Prevent porting of vacuum to the EGR to prevent it from ingesting exhaust gases at partial throttle - a common "fix" for diesels.
While this is likely to help some diesels, it probably does little for a gas engine car ... as you can tell, it is obviously not the problem with your SL.
1991 350 SDL (200K and she ain't bent, yet)
former 2002 E320 4Matic Wagon - good car
former 1985 300 CD - great car
former 1981 300 TD - good car
former 1972 280 SEL - not so good car
a couple of those diesel Rabbits ...40-45 mpg