I think if you do a leakdown test you're going to find a compression leak through that cylinder in the valves. If you did a teaspoon of oil in the cylinder and didn't see a drastic raise in compression, you're bottom end is fine. It's most likely a valve. I'd do that test to 100% verify, but I'm about 90% sure it's a problem with your valve. No reason to replace or teardown anything yet.

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Only diesels in this driveway.
2005 E320 CDI 243k Black/Black
2008 Chevy 3500HD Duramax 340k
2004 Chevy 2500HD Duramax 220k