The document in the diagnostic directory (I don't have the number at home) has a very good fix for a huge number of these problems. It involves disconnecting the wire from the ECU to the starter through the NS switch. By taking the end going to the ECU and grounding it the idle speed is increased. THIS ONLY WORKS ON 86-89 300e. The way it normally works as originally wired the idle increase is removed when put in gear.
This works for those models that the idle is unusually low, fairly common.
Bosch Master, ASE Master, L1
33 years MB technician