Sounds like you have air in the fuel. Compression, hard to start, lack of power etc. if I don't have leaking rings at 500k, I doubt you have a compression issue..

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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