Fuel not being used from one saddle.
 

Hello, the pump is not pumping fuel from the drivers side saddle. Please help. What could it be? Thanks.

bad transfer aka Venturi pump.
When the problem occurred? Usually it is mechanic who makes wrong assemble.

Well, it has always had a problem with the fuel level indicator. It never goes to empty. Recently, I had a mechanic check the fuel system and we found that the drivers side saddle was full and the other was almost empty, realizing thats why the fuel level indicator would never go to empty. A venturi pump, say more please. Is this something I need to change or move to correct place?

The saddle tanks use return fuel with Venturi pump to suck the fuel from passenger side saddle to driver side, where main fuel pump pick it up and sends to engine.
The "snorkel" you see on the picture is just it and if assembled at wrong angle, will not suck the fuel, leaving about 5 gallons of fuel that main pump will never get and engine will die.
Dual level sensor will show average, so you will have fuel shown on the dashboard, but engine will starve.
https://ae01.alicdn.com/kf/HTB18gtrR...pg_640x640.jpg

https://www.meeknet.co.uk/e38/Sender/Tank_diagram.jpg

I have a similar problem on a E220 diesel 2014(RHD). The sender side runs out of fuel while the pump side is full, and car stops. Both the pump and sender were replaced and the same happened again. At the moment I am keeping the tank full to work around the problem. Apart from design issue, the fuel calculation done by software and indicated to driver is wrong. There are two fuel gauges in either side of the tank, and the computer takes the average of two gauges and displays on the dashboard. So when the sender side is empty and pump side is full the reading is approx half and car halts.

At the moment neither the mechanic, nor MB have any convincing explanations. Software is up to date, and no faults indication and car drives fine with full tank.

Venturi fuel pumps.
 

before changing pumps lines and floats.
Change the metal fuel filter at the engine. Mine was bad even though the car would start and run and go highway speeds. Once I changed it the problem of running out of fuel while still marking a 1/4 tank stopped.
I then sawed the fuel filter in half with a reciprocating saw and it was more then bad .even though it idled and drove fine.Mine is E320 CDI diesel .But gasoline cars have these issues also