I had another post here but it was misleading in what it solved so I did not want to send anybody down the wrong path.

I too have a 1985 380se with an idle problem. When it has warmed up thoroughly, this is when it has the issue. It is clearly noticeable in Park or in Neutral with no load. In drive, sometimes it is there. Add AC to drive and I have never felt it. So no load is biggest problem, and the more load you add, the better it gets.

I bought a CIS fuel pressure testing kit. The warm-up compensator did not pass the cold pressure test. I replaced it, the new one is in spec. exactly, that was nice to see, but... all this did exactly nothing for my idle problem.