Diversion to windshield vents when accelerating

[piano_71]

I've had a problem with my '97 Expedition that when accelerating rapidly (especially uphill - and when the automatic transmission needs to downshift), the HVAC air flow gets diverted to the windshield vents. If I ease up on the gas or the terrain levels out, the air flow returns to the panel vents. (Panel vents selected the entire time on the dash control)

I just had my Expedition in the shop to have a few things looked at, including the engine vacuum system. While this problem has improved (it's less susceptible to doing this), it hasn't gone away completely.

Is it normal for the air flow to temporarily shift to the windshield vents under these circumstances? Or is there something they missed when checking out the vacuum system?

 

[01yellerCobra]

I think there should be a check valve or a vacuum canister to keep that from happening. Sounds like they missed something.

 

[AWD EXPY]

Mine does it as well occasionally.

 

[thenaaks]

You've most likely got a vacuum leak. There's a bunch of lines under the hood. Some big, some small. Lots of rubber connectors that get brittle. Just spend some time looking and feeling and you'll probably find the problem. Just patch it with some rubber hose. Good luck!

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