Going thru quite similar with my 89 300E.
Read a volume of material on the issue, about War and Peace for volume.

The high idle seems to be locked into the IAC. I replaced my IAC with a
gate valve which seems to have eliminated my surging idle but left me with
an idle of about 1K.

Not new to working on cars as I go back to '51 Packard straight 8. Have to say this CIS MB is one of the worst pieces of sheeeet I have ever worked on.

On the positive, once I get this POS running correctly again, its is the last MB I will be working on. Done, done done !!

__________________
1985 300D 198K sold
1982 300D 202K
1989 300E 125K
1992 940T

"If you dont have time to do it safely, you dont have time to do it"

"The democracy will cease to exist when you take away from those who are willing to work and give to those who would not."