Air blows out defrost on WOT

[CowboyBob]

On my 96 the air blows out the defrost when I get on the go pedal, but as soon as I let off, the air goes back to were it is supposed to blow out.  I figure the check valve is bad, but it seems to me that the engine makes the most vacuum when it is rev'd high and this problem is opposite a bad check valve.  Before I change the check valve, is there anything else to consider?

[Scott91370]

I would look under the dash where the temp control unit plugs into the vacumm line(s).  One may have a leak. This s'should' cause it to stay on defrost, but you never know what the gremlins are capable of.


MERRY CHRISTMAS!!

[1995DodgeRam1500]

That to me would indicate a vacuum leak.  My Jeep does this (yeah yeah GAS) when a vacuum line pops off and the default blend door position seems to be defrost.

[Motorbrreath]

vaccum leak or bad check valve. they say to check the vaccum lines under the battery tray (drivers side for you) and the ones goin into the vacc pump.

if you cant find a leak anywhere, its the check valve. mine is bad too.