The 1985 was one of the best of the crop. If you keep them maintained they will last a literal lifetime! (617 engine)
As for the later 6 cyls (603 engine), they were reliable as long as they never got too warm and the oil pump drive did not fail!! If the engine ever overheats the original head will crack! This was a big problem because the radiator necks loved to break off and loose all of the coolant at speed and the engine would overheat befor the gauge showed it! I have seen owners put a bb in the line that goes to the EGR to stop the formation of carbon in that area. (It was caused by turbo oil seepage from the front seal going by the EGR gases and harding.) I DO NOT ENDORSE THIS BUT I HAVE SEEN IT WORK BECAUSE IT CHANGES THE EMMISSIONS OF THE ENGINE AND THAT WOULD BE ILLEGAL FOR ME TO DO!
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM