Carl,

Have you checked the oxygen sensor for function and proper ground?  Have
you checked the integrity of the engine bay ground points/straps?  Have
you gone through the electrical checks of the fuel injection/ignition
systems as outlined in the Bentley manual?

Going through the above allowed me to diagnose a bad (severed) ground on
the oxygen sensor on my 87 Syncro.  A new wire terminal and ground strap
from left head to body, along with some judicious cleaning of attachment
points, cured a major driveability problem.  Go through the Bentley
tests, if you haven't already, and find out what is operating within
spec, and what isn't, and go from there.  Try and do the above while the
van is farting if you can.

Best wishes,

Tony

Anthony L. Mourkas
Hampden, Maine, USA
1984 Vanagon Westfalia
1987 Vanagon GL Syncro
1991 Jetta Eco Diesel