Small update. I have been putting 1/2 quart MMO into every other tank of gas for the last 2 months. The fuel gauge problem is almost completely resolved. I think MMO slowly cleans the gunk off the sender parts without having to disassemble and risk damage. It took 3 treatments to start seeing a change. I've used this stuff in rotary engines, which are somewhat delicate, so I have no qualms using it in a Benz. This may or may not work for your problem, but it can't hurt to try.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma