Two questions. Got a 300d fresh out of long term storage. Won't start. Got bubbles in the fuel lines and I don't know where from. Decided to replace all the rubber fuel lines to start.
Easy question first. Two hoses coming out of the tank, right? Upper is just straight 5/16. Lower looks to be 3/8" at the line and 1/2" at the tank. Am I seeing this right? Would MB actually do such a thing to it's loyal followers? Where do I get a replacement?
Hard question. I remove this lower hose and not much comes out. Just a few drops. Then I remove the upper hose and a bunch comes out of the lower hose, like I've released a vacuum. So I'm assuming the lower hose is the feed hose and the upper is the return. Does that make sense? And I'm concerned that if nothing comes out of it then maybe I've got a plugged vent somewhere that's causing the lack of fuel pressure at the pump. What do you all think?

Bypass the fuel tank for now, use a mini-tank and do a diesel purge.
My 83 300SD sat for a year before I bought it.
I used straight ATF instead of Lubro Moly as the diesel purge.
Over the course of a week I ran a container full of ATF as a purge each day, left the ATF in the IP overnight and repeated until I used up two gallons of ATF.
This made a huge difference in drivability.

If you see air bubbles with the diesel purge, you have an air leak from either the lift pump, primer pump, hoses, crush washers, translucent hard lines, etc.
If you don't see air bubbles with the diesel purge, there is an air leak between the lift pump and the tank.

The lower line is usually the feed line and the upper line is usually the return line.
You can switch the lines to bypass a partly clogged fuel tank strainer.
If you do, you will run out of fuel before the fuel gauge indicates empty since you are drawing fuel from a higher portion of the tank.

From what you described, you probably have a partly blocked fuel strainer (MB Part # 1244700106), # 56 on the attached drawing.

You can remove, clean and reinstall it (or just replace it).
If you do, you should also replace the o-ring (MB Part # 1109970145) or source a Viton o-ring of the same dimensions (3mm cs X 35.2 mm id) if you plan to use biodiesel.
If you do use biodiesel, replace all your rubber fuel lines with Viton (or 5/16 Parker Super-Flex FL Series 397) hose, as the rubber will soften very quickly and create a lot of fuel leaks.
Biodiesel is an excellent solvent and will really clean out your fuel system. BTDT.

A gentler approach to clean out your fuel system (after cleaning the injectors and IP with the diesel purge) is to use StarTron.
StarTron diesel fuel treatment and/or StarTron diesel fuel tank cleaner (available at marine supply stores) can also help rid your fuel tank of buildup that limits fuel flow.
This will take 4 or 5 tankfuls of treatment in most cases.
Carry extra primary and secondary fuel filters while you rehabilitate the fuel tank and fuel lines.

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

To check for a plugged fuel vent line valve # 83 in the attached drawing, try running the car with the fuel filler cap removed.
If it runs better without the fuel filler cap you have a blocked vent line valve.

Remove the bell shaped valve, clean out the mud (or wasp nest blockage) and reinstall.

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

Here is a picture of your fuel system on the 300D:

Number 11 is the stepped fuel line that you are asking about; I believe one end is 8mm and the other is 10mm. Part # 1114761526 so you can order it if you wish from Pelican Parts.

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Marc-André

W115 1975 300D "Geraldine"
W115 1976 300D parts/tutorial car

Thanks for all the great information!
I took a quick look at my fuel strainer a couple weeks ago but it was a quick look as I didn't have the tank as empty as I thought I did. I'll check it again.
The fuel cap trick doesn't work on my car as it doesn't run at all yet.
The bubbles are coming out of the fuel pump on the inboard side, next to the engine through the clear tube that goes back to the filter. My camera is dead or I'd show you a picture.

I misunderstood your first post.
If it doesn't start at all, and you have air in the return line from the IP, you probably lost prime and have air in the hard lines going to the injectors.

