Condition of the opposing engine mount will have negligible effect on the one you replaced. As mentioned earlier, check the tranny mount to see if it is causing the motor mount to be off-axis. If this were the case, though, usually the other engine mount will also fail. It could just be a case of bad quality parts. Yes, even OEM are subject to that plague, which is why I tend to buy aftermarket. My aftermarket mount on my 300E is ready for replacement again, but it lasted me about 35K. This will soon be resolved with another $30 on eBay and 30 minutes of my time.

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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