Redline injector cleaners are very good (pep boys sells). BMW sells it in repackaged bottles. I use one bottle a year in each of my cars. Last time I used it in the Benz, it fixed a problem with my fuel gauge (the very common 300E problem where once tank is less than half full the gauge bounces around, cause by build up on sensor in tank).

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2005 Mercedes C230k sport sedan, 6 speed
1987 Porsche 924S - 968-engined track car