1995 E300D W124.131 Fuel System - Air
 

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

Replace the shut off valve o rings ASAP, matter of fact replace them ALL now.

If you want to replace the lift pump o rings, you might as well rebuild it.

Shutoff valve O rings have been replaced, but it's leaking out of the weep hole below the knob.

I've ordered a complete set of lines and O rings for everything between the filter and pump, as well as a new shutoff valve. Amazingly inexpensive from the dealer, maybe $50 total for everything my cost (I've got a wholesale account, but it's still cheap anyway.)

What's involved in rebuilding the lift pump?

Thanks!

Dan

The 95 E300 has no shutoff valve, it has a vacuum pod to pull the rack to zero fuel just like the old 603 or 617

Remove the primary fuel filter and check for debris in it, It also has an O ring (this filter is buried in the filter stand itself, the line leading from the heater to the top of the stand is actually the primary filter inlet, remove that and you can pull it out. Clean it and put it back with a new greased O ring.

Replace all line O rings one by one (start the engine in between every line change so you dont have to fight a system full of air)

Finally make sure the strainer in the tank is clean - it can seriously hinder fuel flow upto the point of losing half engine power. When you remove it, carefully scrape the remainder of the crud with a bent wire in the same hole to remove maximum amount of debris.

Your lift pump is trying to suck fuel through a very very long and very very skinny straw. What you're seeing is not air bubbles, it's vapor being pulled out of the fuel under deep vacuum. We've gone over this issue on the forum, the fuel system is flawed on the 606. Either put a larger-diameter fuel line between the tank and the pump or add an electric fuel pump just below the fuel tank to push fuel.

Anyone done this, in the engine bay? Would the results be similar or compromised to the point of no effect? I would like to do this, but in the snow belt here, just thinking it would be a safer location under the bonnet. TIA.