Your local shop will always be able to help. Mine does that as well and we just figured it was the blender door in the dash(which is why we left it). It’s not a big inconvenience for it but I definitely understand the need for your van to be perfect. Lol

 

Vent controller is vacuum. controlled.
Vacuum leak in engine compartement... could be vacuum hose, could be an elbow.
Common problem... they rot and deteriorate over time.
Easy fix, once you find the culprit.
Rubber hose replacement is fine.

 

There's a hard plastic (or rubber hose) that run's from the driver side of the engine intake manifold to a fitting on the passenger side.
You get to the driver's side fitting by removing the doghouse (interior engine cover).
Common problem, as mentioned, and easy fix.
Helpful:

Google Search

www.google.com

HVAC Vacuum Line Replacement

Replaced the deteriorated HVAC rubber vacuum lines that attach the vacuum tank (ball) to the heater & AC control valve with some urethane hose I had on hand. Disconnected the old rubber line coming from vacuum tank to test fit the urethane hose and can see just how deteriorated the rubber line...

www.astrosafari.com

NOTE: REPLACE THE WHOLE LINE, DO NOT SPLICE OR PATCH. IT WON'T LAST!

 

Check out the vent control issue sticky in this section.

Climate Control

Topics regarding the climate control in the cabin of the van. Discuss things like air conditioning, refrigerant recharging, and dual climate controls, a/c vents, accumulator or defrosters, heater core, or fan inside the van.

www.astrosafari.com