Have we put together a step by step process for finding fuel leaks?
If so, what order?

1. Install 1/4 in plastic line in place of the cigar hose to identify bubbles.
2. Check rear of car where exit and return hose connect to tank.
3. Pump primer and watch for changes to bubbles. (Which means exactly what?)
4.????

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

There is an orifice in the banjo bolt that secures the return to tank T on the fuel filter housing. Even though you do not see bubbles in the clear lines you currently have, you may see bubbles that are in the fuel filter which will rise up that orifice out the T then out your temp clear cigar hose. Try the temp clear hose and report back.

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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

Copy that. One last question -I bought a length of 1/4 ID and 5/16 ID.
Is there a reason for not using the 5/16 hose considering it fits the barbs without alteration?

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1981 240D 4sp manual. Ivory White.

Seeing a bubble at the highest point in the fuel system when the car sits overnight means fuel is draining out somewhere or fuel is draining back to the tank and that fuel is replaced by air.

Next time you shut the car off, crimp the rubber line between the clear primary filter and the lift pump with a long nose vise grip and see if the bubble is there or not the next morning. That will tell you which side of the crimp to look for leaks.

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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

5/16 tubing is too loose a fit and clamps that small are hard to come by. 1/4 tubing will be a tight fit even without a clamp and will insure you have a good tight connection.

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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

I'm going to try taking the clamps off the cigar hose and chucking that on.
Unless I'm missing something, 5/16 is the size of all of the other hoses, and clamps.

Will be report back. Thanks Fun

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1981 240D 4sp manual. Ivory White.

the fuel filter mount is designed to remove some from the fuel system before fuel gets pushed into the IP.


IF too much air gets into the fuel, air will enter the IP.



You will see bubbles throughout the entire system after the fuel filter if the air cannot be removed fast enough there.

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cracks or failed seals in the filter mount can cause fuel from various locations to mix- reducing the ability of the filter mount to remove air from the fuel.

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OKay, I'm a believer.
Have the clear line installed now. It's subtle, and far more apparent during idle, but bubbles do appear. Something like the the carbonation working it's way free from a glass of seltzer that's been sitting for a few minutes.

Edit: Funola, will pinch off the first hose in the chain this eve and see where that leaves me.

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1981 240D 4sp manual. Ivory White.

Getting somewhere now, I think. Applied long nose pliers at the point below, awoke to clear lines. Continuing, I imagine pinching the line to the primary filter is my next move?

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1981 240D 4sp manual. Ivory White.

Warning: do not crimp any of the yellowed factory clear lines! You will destroy them! If the rubber line you crimped was the feed steel supply line (from the tank), and you have no bubble in the clear line you just installed, then your leak is from the feed steel supply line back to the tank. Next thing to do is crimp the rubber feed line at the tank to the steel supply line. If you get a bubble, the leak is probably in the steel supply line, most likely a pin hole due to rust, under a line bracket.

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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

Certainly! They are quite obviously un-pliable.

The line I crimped, was the rubber hose between the lift pump and the prefilter.
Heading upstream, It seems the next place to crimp is behind the prefilter (the first rubber hose in the engine bay) in effect adding the prefilter to the milieu.

As I don't have the sort with the brass nubs, it's possible the plastic isn't giving me a good seal. I would much rather rule that out (and preferably find an issue) than have to deal with hard lines under the car...

Re: the two rearmost fuel lines (under the car) Do you know which is supply and which is return? That'll be my next step. I suppose after that it'll be the rubber line from the tank.

EDIT: How long is it considered safe to keep that clear line installed?

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1981 240D 4sp manual. Ivory White.

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

I have heard of people trying to crimping those lines, that's why I gave the warning. You never know.

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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