I'm not sure where the 'black exhaust' really comes from, but I had a
Rabbit Diesel for years that almost NEVER emitted black smoke.  I think
on an occasional extremely cold morning, it may have puffed a very small
puff at start up, but that was it (I'm talking maybe 4 times in the 10
years that I owned it).  Visibly, anyway, it was extremely clean
running.

IMHO, diesels are very environmentally friendly, but then I'm not an
environmentalist, and definately worth having.  My little Rabbit
consistently got better than 50 mi/gal on the highway (about 55
actually), and 45 m/g in the city.  I could drive from Baton Rouge to
Houston (250 mi each way) and back on one tank and have enough fuel to
runaround while I was in Houston.  I sold it (against my will) five
years ago.  It had 180K on it and NEVER had engine work.  I constantly
had to get new alternator belts (primarily because I couldn't find a
mechanic that could put one on it--they pulled out the manual EVERY
time!), a couple of batteries (primarily due to the alternator belt
problem), had the CV joints redone once, and had gotten a new water
pump.  That was IT!

When I sold it, it would have needed clutch work done soon (say by
200K).  I have never owned a more dependable vehicle.  If I could find
the person that owns it now, I'd buy it back!

Now it wasn't exactly pushing a van around, but it had wonderful
acceleration and power, although it didn't care for steep bridges. 
There was a trick though.  As long as I was going at least 55 mph when I
got to a bridge, I could maintain speed, anything under 55 and I might
as well have been an 18-wheeler because I was slowing down.

Tonya
87 Vanagon GL Wolfs
74 S. Beetle