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KKK turns: On my car, the first 1-1.5 turns did nothing, then the next 1/2 turn was like an extra 2psi, went way up after that. Which will be a real problem for you since it's so hard to access the adjustment! I fiddled with mine about 4-5 times before I got it where I wanted.
603.96x (3.0L): These have weak original heads. Have a new head put on, the engine lasts about forever. The casting number is cast into the head, above #2 and #3 injectors. Something like 603-016-xx-20, or whatever - only the 7th and 8th digits matter. If "xx" is 15 or less, it is the bad/weak head. If "xx" is 17 or above (used from 1990-up on 3.5L engines), that's the good head. Newest version is 22. Theory is that the trap oxidizer caused a lot of the problem, combined with low tolerance for overheating (115C+). remove the trap, keep temps down, and even the old head may last a long time. Test a car before buying to make sure it doesn't have the first symptom - high pressure in the cooling system when stone cold.
603.97x (3.5L): These have great heads, but weak/flawed connecting rods. Either buy a car that has already had the new rods (or new engine); or buy one that has zero oil consumption & immediately replace the connecting rods (~$1500 in parts, lots of labor). With new rods and a good head, this engine will also last a long time.
The cars that have these OM603 engines are soooo much nicer than the older 617's, that most of us who own them feel it is well worth the possibility of spending a few thousand dollars to fix them should something really bad happen. Thankfully, the cost of rebuild 603 engines from Mercedes has dropped drastically in the last few years, from $9-$12k down to the $6k area, so I'm told.
Regards,
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