You can inspect your sparkplugs to identify a rich cylinder.

If your mileage is pretty high, probably best to replace all injectors. It's not too difficult on M103, and injectors are only $36 ea. You need new seals and probably plastic mounts, too.

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