Benzfan is right on. The last time I was plagued with an erratic idle speed I unplugged all the sensors in the fuel and ignition system to include removing the two computers. All the individual components in fact checked out okay. I cleaned the connections, cleaned the item Benzfan mentioned and put everything back together and it ran for the next 8 months before needing idle speed servicing again.
MB has two diagnostic manuals for this engine. the "Mercedes-Benz C.I.S.-E Engine 102.961 & 102.985 Diagnosis & Adjustment 1984 -'86" (part no. 102 584 0110)which is for the 190 E 2.3 and the "Service manual Supplement Engine 102.983" (part no. S-2347-SUP) for the 2.3-16 and items that differ from the 2.3.