HELP!

I have a 1991 Mercedes 420 SEL (8 cyl. engine) 98,000 miles. Normally at idle you cannot tell whether the engine is running, it's SOO smooth. However, recently the idle became slightly rough. The whole car rattles, etc. Engine performance is excellent in acceleration, etc. however. Doesn't "feel" like it's missing or anything.

WHAT I've TRIED DOING SO FAR and NO SUCCESS: replacing the spark plugs (had 30 k miles on them); exterior cleaning of the injectors and replacing the injector seals (o-rings and sleeves are fine); checking for a vacuum leak (can't find any) ------

ANY IDEAS I haven't tried yet??

Would appreciate any help you could offer. THANKS!!!!

Bob Irving
Charlotte, NC