my 96 v6 does the same thing. vents all over the place. cant find any issues though. no leaks that i can see. im doign the plenum today, will keep an eye on it going forward and see if that fixes it

 

^ when the plenum is done, replace the little check valve(s) on passenger side of the intake, in and around the firewall. also check vac lines for crack or break.

not sure about yours, but on my 01 there is also a vac canister under the cowl, with more vac lines in/out of it.

 

He was saying that the owner said it doesn it as a fuel saving feature. That's BS. Mine does the same as your but thats because it has "wild vents"

 

its totally a fuel saving issue. Once you buy it and it breaks, it wont use anymore fuel..

 

You got me there...

 

lol. So someone mentioned this is a common vacuum leak. [the heater vents blowing out the defrost under load] Then how do you fix it? I love the reference to common problems without any reference to common solutions =] I am going to check and do my plenum too, I am pretty sure it is bad that and I need to remove a valve cover anyway to see whats clacking on the passenger side. I also have no vacuum canister. does a 96 5.9 have one? Would this be a cause of my vacuum problem assume nothing else were wrong?

 

The common condition is known as "wild vents". I think there is a thread about just this problem in the faq section. Basically, eliminate all the vacuum leaks, and replace the check valves in the lines by the intake, on the passenger side.