MB W211 220CDI Fuel return issues

[wevv]

I have a MB W211 E220 CDI 2004 and recently I had a problem with the lift pump in the sending unit that stopped working.

I replaced for a new pump and everything was working correctly.

For some time now, I noticed that the dashboard said there was 1/4 fuel, but the car don't start.
I checked that the left side of the tank had fuel, but the right side (where is the lift pump) hadn't.

I put more fuel and the car starts well, but after a few minutes, the engine stopped.
The right side of the fuel tank was dry again and left side had even more fuel.

I think there is a problem with the fuel return. Does anyone know what is a possible cause for this?

Best regards.

 

[Steve@Avantgarde]

2 things.

Check that the arrows on the sender units line up with the markings on the tank, you will need a torch to see this. If you dont line them up the senders get stuck on the side of the tank and register 1/4 full on your gauge.

Seen that a few times.

or

Possibly a valve jammed in the high pressure pump causing the venturi effect to stop on the leak off system.

 

[wevv]

Thanks for your advices Steve@Avantgarde.

I just confirm that arrows on the sender units are line up with markings on the tank.
But the main problem is why the right side of the tank is empty.

You said it possible that a valve is jammed in the high pressure pump. Can you tell me which valve is?

 

[Steve@Avantgarde]

The right hand side is the supply side to the engine, the left side is the return. Fuel cycles from one side to the other using a venturi effect. The valve i refer to is inside the high pressure pump itself, you either replace it or have it overhauled by a diesel specialist.

 

[wevv]

After replace the valve, like Steve@Avantgarde, the engine response was much better, but the problem still persists.

I performed some tests to eliminate possible causes. I connected directly with a tube (in the fuel tank), the fuel supply pipe to the return pipe. I noticed that everything worked perfectly and that there was some fuel pressure in return pipe (above the lift pump) that brings the fuel from left side to rigth side, in this case.

After, I connected the tubes as they were previously and start the car, when I double-check the return pipe that is above the pump, the fuel that arrived was very little and sometimes none.
The problem happens even with the key turned on.

Is there some problem in the engine section to induce this drop in pressure?

PS: I checked all the pipes to see if they were obstructed.