For a couple of months now, I have been trying to troubleshoot an 87 syncro
that persists in running rich.  It always starts right up, but will die if
left idling as the mixture gets richer.   It can be driven (sort of) but
usually dies at stop lights (always restarts) and has little power-hard to
get above 50 mph or so.  I've spent many hours on it, so has my local
independent vw mechanic (he's not bad, but doesn't see many late-model
vanagons).  I have a good-running 87 van to swap parts from, but I STILL
can't diagnose this problem.  When I bought it it wasn't running
particularly well, but I was confident that I could fix it.  At the time it
had the cat converter replaced with a straight pipe, no oxygen sensor.  It
also had a bad afm.  I installed a converter and oxygen sensor and a good
afm ( works fine if I swap it into another van) , but it still runs poorly.
  Here are the other things I've done:

Checked fuel pressure (30 psi), swapped fpr to be sure.
All voltmeter/ohmeter checks at the ecu connector per Bentley OK.
Cleaned ecu connections, checked and cleaned all grounds, replaced ground
strap from left head.
New plugs, wires, cap, rotor, installed afm harness upgrade.
Replaced temp sensor II- checks out OK with ohmeter.
Swapped from good-running van: ecu, afm, coil, idle control valve.
Emptied contents of converter to make sure it wasn't clogged.
None of the above new parts or swaps had any effect on performance.
A voltmeter on the oxygen sensor shows predominantly 0.8-0.9 v. with the
usual fluctuations, so the sensor is definitely trying to tell the ecu that
it is too rich.  Disconnecting the oxygen sensor has no effect on the way
it runs that I can discern.  I'm starting to be suspicious of the main
wiring harness but replacing that on the syncro will be a major
job-probably will require dropping the gas tank and tranny.

The engine is not in great health, but I don't think it's bad enough to
blame for the poor performance.  Compression is 135-135-90-120.  Head
gaskets had apparently been replaced (but not heads) by a real schlock
mechanic-when I got it the ground from ecu to engine was disconnected, and
the protective covers over the pushrod tubes were missing.

I've gotten lots of advice from people on the list already, special thanks
to Darrell Boehler and Kyle Wade, but I'm still stuck.  Any ideas are very
much appreciated.

Bill