Odd engine problem on a rainy day . . chug, chug, chug, chug -- SOLVED!

[SJ]

Patient is 1990 100q.

I found the problem! Carbon pathways inside the distributor cap from center
electrode to one of the spark plug wire electrodes.

A cold rainy day . . between 35 and 40 degrees. Been drizzling on and off
all night.
I had to move the car to let the wife out of the driveway. Car started fine.
Pulled  car out . .let wife out . .put car back in driveway.
Came back in 30 minutes to start up car for trip to store. Cranks but wont
start . .. shit!
I get the truck out and come back to the house after 2  hours.
Try starting car . . .Vroooooom!  Pull out car and put it in front of house
while I park the truck in driveway.
Try starting the car . . . .nada . . .SHIT!!!  The engine sounds like one
cylinder tries to catch.

Well, it stopped raining . . .good time to do some checking and testing.
Weather is miserable . . around 35 F, cold, wet, drizzling. Very similar
conditions to when I have had starting problems before.
I spray in starter fluid . . .crank . .. same as before. Try starter fluid
again . . .ok, problem is not fuel related.

I get my can of WD40 and start spraying the ignition wires and ignition
parts. After the spray, I use a hair drier to dry things up.
Try starting . . .no start.
I take off the distributor cap to clean out the insides with WD40. AHA!  I
see two faint carbon trails leading from the center electrode to one of the
spark plug electrodes.
I clean off the carbon path with the WD40 . . .dry the insides with the hair
drier . . . hook it up . . . .start the car . . Vroooom!

I believe my unusual problem was caused by a partial carbon path inside the
distributor cap. The path was completed only when the due point(air
temperature and moisture content) was reached to cause condensation inside
the distributor cap. The moisture, when mixed in among the carbon in the
path, was enough to cause the spark to jump the gap. Everytime the right
piston was up on  its compression cycle, that plug would fire and hence the
"chug chug chug" when one piston was trying to fire or was firing.

There also must be some relationship between  the due point and ozone or ion
creation inside the distributor cap that will cause shorting after a short
ride. The car would start up fine . . .then conk out after a short trip or a
short engine run time. If left alone for 30 minutes or so, the ozone or ions
would dissipate and the engine would start again.

My parts guy says he sells a lot of distributor caps. I bet they fail in a
similar fashion, carbon paths and arc overs.

The failed distributor cap had about 20k miles on it.

I cleaned out the new distributor cap with gasoline, then cleaned with
alcohol to make sure there was no residual molding release or some other
contaminant on the inside surface of the cap. I then sprayed silicone inside
and dried it off using the hair drier.

Took the car to Canada and back this past weekend. The weather was cold and
wet for 2 days. No problems.

SJ
85 Dodge PU, D-250, 318, auto
85 Audi 4k - - sold but still on the road
88 Audi 5kq
90 Audi 100q


> Historical Summary so far:
>
> 1 time - cold damp day, cold start, short drive, shut off engine, restart
in
> 15 minutes, chug chug chug for 6-10 seconds
>
> 1 time - dry warmer day, cold start, short drive, shut off engine, restart
> in 15 minutes, chug, chug for few seconds.
>
> 1 time - wet cold day, cold start, wont start . .. sounds like it tries to
> start on one or two cylinders.
> next day - cold dry day, cold start, starts right up.

1 time - wife starts car . .drives off . .it stalls out about 3 miles from
the house. Wife sits around for 30 minutes, starts the car . .Vroooooom!
Works fine the rest of the day.