Diesel emissions have been discussed before. Check out this thread for example:
http://www.peachparts.com/shopforum/showthread.php?t=75194&highlight=emissions
I think this test is a joke for the most part. My 87 300D which I used to own tested at 0.0% as well, but that's because minutes before the test I did an italian tuneup on the highway, leaving behind a thick black cloud! Also I've seen how this test is done and it's quite subjective as it depends on how much the operator floors it on the dynometer. An operator with an aggressive foot will mash the pedal to the floor instantly, resulting in a higher maximum opacity reading. Others will accelerate more gradually. Anyway in my state the limit is 40%, I don't see how anyone can fail this test and still have a driveable vehicle.