Gremlins in my vents!

[dgoodall]

1. I have an E-350 van front end that is carrying a Tioga motorhome on the back end. I believe that it has either a 7.3 or 7.5 liter engine in it. It is a 1992. So here’s the problem. I am driving along with either the diverter setting set to Vent or AC on (through the vents) and for the most part the air blows out of the vents just fine. (note I do not believe that this is related to the AC circuit as the same behavior happens when on vent only) so every now and again, the air gets diverted to the windscreen for about 30 seconds and it then comes back through the vents again! There is a little access panel in the top of the dash which when opened shows the vacuum actuated valves working. The one closest to the front of the car seems to operate the vent / windscreen direction and you can see it moving of it’s own free will from time to time!fficeffice" />>> If when it has self diverted to the screen I take my foot off the accelerator – even for a second, it will immediately move back to the vents…. Makes for an interesting journey J I have to believe that this is a vacuum related problem, but where do I start to diagnose this issue further? Where is the vacuum source on this beast? My first thought would be to trace the vacuum lines – any thoughts?>>