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!
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?

 

With a loss of vacuum, the system diverts to the defalt setting (DEFROST/WINDSHIELD). You could have a small vacuum leak in the system. I would check all under hood vacuum lines and also the vacuum canister (usually mounted behind an inner fender. Sometimes the line will be chaffed where it passes thru the fender. Also double check the lines under the dash and at the dial(s) for a loose or cracked line.

 

So i have not played with this for a while, but got round to it tonight. It does not seem to have anything to do with the vents - this is just a symptom. the vacuum line comes through the firewall and connects to the vent system / manifold etc. I disconnected this line and at idle there is good vacuum to the vacuum tree on the manifold. so for sure there are no leaks in the system from the tree to the dash. What i notice is that as i am driving with little load on the the engine, again i get good vacuum, if i open the throttle to accelerate / go up a hill, the vacuum to the system is lost! - ie throttle plates open - vacuum goes away! Why would the vacuum go away - this means that there is not vacuum / low pressure in the manifold! arghhh! is this right?

Also in the vacuum diagram, there is mention of the branch off the vacuum tree that goes to the 'climate control system' via a check valve. i do not see a check valve. The presence of one would certainly compensate for loss of vacuum as i go up the hill, then vacuum will be restored again!

HELP!

 

Look for your vacuum canister. These were made from metal and most often looked like a coffee can painted black and mounted on the inner fender area and sometimes between the innner and outer fender. If you have this type, it may have a hole rusted thru it. Most of them are now made from plastic and you only have to worry about a bad line leading to the canister.

The vacuum canisters store vacuum supply for vacuum operated controls, such as switches on controls and vacuum actuators throughout the dash & engine compartment.

The canisters keep a constant storage of vacuum so your a/c & heating system can operate correctly even when your engine vacuum drops while under throttle.