Flummonxed: why does my 93 850 stall when warmed up?

[JimBee]

Cool trip down memory lane from those links!
Just to recap,
starts and runs fine in dry weather (no stalling) in all seasons with no snow on the ground.
starts and runs fine in weather when raining or has rained recently or snowing or snow on ground. However it has trained me to let it sit and warm up because due to moisture around it will stumble and die, usually when it's near or fully warmed according to the gauge.
Then, like Erik reports, it dies and won't start—until it cools, then will start and run fine through a day of trips, engine off, restarts, etc, no problem.
It has to be atmospheric moisture related.
I don't know how moisture could get into the distributor cap, but I don't think I've tuned it for years, though there isn't much mileage on it, probably 3 or 4k, mostly the first 3 years I drove it exclusively before 2011.
In all weather, starts instantly, usually when the first plug fires, even after sitting out by a snow bank for days, and there's never a misfire when driving (after it's done its thing) which is what leads me think it's not ignition, but when the birds start chirping I'll probably do the distributor and rotor just to see. If that fixes it, I still won't understand why, but neither will I care.

[abscate]

The cap is not sealed and Any moisture inside will steal spark to ground. Another source of moisture can be blow by from the end cam seal working its way into the cap. Time to have it off and show us the ugly, corroded terminals inside

[JimBee]

Okay, that's the most persuasive rationale I've seen yet. But positive vs negative forces. I replaced all the camshaft seals when I got the car 15 years ago and don't have tons of mileage on it since, but it can't be ruled out 100%
Versus, atmospheric moisture somehow getting into the cap, but wouldn't there need to be negative pressure drawing it in?
I'm positive the stall has something to do with atmospheric moisture—however it is actually causing the problem.

More evidence. There has been quite an accumulation of snow on and around the car, so with normal evaporation, there will be moisture in the air, though not enough that I can feel it when there's no precip, but nature being what it is, there will be evaporation, so moisture in the air. And the stalling has been consistent with either rain or snow around.

Surprise: Two days ago a lot of the snow immediately around my car had gone away, it was warm and clear. I started the car prepared to wait it out and let it rest before driving it (as it has trained me to do), but decided to drive around the block and park in a different location where it was sunny, and I would wait it out there. It didn't stall, which was a bit unnerviing as I wondered it I got back on the road it would then. It didn't. I let it warm up again and drove around the block a couple of times; it was fine for the rest of the day.

WHY DIDN'T IT STALL? There just wasn't enough moisture immediately around the car or in the air to cause the issue (I assume).

So, checking out the distributor will be another of the accumulating car related tasks for when the birds start tweeing.

And then there's the question of why does it start instantly and run normally until it's warmed up?

[abscate]

The air in the crankcase is hot and moist from all of the water of combustion. The tiniest bit seeping into the cap will cool and condense , in warm weather it will evaporate. In cold weather, it will accumulate

[erikv11]

Or ... as the engine runs, steadily puffing crankcase gases past a dislodged cam seal into the distributor cap, eventually enough moisture condenses in there to quench the spark. So it starts instantly and runs until enough moisture accumulates. Shuts down due to moisture, is still too wet to start. Once it dries, it starts again.

Or ... if there is a problem with worn or crusty contacts, remember that electrical resistance goes up in a conductor as temperature increases. Super common problem with automotive electrical parts. This kind of problem is notoriously intermittent. So it starts instantly and runs until it gets above a certain temp. Cools down, resistance goes back down, it starts again.

Any and/or all of these speculations could be in play. Or some we didn't think of. You don't have to replace it per se - just pull the cap and take a look for starters.