Air only from deforst

wranglerrambler said:
Like the title says... Under throttle only the defroster works. Under idle, air/vent comes through normal vents. What valve is the problem
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It's definitely a vacuum loss. Not knowing the vehicle will make it tough for anyone to pinpoint. Basically, system is designed to spring to defrost mode for safety reasons. Vacuum is what pulls the doors to other positions. You may just have a small leak
 
You can pull off the vacuum line at the fire wall and add a check valve. That'll keep the HVAC from changing at WOT.

Might be a good idea to draw vacuum on the HVAC side of things and see if/how quickly it leaks down. A hand held mityvac will do the trick.
 
Mine was doing nearly the same thing but it was always blowing through the defrost duct. I used the mighty vac to test individual sections til I found the culprit. Mine ended up being the actual selector switch in the dash not holding vacuum.
 
There is a black vacuum hose that goes I to the dash through the same hole the AC like we pass through the firewall.
From under the hood, disconnect that hose and see if there is vacuum present. If so, continue your search for a leak under the dash. If not, Chase that hose back to the intake and see where the leak is.
There is a check valve that can be installed on the HVAC supply house to maintain vacuum on long pulls like @shawn said
First I'd check for leaks like I stated
 
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