Just finished doing an a/c evac and recharge on the wife's '03 Expedition XLT.. The entire climate system works just fine, EXCEPT a/c on the MAX (Recirc) setting. Nice, cool air comes out of the dash vents at idle, but smoothly transitions to the defrost vents when I hit the accelerator. Soon as I take my foot off the gas, it transitions back to dash vents. I traced the vac line from the intake manifold, replaced a splice that seemed to have "collapsed" at the check valve / tee, but no joy on the problem. The end of that line is the vacuum reservoir tank next to the battery. Could I have a cracked line under there that ONLY affects the recirc door vac circuit, or is it more like somewhere in the cabin under the dash (closer to the door mechanism)? Thanks in advance!

 

My issue on my F150 (2002) was in the electronic control unit. It has a vacuum manifold inside and the orings had dried out. I took it apart and replaced the 4 orings and it is working fine now .