Okay, so here's the story. I have a 1997 E-250 with the 5.4 V8. It's had the issue where the hvac would fail over to the defrost vents when the motor is under load. It's done this since I bought the van a few years ago and I never bothered to fix it.

However, about a week ago the air stopped blowing from the dash vents entirely and would only come from the floor vents. I figured that the line or tank had let go entirely. I took the red line coming off the manifold and bypassed the tank behind the evap coil for now, running it straight to the black manifold underdash feed line. I understand I will still lose vaccum under load, and I do plan to add a surge tank & check valve once this is all sorted.

However, I still have the same issue. I cracked the service cover on the top of the dash and I can see the actuator moving when I select "Vent" "Norm A/C or Max A/C", but the air keeps coming from the floor. The recirc door closes fine when Max A/C is engaged. Moving to the "Floor" position results in the pass side valve opening up. No change in the amount of air coming from the floor vents. Using Mix engages the drivers side actuator, and going to Defrost moves it all the way. Defrost vents work fine.

The only thing I can figure is the blend door in the diverted box is broken somehow and is not moving or sealing. However, due to the extremely common vacuum issues I can't find any information regarding this.

A final thing to mention is I do get a hissing sound like there's a leak only when the HVAC is in the off position. I have replaced the controller with a junkyard one already and it does the same thing. Not sure if that's normal. My Super Duty doesn't do that.

I'm just trying to get some more thoughts before I rip the entire damn dash out of the truck to get at that box.

Your time is appreciated.