It appears as though the harness was not the only reason - yesterday, after a few days of non-usage I fired up the 140 and drove it for a good 10km. The rough idle problem returned and would not go away.

I left the car in the office car park. When I returned in the evening everything was back to normal. I decided to take the car in for further diagnostic testing. Oddly enough, during the entire journey to the MB garage (around 15km), with multiple stops at traffic lights, there was not even the slightest of hiccups.

I will inform of progress when the problem is FINALLY resolved...

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Mateusz
1986 126 500SEL
1986 126 300SE
1992 140 500SE
1996 124 E200T
1999 210 E320T
2017 205 C200T mit Schaltgetriebe!
2010 639 CDI 3.0
2008 204 C180 KOMPRESSOR