'86 cargo 5spd goes down to idle in the rain

[Flounder]

Good day everyone, one of several vans I own is an '86 2wd cargo 5spd. (Llamavan, you know him as "Satriani.") I searched for all threads related to "idle," but in 9 pages I didn't see any thread title related to the curious problem the van is having.
When it is raining and there are puddles on the road, the van goes down to idle and stays there. Never stalls, but it takes a period of time driving slowly around a parking lot to dry out something.
I suspect a problem with the TPS. Anyone out there ever have a similar issue?
Thank you for any adult guidance you can provide.

[Burntboot]

What have you checked so far?
Most drivability problems, during wet weather, are usually traced back to the ignition system (cap, wires.....)
Are all the maintenance items up to date?
Is this a new or all of a sudden problem?

A spray bottle filled with water is an awesome diagnostic tool, for these sorts of problems

[Flounder]

Thanks very much for your reply. It's been a problem for a while. (This particular van is as simple as they come.) My job has been keeping me busy for the last 3-4 years; my profession has had a dire worldwide shortage of qualified engineers for the last 15+ years. The van is in storage, but in 2-3 weeks I'm buying a new battery for her. Then, my professional mechanic brother-in-law and I are going to start her. I have checked the distributor. Replaced it about 6-7 years ago. Also replaced distributors in 2 of my other vans, all within 3 months of each other. ??? All the other maintenance is up to date. I have spare spray bottles, will fill one and spray the distributor then open it up. Will also check all the plug wires. But she hasn't ever had a problem with rough running.

[Burntboot]

I would probably think about dropping the tank before trying to fire it up.
3 yrs is a long time for modern gas to sit around.

An empty tank will have internal surface rust, a full tank will have varnish in the bottom (at least that how things go up here, YMMV)
You really dont want pump whatever is currently in the tank, thru the injectors.
There are several threads on plugged injectors and rusty tanks, the problems they cause and the difficulties of addressing them.
All sounds like something best avoided, if possible.
Tim has an excellent write-up on his experience, definitely worth the read.

The beauty of the spray bottle is you can (hopefully) replicate the fault while it's safely in your driveway.
Puddles can be so inconvenient for diagnostic work

You want to do targeted sprays, work front to back OR side to side, I usually work bottom up so I don't have to contend with gravity, but be methodical and leave no stone unturned.
All electrical connections and grounds are suspect, as well as ignition wires, distributor cap, +++++