After much thinking, I decided to trust my diagnosis (I'm not a mechanic, I just play one on the internet) and decided that the fault was most likely in the turbo boost solenoid (which Mercedes Benz has several names for), and I decided since we were in Adelaide to stop in at the wreckers there and see if they had one. They had one for $100, a far cry from the brand new price (a whopping $425 I was quoted from MB), and I decided it was worth the gamble.
Immediately after plugging the new unit in, I had vacuum the other side of it and I knew I'd have a turbo again. Hooked it all up, put the headlight back in (MB documentation suggests removing the air cleaner to access the boost solenoid stuff, I think the headlight is far easier, even with the pain in the arse of angling the thing past the bull bar) and off we went, complete with turbo boost.
On the way home from Adelaide, we stopped at Tailem Bend for the night because I simply wasn't going to drive all the way home (~500KM) at about 7 o'clock. We found a nice little spot in the center of town near the toilet block and went to bed - it would have been perfect if not for the trains going by, but what are you gonna do.
The next morning as we took off, and noticed things in the cab smelt a bit "diesely" when the vent was open. Not overly bad, but there was a definite odor. At this point I kind of had my suspicions, as I know what a diesel smell inside the cab or engine bay typically means for a Mercedes 5-banger. When I inspected the thing shortly after buying it (and researching, probably should have done that beforehand) I noticed the hallmarks of "the black death", but it's been running rather good so I figured it was a historic problem.
We get home, I again remove the injector cover, and it's plain as the light of day - a very definite spray of fumes from one of the injectors twice per cycle. Fuck.
I considered doing the job myself, but breaking one of the injector hold down bolts off in the head just scares the shit out of me. I even actually ordered the seals and hold-down bolts from MB at about 4PM on a Thursday, because apparently Toll would have them to me on Friday - except the guy at MB stuffed up the address and Toll returned it, so the weekend was a wash.
In to the local diesel mechanic in Maryborough on Monday, who gave me a quote I could live with so we left the van with him. He removed all the injector bolts, replaced the seals, cleaned the threads and put it all back together with the new bits I'd ordered from MB.
He noted that it very obviously had been done before - they hadn't cleaned all the "black death" off the valve cover, but it's not the sort of thing that goes away on it's own. He also noted that when cleaning the threads that there was far more shit in there than there's supposed to be. Given that the injector hold-down bolts are a one-time tighten affair with very specific torque specifications, that's immensely bad and probably the reason for the failure this time.
The image above left is the culprit seal, you can clearly see where it burned out. Hopefully the guy's done a good job and it won't go again for a while.