My 91 Syncro Camper had the same problem you described - black exhaust and
stumbling when warm.

I was able to correct the problem with a very simple fix.

There is a ground wire that attaches to the top of the engine that had
corrosion on it, and this particular wire was "grounded" at the one of the
oil breather housing's mounting bolts - a rather foolish place for a
ground wire, considering the oil breather is made of plastic. This wire
may terminate in a different location on your van - I have seen them
mounted on the left top of engine case, below the air conditioner
compressor.

The wire in question is the brown wire that is included in a small sheath
that also carries the 3 wires that go to the Hall Sender, which is located
at the base of the distributor.

I solved the problem by cleaning that ground wires' terminal, and by
making an additional "jumper" ground wire that I terminated at an unused
wire connector at the base of the distributor.

The above "fix" took about 20 minutes and took care of the problem.

Good luck, and I hope you are able to begin your vacation trip without too
much delay.

Michael

My VW web page: http://www.velocity.net/~vickrail/moshevw.htm