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Not a huge problem just something I cant figure out and has getten annoying with all the heat we are having.  My AC works great, the only issue is that when I am on the interstate it will all of a sudden switch from blowing out the vents to the defrost vents when the AC is on max. In order to make it start blowing through the vents again I have to let off the throttle.  Its really annoying when I am in the mountains and have to downshift and the RPMs rise.  I know it has to be vaccum related since the problem seems to be related to engine RPM's. Any thoughts?