MB used high tech engineering on the SDL's in the mid 80's. They run the exhaust straight out the back, not pointed down. Thus, there is no soot spot on the driveway - - -it collects on the rear bumper! I guess that MB figured that it is easier to clean a bumper than clean soot off a driveway. :D :D

Don't let it idle in the drive way for 2 minutes... problem solved!

Post glow relay could help. Clean airfilter, proper camtiming/IP timing and yes even valve seals. But if everything else seems sound- live with it until there is some other appreciable reason for spending $$.
The post glow kit is really nice in my car, but some are suggesting shorter gp life.

Michael