Have you checked carefully for any leaks? My wife's C280 had a tiny leak, so small it created a mist that was barely visible, in the hose going from the filter to the feed line. I had issues with the sender for a couple of weeks before finding the leak and the issues disappeared, perhaps because of what mbdoc mentioned?

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1983 380SL
1995 C280
1995 S500 coupe
1990 Mazda Miata
2004 Suzuki Hayabusa