How many miles constitute the actual life of a Mercedes E350 ?
I need to know when and at what mileage it should be considered that the E350 engine is dead or must be tuned up to continue performance.
No set mileage. Failure or wear of the engine parts can be sooner or later depending on luck, oil used and frequency of change, how it is driven, etc. Usually around 100K you will be swapping out the plugs.
It all depends on how it was maintained. 350000 miles or more if taken care of. Synthetic oil is key.
The transmission is the weak link. Change the fluid ONLY with Mercedes recommend transmission fluid.
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