A/C switches from vent to defrost when going up a hill

[Convict1997]

In my 2002, when driving up a hill, the a/c will change from the vents to the defrost. Once I get up the hill, it will switch back to the vents. Sounds like some kind of vacuum issue. It only does it if I stay off the gas and make the truck stay in o/d. If I use the button to turn o/d off or give it enough gas to down shift, then it will stay on the vents. I've also noticed a hissing sound coming from under the dash when I initially press the gas. Could these two be related? Maybe a leaking vacuum reservoir, or something?

 

[general x]

Sounds like your on the right track, but it seems odd the hissing only starts when you begin to press the gas, it should be most prominent when at full idle. Perhaps the gas pedal linkage happens to be stressing some nearby vacuum lines under the dash? I am not familiar with how the vacuum lines are run so you would have to check. One way to check for leaking vacuum lines it to spray a combustible gas around them (Think brake cleaner or WD-40) If the idle surges when sprayed then your in the right area. That may leave a nasty smell if you do it INSIDE the cabin so you may want to play it by ear and see if you can't find the leak by sound.

It may also mearly be the way the vacuum setup is designed. I have a 88 grand prix that will do the same thing no matter what you do, it's vacuum setup is simply not designed to handle full throttle and maintaining vacuum to all the accessories. It has no leaks however so that would be a good place to start

 

[mulletcar]

sounds like you either have a loose/leaking vacuum hose or a directional door with a pinhole in the diapgragm. Oh yeah, 3rd gen X's are drive by wire so you can scrap the throttle linkage idea

 

[Convict1997]

it's vacuum setup is simply not designed to handle full throttle and maintaining vacuum to all the accessories

It's weird, though, that it only does it when I keep the rpm's low and have a load on the motor, such as when climbing a hill. If I keep the rpm's up, then no problem.

 

[mulletcar]

believe it or not, the vacuum is higher at idle than full throttle because when the throttle plate is closed it introduces a restriction which increases vacuum