It's probably a clogged injector. WVO has higher cetane and in your case it burns sufficiently well even with a bad spray pattern that you don't hear the knock. Try running some diesel purge. I've read the VO can coke injectors after some time.

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2004 VW Jetta TDI (manual)

Past MB's: '96 E300D, '83 240D, '82 300D, '87 300D, '87 420SEL