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!<?xml:namespace prefix = o ns = "urn:schemas-microsoft-comfficeffice" /><o></o>
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?<o></o>

 

The 7.3 is a diesel, the 7.5 is a gas 460-
It sounds like your vacuum reservoir is bypassed, or there is a vacuum leak. I don't know off hand where they hid the reservoir on that van, but chances are it's a plastic sphere that's attached to the underside of the hood, or a coffee can looking thing on the right side fender liner.

 

You most likely have a leak, when you are at WOT or near it, the vacuum signal drops between 0-5 Hg. So, the engine is not able to supply enough vacuum to make up for a small leak. Once the engine is not under load, the vacuum signal returns.

Now, it might be the valve too, it might be cracked where the stem meets the body of the valve.

Look at the same place on the tank too. Smearing JB Weld or Super Glue over the crack works as long as it is not too bad.