Fuel gauge drops to 3/4 after driving 10-20 miles from a full tank
 

2004 ML350:

1. Fill up the tank to the max.
2. The fuel gauge shows it full.
3. Drive around 15-20 miles.
4. The fuel gauge shows 3/4 full.
5. Once it drops to 3/4, the gauge works normally.

What should I check?
I put key to ignition position II and removed & plugged back in Fuse 5. But I think this works when the fuel gauge somehow drops to 0 when the tank is full, which is a diff problem.

https://cimg8.ibsrv.net/gimg/www.mbw...225f31c438.jpg
https://cimg5.ibsrv.net/gimg/www.mbw...b92becfe89.jpg
This problem usually occurs because of a faulty float which is part of the fuel pump.

Wait until the tank is 1/4 full and remove the fuel pump. Clean the grid with a pencil eraser and reinstall the pump and hope for the best.

Ordering a new locking ring prior to the job is necessary.

Agree, I had the exact same thing on my 1999 ML430 and fixed it by replacing fuel pump (which has fuel level float integrated into it).

Thank you so much for the info!
I know the opening is under the rear passenger seat.

Does it require a special tool to open up the entry area?

I don't recall needing anything special for it. I did reseal it with some caulk to make sure no fumes came out (it had been sealed before). In my model they changed the pump, added a whole additional hose, changed the filter,... so it was much more of a project than just swapping the pump. Hopefully you just have to replace the pump which is easy.

I do recommend running the gas tank fairly low and having a fire extinguisher nearby just to be safe. Lots of gas fumes and the pump is obviously covered in gas when it comes out. I had no problem doing it safely myself but be careful and take your time.

Here is the procedure.