One thing I've heard, but don't have direct experience with, is that the
advance or retard of the spark can make a big difference in emissions testing.

Apparently VW designed these engines to meet the smog requirements of the
time by specifying a retarded spark--10* ATDC. Most drivers seem to run
their vans at a 5* or 10* BTDC setting because it increases power, and
retard the spark to pass emissions.

I'm there are others who can give more information on this arcane subject.
At least it's cheap to try--no new parts.

Geza