My favorite way of judging blowby is to loosen the oil filler cap and watch it for movement. If there's no movement (make sure the cap isn't sticking) then that's excellent. If there is, try raising the RPM to eliminate engine vibration as a factor. A little movement is ok, but if the cap flies off like a bat, it's time to run!

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2004 VW Jetta TDI (manual)

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