I will swear by BG 44K. It fixed the knocking caused by a fouled injector.

My mechanic and Mercedes recommends doing it every oil change, so i do. It's $15 and hard to find, but the dealer sells it. I add the whole bottle to a full tank of 47 cetane.

cheers
eric

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1995 E320 Sportline Wagon "Bernard"
black on black leather

http://i.imgur.com/BdZ7jM3.png