On day 2 of a road trip with my newly purchased 09 550 cabrio. Yesterday after 1 hour of driving I noticed that the fuel gauge read empty. Checked range and it read ---. Got off a pumped 5 gallons and pump shut off. Gauge came back on and read full. No other problem for rest of the day. On the road today, wife currently driving and I watched the gauge slowly sink to empty, range ---- after 2 hours driving on full tank. Stopped for 10 minutes, restarted car and gauge back to 3/4 full. Any suggestions?

 

Seems like I recall others having this issue. You might try some searches in the forum. Absent a clear answer based on others' experience, my first suspicion would be a faulty sending unit in the gas tank. It is accessed through a hole under the rear seat.

 

I had the same issue on our trip back from Florida last week. I also had periodic warnings to check tire pressures. I know how sensitive Mercedes are to at least 12.1 volts from the battery and thought perhaps I should have the battery checked.