Lots of guessing here. I especially like guys that think new filters are the cure for everything.

Ours recently exhibited a significant "knock" at idle and some nailing coming off idle.

The dealership tech immediately suspected fuel injector and there is an electronic test (with printout) for fuel injector performance.

We got a new #2 injector ($750) and all is now well.

Understand that the injector test indicates a range of diagnositics and an injector could be approaching but not at the cutoff point where replacement is required--that is, covered by warranty.

Injectors on these engines are not the simple calibrated holes of earlier days, but rather are sophisticated, complicated electronic devices not necessarily going to last forever. Our #2 made it 38K miles.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles