Originally Posted by S-Class Guru

There is indeed a neat way to increase the idle on the 103 engine.
It's sometimes referred to as the "purple wire" trick. I did mine, temporarily, about 10 years ago, so I can't remember the details, but a search using that term should turn it up, or look under some of my old posts. Sorry, Old-age syndrome.
By the way, it turned out the rough idle at 600-700 was the infamous right motor mount failing for the tenth time. I didn't suspect that because the mount at the time was only a few months old - but the aftermarket parts on the right side under the exhaust can collapse in only a couple weeks. iI'd check that first.

DG