My car has a slight idle knock when cold after the glow plugs stop glowing (about a minute or two after a cold start) and I believe that's pretty common and I've seen that on other OM606 cars. What concerns me more is some major nailing that I have if I accelerate past 3000 RPM and ease on the throttle when the engine is still cold. Does anyone else get that? Once it warms up it's quiet at all RPM's. I suspect there may be some carbon buildup in the prechambers or I have low compression in some cylinders, but the engine certainly doesn't behave like a worn-out one. It's smooth and has plenty of power (for a NA OM606 that is). I have checked all my injectors for spray pattern, I have no fuel leaks, have tried many injector cleaners, and all other maintenance is up to date.

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2004 VW Jetta TDI (manual)

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