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About the 3.0 liter six cylinder diesel engine in the later W126 cars - I know that most here on this site are concerned about cracked heads and overheating on this engine, but I've discussed this with the legendary Enrique at Mr. MB Motors in Southern California, and he swears that the newer engine is a better engine than the five cylinder, and doesn't have a high failure rate of cracked heads and overheating.
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Paul, I agree with Enrique. The OM603 is a much better design in MANY ways over the OM617. Much more refined, quieter, less maintenance, more power, better MPG. But it just won't tolerate abuse and/or overheating like the OM61x, which was almost impossible to damage! Anyway, the 1986/87 six-cylinder 3.0L engines (OM603.96x) all came from the factory with cylinder heads that were weak - not strong enough in certain areas to withstand temporary hot running conditions. They will eventually crack. MB fixed this on later 3.0L engines (not shipped to the USA), as noted in the service manual, by making the castings thicker in some places. But all these USA 3.0L engines (86/87) ARE prone to cracking! I personally know of a good half-dozen or more, mine included. The fact that there were FIVE redesigns of this head (six part or casting numbers), and also FIVE redesigns of the head gasket (again, six total part numbers), shows that MB realized there was a problem with the original design back in 1986 and worked to perfect it as years went by.
The 1990-95 six-cylinder 3.5L engines (OM603.97x) all have a stronger version of the head, and these very rarely have head or gasket problems. However as you probably know, *all* those 3.5L engines had weak connecting rods, which oval out the cylinders, requiring a MUCH more expensive fix - which is NOT a DIY project like the head. Luckily, the 3.5L engines provide good used heads for the 3.0L! :p They're scarce and not cheap ($800-$1200 typically, used, with cam & lifters if you're lucky).
Now, the OM606 engine used from 1995-up in the USA does seem to be a much better engine, with no head or bottom end problems that I'm aware of. Perhaps that is what Enrique was referring to!?
On a side note, I have heard of very, VERY few OM61x heads cracking, or gaskets failing. Check this forum and you'll find lots of threads on OM603's cracking, but few if any 616, 617, 601, 602, or 606.
Regards,