I have a 1999 Dodge Ram 2500 V-10 4x4 I was on the freeway with the windows up and whenI went to pass I felt air comming from the defrost vent. But the defrost was off.If you have the ac on going up a hill you can hear the blend door shut. SoI checked for vac leaks found nothing. seems to go on under load only.

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Chrysler products use a check-valve in the vacuum line that operates your flaps in the airbox. You can get a replacement one at a dealer or parts store for less that ten bucks. That should stop that flap problem.

 

will see

 

Thank you for the info. I'll see if it works.