

[thebrakeman]

I believe those louvers are vacuum controlled. The default position under a loss of vacuum, is the defrost position. This way, if there is a severe problem, you'll never be in a situation where you can't defog the windshield.

So, I suppose under hard acceleration, you are losing enough vacuum to those louvers that it's not able to hold it in the selected position. Sounds like a vacuum leak.

 

[K-2]

THNX brakeman; that makes quite a sense ...
I would further look in that direction, thnx for your time n help

 

[snakester]

Please post a solution if you get it fixed. THX

 

[banditxp]

mine was the vac check valve under the hood. pass side. its a tiny thing between two vac lines kinda sits above the back spark plug area. i can take a pic later

 

[snakester]

Fixed this with a new vacuum check valve. No guesswork.

Purchased for $10 here : http://www.rockauto.com/catalog/moreinfo.php?pk=380402&jnid=2&jpid=0

It is located here:

Just pull it apart and replace it with the new one installed in the same direction and its done. 5 minutes including the test drive.