Oldest son’s 1995 E300D has exhibited some weirdness I’m trying to understand:

As those of you who know these cars, the fuel filter assembly has five lines attached to it:

1 - clear line from fuel heater (main supply)
1 - clear line to injection pump, engine side banjo fitting
2 - clear lines to side of injection pump (lift pump?)
1 - dark colored line (return?)

He has issues with it losing power to the point where the engine died on him. We’ve noticed the weep hole in the shutoff valve on the top of the fuel filter has been leaking fuel. I’ve got a new one on order with the dealer.

My question is based on observations I just made to the fuel system.

I’m seeing bubbles/air clearly moving from the filter assembly to the clear lines to the side of the injection pump and the dark colored return line. The return line I’m not concerned about, but the other ones I am. The supply from the fuel heater and to the injection pump have no air in them, and the engine seems to be happy. I’m just wondering why there would be air in these other lines, and where it might be coming from?

If the shutoff valve is leaking (it is) despite fuel coming out of the weep hole there wouldn't be any air getting in, I would think. O rings on the center bolt are new and in good condition. The air seems to be from the filter assembly to the lift pump and back and the return. I'm wondering if the fittings at the lift pump are leaking? Are there O rings at these fittings, and how are they disassembled? Does the black metal or plastic clip just come off the related the hose/fitting?

The plumbing on this thing is hardly intuitive.

The car is running, but it's acted up several times in that it's died and been difficult to restart as if there is air in the fuel system.

Thanks,

Dan