The more I think about this, the less sense it makes...

Doesn't the pressurized system begin at the lift pump?
If I have a hole in a hard-line under the car, I don't see how that would affect the pressurized side of the system when the car was off.

My issues is that I'm finding a large bubble in my IP return line after prolonged shutdown.
Crimping the hose where I did is performing two functions: isolation of the two parts of the system, and preventing back flow to the tank.

My understanding of the lift pump is that if the valves are seating properly, it's a one way device. Since I rebuilt mine, it certainly should be. This pinch test may have inadvertently prevented some degree of back flow to the tank which is possibly causing my minor loss of prime. Unless I'm totally barking up the wrong tree here. And if I am losing prime through a lift pump valve, well that reallllly sucks because that was a job.

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1981 240D 4sp manual. Ivory White.

If I read your post correctly, after you crimped the line, you awoke to no bubble. That means you have no leaks after the crimp, on the engine compartment side. The leak is before the crimp, on the tank side. That's why I suggested you move the crimp one step closer to the tank, to see if the leak in the steel supply line or after that. The supply line is the one that goes to the fuel strainer.

If the clear line is 5/16 i.d. I would not trust it not to blow when the engine gets hot. If 1/4 " i.d. I'd trust it for short term.

Edit: I see you have no clamps on the clear line. Air can get in on the relaxation stroke of the lift pump when it is under vacuum. I already explained to you how to make a clamp with rubber band.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Yeah, apologies this part me be unclear. I haven't isolated the primary filter, that's what I'm trying to express. I pinched the line *directly* before the lift pump so I'll be working backwards, section by section. I'm not convinced just yet that the primary filter isn't the problem.

Edit: the clear line i installed was the 1/4 and boy was it a snug fit. I can certainly wrap some rubber bands around it but to be honest I'm more concerned about when I have to remove it.

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1981 240D 4sp manual. Ivory White.

You remove it the same way you installed it. Microwave a cup of boiling water and slowly pour it over the tubing to soften. Yank it off with a pair of pliers.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Okay, engine bay system has been eliminated... No bubbles this morning. Working back this evening to the short rear supply line.

I realize this is a slow moving narrative with a questionable pay-off. Hoping it's future-helpful.

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1981 240D 4sp manual. Ivory White.

You did test by clamping the line also just before the pre filter? Common area for a lot of grief. Normally but not always when an air leakage is back farther. You see a tiny stream of bubbles processing through the primary filter. When the engine is running.


Even if you see one tiny bubble in transit it is too much in your case.

Questionable payoff? Still don't trust the process? I am going to guess you have a bubble this morning after clamping the supply line under the tank.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Waiting to see what the end issues is......

__________________
-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

Ha, absolute trust. “Questionable payoff” was referring to the end of this story for our readers, not me. I of course have everything to gain.

Last night I crimped the inlet rear fuel line. And this morning?






No bubble still.



This is getting interesting. I’m left with either the rear hose clamp on that same line, the hose clamp on the line from the tank screener, or the tank screener gasket itself.
My money (and hopes and dreams) was on the primary filter. Now I’m really hoping it’s not the screen gasket because that was not a fun job.

Logically this makes no sense, but to be thorough -would a full tank and or a narrower “temp cigar” have any bearing on this? The cigar hose is meant to accommodate surges in fuel, which would mean (in my mind) between surges the flow wouldn’t quite fill the circumference allowing bubbles back? Into the IP return line?

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1981 240D 4sp manual. Ivory White.

The smaller diameter clear hose (1/4" i.d. for 1 ft) is a little more restrictive than 5/16 but not enough to make a difference when running IMO. Static wise, it makes zero difference.

Next thing to try is do not clamp anything and see if you get bubble (this tests the real cigar hose you removed and the 2 clamp connections). If you get a bubble, next thing to try is no clamp again but leave the fuel cap loose to simulate a working tank vent (I do not know if your tank vent is working. If you get bubble, next thing to try is clamp cigar hose (your clear line). If you get bubble, then clamp the return rubber line at the tank.

If none of the above tells you where the leak is, start testing each component with a Mityvac under both vacuum and pressure.

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

What if it is the cigar hose? What would that mean for the T junction at the secondary fuel filter. Something would have to be off...
The funny thing is, everything is running very smoothly at the moment.

About a year ago, I replaced the cigar hose with a regular piece of 5/16 when I found it leaking after a diesel purge. Then I started reading about overflow valves, and shaky idle, etc, and found a compelling case to reinstall one. I haven't noticed an ounce of difference. If anything, running on a restricted hose has been potentially the smoothest running I've had yet.

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1981 240D 4sp manual. Ivory White.

Well, it looks like it must be the cigar hose, which is bizarre and to the best of my knowledge, nearly impossible. I didn't pinch anything off last night and awoke to clear lines all around.

I'll reinstall the cigar hose and see what happens tomorrow morning. I really have no idea what to make of this.


*Edit: It only took a couple of hours. An inch of bubble has appeared. This is interesting... There are several channels running through the secondary filter. Some device or another keeps them discrete. Whatever device that is seems to have failed.

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1981 240D 4sp manual. Ivory White.

Hypothesis- Internal leak within the secondary fuel filter housing that causes return air from the cigar to mix with supply fuel. There appear to be crush washers between the fittings on all three ports (2 brass fittings on the side, three way fitting on the top). Beyond this, anywhere else to look? Alternatively, barking up the wrong tree entirely?

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1981 240D 4sp manual. Ivory White.

The cigar hose should be full of fuel after you ran the car and all air in the system was bled out to the tank, same with the temp clear cigar hose. If there's a leak within the secondary fuel filter housing, you'd see a bubble in the clear cigar hose also. But you didn't? Only with the black cigar hose?

__________________
85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked

Never any bubbles in the clear temp hose and yes the cigar hose is new..

Afaik, diesel fuel (or liquids in general) do not compress. If the cigar hose was 100% full of fuel 100% of the time, what would be the point of the expansion space in that design? What’s funny is the engine did feel nominally smoother on the smaller ID temp hose. The overflow spring is to spec, the lift pump is fresh, would the further restriction of fuel leaving the IP have an effect beyond the Overflow valve?

In any event, I’m traveling for a few months so I’ll have to dig this up when I return. Thanks funola -looking forward to reading your thoughts.

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1981 240D 4sp manual. Ivory White.