Re-establish prime - hook up a mini-tank with fresh diesel (or Lubro Moly, or ATF) - this bypasses bad fuel that might be in the fuel tank.
Use the primer pump to purge the air between the mini-tank and the IP elements (about 50 to 100 strokes will do it).
This won't remove air trapped between the IP elements and the injectors.

Try to start the car, if it runs...great!
If not, you need to bleed air out of the hard lines going to the injectors.
Loosen the hard lines at the injectors, unplug the glow plug harness and crank the engine (no more that 15 seconds each time with a few minutes rest between each time).
Fuel will eventually come out of the rearmost injector line at first.
(This can take a while and the battery may run down and need to be recharged...unplugging the glow plug harness gives you some additional cranking opportunities)
When fuel does come out of the rearmost line, tighten the fitting.

Repeat the above procedure until you have purged all five injector hard lines of air.
Plug in the glow plug harness, use the primer pump again, and then crank the engine and it should start.

If it doesn't start after re-establishing prime and purging air from the injector hard lines...then there may be a problem with the glow plugs and/or poor compression (a valve adjustment can help improve compression).

Also it's not a bad idea to have fresh fuel filters as well.
Let us know how you fare.

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

Hey, I found my old camera. Here's the line where I keep getting bubbles.

When you push the black primer bulb do you see any other of the clear lines with air bubbles? Look carefully at the line that goes into the IP (it's almost directly across from where your finger is, is comes from below the fuel filter bowl), the line that goes into the black primer bulb and then the clear primary fuel filter. Old hose can have a bad habit of not leaking fuel but will suck air in. When I first got my car I decided to use a diesel purge type product and boy did it clean out the system; it also showed me where to look for leaks as it seemed to have loosened all the years of old crud in the lines and they started to leak.

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Marc-André

W115 1975 300D "Geraldine"
W115 1976 300D parts/tutorial car

Not seeing any air there but all these hoses look like they need replacing. I should just start there and then see about the bubbles.

Do you see any wet spots on the top of the IP when you push the primer? The reason i ask is that my IP is currently leaking on the first element and I get the odd bubble from that same line. I have to pull the IP and send it to a shop to replace the orings under the elements.

NExt check that the banjo bolt that that line attaches to on the IP is tight and has the aluminum washers. That bolt is also the fuel pressure relief valve. It looks like yours is a rebuildable type.

When you push the primer bulb do you hear an audible buzz? If you push it and all you hear is a light swoosh and then what sounds like air bubbles in the fuel tank you have an air lock on top of the secandary fuel filter and you need to get that out. Use a 22mm wrench on the bolt on the fuel filter and push the primer until all the air is out and you get diesel fuel then tighten the bolt. Now you should be able to push the primer and you will eventually (5-10 pumps) get the audible buzz.

Now you can glow the car and try and start it. No more than 30 seconds of starting at a time and let it cool for a few minutes in between.

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Marc-André

W115 1975 300D "Geraldine"
W115 1976 300D parts/tutorial car

1. First thing is to verify glow plug function.
2. If you have glow plug function, and it won't start, then to verify it is a fuel problem you can spray wd 40 directly into the intake while cranking with glows hot. It should run on the wd40 as long as you are spraying it in. If it runs on the wd40 then you have a fuel supply problem.
3. If you have a fuel supply problem you should put biocide in your tank to kill any bacteria growing there. Once they have been killed they should flow out and the car should run if all the hoses and such are intact.

Only after doing these simple things would I suggest going to the more advanced actions suggested above.

If you have fuel and compression you can sometimes pull the car to get it started without glow plugs. I've done this with success.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

The glow plugs get hot when I jump them. I'l be doing a relay bypass switch soon.
It won't run on brake cleaner but I'll try the WD this weekend.
This car just seems to have lots and lots of little problems. I'll eliminate a couple of variables every weekend.
Now that I've looked at that stepped fuel line really hard, it's dripping a little. Didn't do that before...