Wow JHZR2, thanks for the write-up. I was already in the camp of "oil technology from 30 years ago can't really be compared" before I read it, but it's interesting to see the reasoning.

Quick update; I did a full inspection of my valve work this morning, and except for correcting a gap on the second intake valve, everything was perfect and well-tightened. It's still idling rougher than after the first valve adjustment and seems to be a bit down on power, but I can at least say the engine is in good health.

VWNate1, I think you're right on the money with the clogged fuel filter diagnosis. That seems to fit with everything I'm getting right now - reduced power and slightly shaky idle. I also forgot to mention that on my last stop before the highway the other day, I filled up the tank with diesel. Maybe that contributed to the dislodging of some sludge.

I'm planning to replace that prefilter and see how she runs

EDIT: Just looked over the plastic overboost lines; thie insides toward the manifold are black with soot. Planning to clean the banjo bolts and lines; that should make a difference

No chance that could be significant at all . You probably just got some bad fuel or fuel laden with crud. Or maybe there was some water in it that got strained out by the primary filter.

Go get a gallon of fresh D#2 from a different station and plumb the fuel intake/return into it. Drive it around on that a bit and see if the problem goes away.

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'83 300DTurbo http://badges.fuelly.com/images/smallsig-us/318559.png

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Update #2: Finished cleaning the banjo bolt and lines from the ALDA to the intake manifold, and there was a ton of crud blocking it up! Now that it's all cleared out, the Benz runs really nice and smooth with a surprising amount of power. Sounds like I got my turbo back!

Huge turnaround from 6 days ago, and thanks to everyone who posted with your invaluable advice