Its possible its leaking into the exhaust. If you didnt top it off you would eventually have a problem yes? Same symptoms on my 740i, we kept adding fluid until it let go. Then is misfired.

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1981 300TD
2005 C55
2004 E500
1999 ML430
1992 400E
1998 C43
2001 SL500
1988 300SEL
1986 190E 16V
1991 190E 2.6