Thanks everyone for the suggestions. The engine was covered with a layer of
windblown snow when I opened the hatch:
https://photos.app.goo.gl/PVZnD0hrPiisjhWH2

I checked the TPS, it was good, and measured the voltage from the OXS as
0.43V with the key on.

Reseated the TPS and AFM connectors. Got a bottle of Iso-Heet and poured it
into the gas tank.

I've driven about 5 miles with no incidents of hesitation. Water in the gas
sound plausible, the trouble started immediately after fueling up in
Missoula. Keeping fingers crossed, heading into Yellowstone Park tomorrow.

-Mark

On Sat, Dec 23, 2017 at 8:45 AM, Mark McCulley <mark.mcculley@gmail.com>
wrote:

> I'm on a road trip for the holidays, van has been mostly running great but
> have had some intermittent episodes of roughness. Hoping to get some
> suggestions for things to check when I have an opportunity to do so (single
> digit temps today, no garage).
>
> First noticed on beginning of second day, leaving Missoula, just a couple
> of hesitations when accelerating onto the freeway on ramp. As long as I was
> holding at steady cruising speed, very smooth but if I let off the gas for
> any reason, then hesitations again.
>
> I noticed the symptons again yesterday, this time mostly when changing
> gears. Smells like it's running rich. Come to think of it I noticed that
> smell a few weeks before setting out, well before I noticed any
> hesitations.
>
> - The first thing that comes to mind is the TPS, I will check that.
> - I have spare distributor cap and rotor, but only 7K miles on the
> existing ones.
> - I have spare plugs, 17K on the existing ones (W7DTC). Inspected 7K
> miles ago and looked fine.
> - I have the AFM harness with the capacitor, and have never had a problem
> with Vanagon syndrome.
>
> It probably wouldn't hurt to change all those ignition components...
>
> I'm a bit suspicious of the O2 sensor, is there an easy way to confirm if
> it's OK? What does ECU do when O2 sensor is unplugged?
>
> Thanks in advance for any suggestions.
>
> -Mark
> 87 syncro Westy
>