Your tensioner may be shot. Do a search here, somebody took a failed one apart and determined that the poor design is prone to having the rubber material lose shape and distort early.

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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