I had Dexcool in a Peugeot many years ago and after 3 years or so, I noticed a lot of red crusty buildup at all the hose connections (after hose removal). Don't know how Dexcool reacts in W123's. Per our phone conversation earlier today, you said when you changed to Dexcool from Green coolant, a handful of black gunk came out. I would suggest draining the Dexcool (capture and see how much gunk comes out) and do a flush, then put in the proper coolant (Mercedes or Zerex G05 available from NAPA)

Don't put cold water into a hot engine!

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked