For what it's worth, I was playing parts swap trying to pin down an ABS issue on the 87 (solved!). To get to the ABS module, the ECU had to be removed/installed. When all was done and car restarted, it idled low, would bog upon giving it throttle, but clear up and run at high RPM. I had installed the electrical connector on the ECU but not all the way (some pins connected, some not). Another 1/4" and latched down and all was well.

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Hanno
'79 6.9 Sold (after 27 years)
'83 280SL, 5 spd.
'94 E320 Sdn. 5 spd conversion
'02 E320 Sdn.(on loan to mom!)
'87 300E (5 spd. conversion) Sold
'05 E500 Wagon