BenzWorld Junior Member
Date registered: Oct 2011
Vehicle: 1981 300TD
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Here's a follow-up post, in the hope of saving others the same trouble that I went through. The problem turned out to be the thrust piece at the top end of the shaft that drives the oil pump and is itself driven by a gear from the fuel injector. The brass ring against which the shaft turns had worn from ~3mm protruding to ~1mm indented, which meant that the gear from the gears disengaged and the oil pump was no longer driven. Replacing it was very easy. Here are a few points to note:
1. The thrust piece is very easy to access, much easier than either the oil filter housing or the oil pump, so if I saw something similar again it would be the first place I'd look.
2. There are several different variants of the thrust piece so make sure you get the right one for your engine. Luckily I bought mine from proper MB specialists (MBSpares in Canberra), because the part number that the web ordering sites recommended was the wrong one.
3. With a new thrust piece the engine stopped making a strange knock just as it turned off. I had suspected that the noise was something in the suspension since it coincided with that final shudder before finally stopping. But the knock was apparently a symptom of the worn thrust piece, even though my oil pressure was still fine at the time.