I suffered the exact symtoms on my 300SL. Then is was the EGR-vent that was leaking. I could hear the leak when listening to the engine. Now I'm no expert so I don't even know if there is a EGR-vent on the 500 (even though I own one).

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Emil Österman, Stockholm - Sweden

MB SL500 -93 - Silver
MB 230CE -84 - Silver
MB 230E -82 - Yellow
MB 180B -59 - Green