I have an update...
So, I just completed my road trip from NH to Moab, UT (and back)... For the trip out to Utah and back I was using my WK to tow my XJ. I had the o-rings in the Wk's fuel system fail on me a few times and leak during the trip and now I can explain what it is -- I had to stop on the side of the road and fix it 3 times!
The problem is that the Mercedes engineers who designed the fuel system in the 3.0L CRD cheaped out when they designed the fuel injector fuel return system. There are 6 metal stems which come out of the top of the engine, adjacent to the fuel injectors (one per injector). At the tip of each stem is a flange and an o-ring. The excess fuel that wasn't pumped through the injector comes up these tubes then through a T-fitting (plastic) and into some 1/8" ID tubing to return the excess fuel. All these fittings are linked together in a daisy chain on each side of the engine, so if one goes the whole system loses pressure.
The problem is the stupid plastic T-fittings are retained with a collar you push down (it "locks" in to place) to squeeze the plastic fitting against the flange at the end of the tube, which is presumably sealed by the o-ring... Somehow these stupid fittings come loose on my engine and result in leaking fuel. When they come off the engine completely loses fuel pressure.
This whole mess is located under the two black foam pads on the top of the engine under the engine cover, so it isn't too hard to access (couple of 10mm bolts to remove the cover, one to release the air intake, and loosten the intake clamp where it goes into the turbo).
The first day of my trip I pulled onto the highway after 1 hr of driving and a brief stop and one of these clips let go causing a fuel leak and no pressure. I was able to clip the one which came off back together and discovered a 2nd clip was loose. I checked the other 4 and they seemed to be tight. I also disabled my PowerBox CR+ (set it to 0 -- stock mode).
The 2nd day of my trip I got a check-engine code and get stuck in limp-mode... I cleared the code w/ my trusty code reader, but checked the fuel returns because the code indicated "low fuel pressure" -- no leaks, but instead I discovered an exhaust leak around 2 of the fuel injectors, which had come loose. After tightening up the fuel injector retainer bolts (one per injector) it seemed the engine was OK -- no more exhaust leaks. No other problems were had on the way out, but the mileage sucked (10-13 mpg, though I was towing 7200 lbs... On the way back it was 2-3 mpg better).
Fast forward to the first day of my drive back, 6 days after I arrived in Moab... The WK had sat for the week and only been used 1 day immediately before we left to return home (no towing - just driving around town). While I had the engine idling in the parking lot of the hotel I smelled diesel... Looked underneath and saw a puddle of diesel... Pulled the covers off and sure enough -- another of these stupid clips had somehow come loose and started leaking, different from the first 2. I pulled the clip on the leaking injector return, reseated / re-clipped it and the leak stopped.
I'd give a lot to meet the guy who designed this rinky-dink POS fuel system...
Over the summer I had done some medium duty towing with the WK -- pulled an 18' Searay ski boat -- after the original incident. Hadn't noticed any fuel leaks or smelled diesel. Hadn't had any problem since the first incident, which also occurred when towing my XJ.
Only good news is that other than these issues the WK did an awesome job as a tow vehicle for my XJ + 24' car trailer.