Mufflers are almost always replaced - it is not recommended to flush them since you cannot get all residue or particles out of the baffles. I recall reading that somewhere not too long ago. Actually I read it in 3 or 4 different places (one of them most likely here) so I take it to be a pretty good rule of thumb.

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280