Pardon my ignorance, but how does one accomplish this?

I don't have any external fuel leaks. If there's a leak, it has to be small enough to allow air in, but not fuel out. And that's what I can't get my head around. It's all pressure after the lift pump... Why would anything *want* to get in under a pressurized environment?

The other thing is, I realize it's not a perfect seal, but I thought the OFV was supposed to prevent leak back?? And unless I'm missing something, this should be a liquid pressurized circuit. IF the fuel is leaking back, there's either suction or pressure defeating some sort of valve. Hence my original theory of internal leak.

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1981 240D 4sp manual. Ivory White.

So if you break the line, all that air instantly flows out? If so, that sounds like your fuel tank has a lot of pressure in it or you're working against a clog of some variety in the return line. You might try running the engine with the fuel cap off, then let it sit overnight with the cap still off and see if you have any change in behavior. The breather valve on the tank vent should allow no vacuum, but maintain ~1PSI of positive pressure. If the valve is wonky, it's possible you're building more than that in the tank. Do you get a puff of air when you open the filler cap?

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I have had this thought, but no hissing when I remove the cap.
Regardless, I'll try this tonight.

I wasn't clear before. When I break the line, the air doesn't flow out, The level of fuel in the clear return line rises. It must be falling somewhere else to compensate. If I was releasing air by breaking the line, the fuel level in the clear return line should drop even further. I think what I released was a small amount of vacuum, but I can't understand how that's supposed to work.

__________________
1981 240D 4sp manual. Ivory White.

The return line, the banjo fitting on the rear of the IP, the small rubber return lines from the injectors, and the secondary fuel filter canister should all be at the same pressure as the fuel tank when the engine stops running. They are all tied together (the secondary fuel filter canister through that tiny bleed port in the bolt casting) and run straight to the fuel tank. There is no valve in there and no way to create any sort of vacuum or pressure in the line unless there is a restriction/blockage in the line or vacuum built up in the tank.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

So the excess pressure from operation bleeds back into the tank... makes sense.
So why am I getting drain back? I hit the primer a few times and it pushes the level up, I come back an hour later and it's dropped again. I just can't figure out where that fuel is going. If it's simply moving (and not getting lost somewhere), then the entire fuel column is moving down through the pump, through the spin filter and back all the way through the lift pump valves and onto the tank. My understanding was that the lift pump was supposed to be one way, as was the OFV.

__________________
1981 240D 4sp manual. Ivory White.

The lift pump and the OFV are one-way devices. The return lines are all connected together and open to the tank. If your OFV is any good, it is impossible for fuel to drain back into the IP.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Alright, now we're getting somewhere.

So looking at the photos I posted, that situation, at the very least, is indicative of a faulty OFV. Am I right?

I just figured a spring and a bearing could never be totally water(fuel) tight. I replaced that spring with one from Greazzer. I even replaced the bearing.
Assuming there's an issue there, there shouldn't be any dead space in the pump for that fuel to go to. In fact, there shouldn't be any dead space anywhere in the circuit. Which brings me to the next set of one way valves. My lift pump.

I rebuilt that damn thing... replaced both valves. In order for the column of fuel to move as it is, I'd have to be getting a very small leak down in both of these points (OFV and LIFT PUMP). ?


PS* thanks for hanging in there. I need to solve this, its driving me crazy.

__________________
1981 240D 4sp manual. Ivory White.

You could have air leaks at the fuel filter head end and the IP end of the clear hard return line.

The pressure relief valve (OFV) has a small sealing washer under the cap nut that holds the spring and ball bearing in place.

Try renewing the 4 crush washers for the 2 banjo fittings and the additional crush washer for the cap nut.
Alternatively, first try to get a really tight crush by simply re-tightening the cap nut of the overflow valve, the overflow valve, and the hollow bolt at the fuel filter head.

__________________
78 W116 300SD 'Desert Rose' new as of 01/26/2014
79 W116 300SD 'Stormcloud' RIP 04/11/2022

Real problem,nailing and engine miss. Could air in fuel return lines cause this?
How would one find out? Try pumping up system with primer pump before
starting.If still nails and misses different problem. Ck this and then when that is
known other problems can be found.

Alec makes great points.

I'll add to the mix: I've always had TERRIBLE luck getting the copper crush washers to seal worth a damn. If you're using copper washers, hit the trash with them and get some of the aluminum ones. Copper are one-use only and I doubt they seal all that great on the first use. The Aluminum ones can be used until they leak and even then, they can be sanded flat on some 600 grit sandpaper on a piece of glass and used over and over and over.

BTW, pumping up the hand primer before starting won't do anything for nailing and missing. If you already have air in the hardlines, the primer pump will not push it out.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I appreciate it guys, but this doesn’t make sense.

*i am not an engineer or a physisist*

All of the areas you describe are under pressure. Air wouldn’t leak in, it would have to push its way in. And as I understand from Diseasal, the return portion of the fuel system eventually takes on standard atmospheric pressure by way of the fuel tank (which is open air). The second problem is that even if the fuel system somehow had less atmospheric pressure (effectively, vacuum) than the air outside it, the mass of whatever air forced its way in would positively displace fuel. Ergo, the fuel level in the return would *rise*. This was easily confirmed by cracking open the banjo bolt at the spin filter.


Again, air is not rising upwards from the pump. The fuel level *as seen through the clear return line* is falling.

I really don’t see how this can’t point to valves that aren’t sealing properly.


Edit: diseasal, interesting about the aluminum washers. I think I read on here from diesel911 that it was the other way around. I actually have replaced all of the original aluminum washers with copper. I’ll order a set of aluminum, but reasoning above still applies.

__________________
1981 240D 4sp manual. Ivory White.

Fascinating to say the least .

" *i am not an engineer or a physisist*" ~ but, do you play one on T.V. ? .

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better

Fluid rushing past an opening can draw in another fluid (air in this case) by aspiration due to the difference in pressure across a gradient. Your temperature sensor for the HVAC works exactly like this, high pressure air is blown through a venturi which has a suction port connected to the sensor. If air can find a way into your system, it will.

It definitely points to your OFV not sealing positively, or having a weeping leak at the banjo bolt. The fluid is seeking an equilibrium with the air. If the OFV were sealed like it should be, fluid cannot leak back through the pump and any air that is in the lines will collect at the top of the secondary fuel filter canister and be burped out harmlessly when the lift pump starts pumping.

Aluminum is softer and more easily distorted than copper, it allows a better seal with less clamping force. There's a reason it's still factory equipment for oil pan drain-plugs that use them. Copper is a good way to strip the threads out of an oil pan since you have to torque it so tight to get a good seal. Formed copper rings are a different story, but you rarely see them for applications like this. Solid copper rings suck for trying to get a perfect seal.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

How does air inter hard lines?
Foot valve leak
Injector leak
external leak
Fuel system will return to little or no pressure after a period of time,not hard lines.
Owner, major concern, nailing and missing,air in return lines can not cause this problem!

My judgement on fuel leak back is defective OFV,yes you have replaced spring and
ball but how about the ball seat.Over the years the seat pounds down, wears and
becomes leaky. Install new valve.