NZScott, I realize this is a late response on this thread, but I have one of the IP's where Oil has to be manually added. I've had a lot of loud clatter when first starting up my 72 220D and recently drained the oil from the IP (finding it was pretty much just diesel) and refilled with oil. Wow, suddenly my car sounded like new.... after a couple quick drives it is clattering again. I'm assuming at this point (as I'm no IP expert) there are some worn out seals in the IP which are letting the oil and diesel just mix right together. Does this sound logical? Any idea on repair options?

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1972 220D
1981 300TD
1982 300D
2004 Hummer H2