Thanks for that explanation. I wondered about the AC/Defrost connection. I have to say I prefer a system that is more independent such as my typical cars have been. That is, a temperature function, a button that lets me turn the AC on or off (but does not tell it where to exude from), and a selector from which I can tell the air (hot or cold) where to come out from. I mean seriously, I'm not so stupid that I couldn't put AC on the windshield if I needed it to help defrost. OTOH, I don't necessarily want the AC compressor on when I'm towing up a hill, and I might like the AC to come out a specific position....
Okay, got that over with. Anyway... I think I might have two issues, but perhaps you (or someone) could give me your thoughts. Since I'm traveling, I don't have a good/known mechanic, so I usually try to either do my own repairs, or at least know exactly what I want done (if possible) if I'm taking it to someone I don't know (OTOH, if I had a good/known mechanic, it's not that I need or want to do this work myself!).
Anyway, so after reading the links that 86 Scotty provided to FTE (thanks!), I think I have both of the following:
1) The vent/heater core/design problem, and...
2) Perhaps a vacuum problem. I think the vacuum problem is a minor one, not a complete failure.
Symptoms of #1 are heated air coming out the vents even in cool weather (especially after stopping, but all the time to some degree), and a general area of hot air coming out from under the dash above where the Scan Gauge plugs in. Also although the AC always works, it seems less than icy in hot weather.
Symptoms of what I think is #2:
a) One time when I was towing and had the AC on (or actually it might have been the vents since I don't typically go uphill towing with AC on - - this might have been before I gave up on the vents), I was accelerating up a hill and everything automatically switched over to "defrost" position, then "mysteriously" switched back. This happened a couple of times last May, but then never again. I did wonder how it had "healed itself." Now I think I know: I have quit using the vents as they are so uncomfortably hot, and I typically now just switch the HVAC to "Off" when climbing a hill while towing.
b) I have had an EGR code (0455) set from time to time but with growing frequency. I had part of the EGR system replaced (for some annoying reason they didn't do it all but didn't tell me until I was on my way out the door and sure enough, about 50 miles later.... 0455). However now I think it might be (at least partly) due to a vacuum loss under acceleration. Because I thought I had noticed that it would often set if I was climbing a hill but midway up let off the gas slightly. Then "bing" I'd get the CEL and Scan Gauge would show it to be 0455 (I'm also getting occasional 0401, but I think that is separate?). The 0455 has been getting more frequent (but I did just finish a 2,000+ mile mountainous tow, so lots of accelerating up hills).
From what I read in the links, there is a hard-to-access vacuum canister and an impossible to find check valve. If Ford fixes these it's thousands of dollars plus (almost worse, to me) they take your whole dash, etc. apart. I find that often leads to annoying scratches, loose parts, etc. Anyway, when you are accelerating hard that pulls a lot of vacuum and so if there is a leak or problem, there is not enough left over to "run" the HVAC selector system, and so it goes to its default position, which is Defrost. Apparently the EGR system also uses vacuum, hence my hunch in (b), above.
I figure it must only be a smallish leak, not a complete failure, because I only notice it when towing/accelerating uphill, and also when I turn the HVAC selector from Off to another position, I can hear the "hushhhhh" vacuum sound (plus a wee squeak). So that must mean there is at least enough vacuum to run it most of the time.
I think this is separate from the "heated vents" problem as that occurs all the time, just by virtue of the engine running (no need to be towing, going uphill, etc.). So I likely need to fix both. It seemed like the best fix for the vacuum system was to make your own, generously sized canister out of PVC and mount it above the passenger footwell, along with a check valve, bypassing the original system. Some folks cut a hole in the passenger fender liner above the wheel, and were able to re-attach a vacuum line to the original canister, but that doesn't do anything with the check valve, which no one can even find. There are vacuum canisters you can buy, but it sounded like they might not be large enough (?).
Okay, if anyone is still with me, a question just out of curiosity (and back to issue #1, the heated vents): If the HVAC knob is in the Off position, is there still circulation through the heater core? Not that it would be practical to drive like this, but I was just wondering.
It is a bit irritating having to re-engineer the working-as-designed HVAC system of a van that has been in production for decades
So in summary, does this sound reasonable for my rig:
1) Fabricate and install larger vacuum canister and new check valve above passenger footwell; abandon OEM canister/check valve. OR... if/since the leak seems to be small, is there something else easier I could check first?
2) Install Max Air, then install The Cold Rush which is really what I want, but you have to put in the Max Air first.
(massively intimidated by doing any of this!) (if anyone knows of anyone really good/trustworthy - preferably a one- or two-person shop in N or NW Arizona, please let me know).