Have a 1973 220d, I rarely drive it, maybe once a month. It starts, runs, then after 20 minutes of driving it starts to struggle, I lose power. If I pull over and crack the bleed screw on top of the fuel filter and pump the primer pump the filter seems empty and it takes lots of pumps before any fuel comes out the bleed screw, once it does and I successfully bleed all the air out and get it full of diesel fuel then the car runs fine again.

So I am wondering if 1. Fuel is draining back to the tank while sitting (like I said I don't drive it often) this seems likely but I should note that I had replaced the primer pump and rebuilt the lift pump less than 200 miles ago. or 2. I am getting air in somewhere, this could be but I replaced all fuel lines except the clear plastic ones. Not sure if you can buy those anywhere? or 3. some other possibility I haven't considered yet?
Any help appreciated, I think I may bleed the filter and then let it sit over night and then check if the fuel drained out of the filter. Also may just start bleeding it before I drive it anywhere.

After the manual pumping to refill the filter, does the car once again run for only 20 minutes, or for an indefinite amount of time?

Seems to keep running fine for as long as I have ran it after the filter is bled. But then I usually just try and get it home and never drive it for too long after. I'm thinking this weekend I may try to get it out and put a good hour or more on the car to see if it runs fine for extended periods after bleeding. Also every time it has happened its been going up hill that I started losing power.

Still haven't figured this out, but really haven't had time to tackle it. Like I said in the original post the car will run for 20 miles and then the secondary fuel pump is empty and I need to crack the bleed screw and prime the system to keep it running. All I can think is the fuel pump/lift pump isn't pumping enough fuel to keep up?

Personally, assuming you have plenty of clean fuel, filters, lines, etc….
You already know air is getting in there. Unless you are running the tank dry, you have a leaky hose fitting, crack in your plastic secondary filter, cracked lines, etc. on the negative/suction side of the the LP pump inlet. A compromise anywhere on the positive side reveals itself as a fuel leak.

My son recently bought a 76 240 with the same issue. It ended up having a seized lift pump. The next time it dies, also remove the cap from the tank. A clogged tank vent will also give these symptoms.
The lift pump should always supply positive pressure to the secondary filter, so air indicates it can't do its job. A leak in that area should be wet, not allow air when the car is running.
Either the lift pump is not working or its being hindered by a clog or negative pressure in the tank.
The IP creates its own suction, so even with a non working lift pump the car will run for a while, but won't be able to keep up with the volume it needs.

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1985 300 TD 448K
1984 300 TD 278K

1983 240D euro 240k
1994 f-250 idi turbo 330K
1986 f-350 IDI
1987 F-350 IDI

1985 JD 1050 4wd
1965 IH 3660

I agree with what you and others have said. Air leak, dying Fuel Supply/Lift Pump or the Fuel Tank Vent are the most likely suspects.

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84 300D, 82 Volvo 244Gl Diesel

Thanks everyone, I am not very knowledgeable on diesels or really cars from this era so the help is greatly appreciated.



I will shoot some compressed air through the vents in the tank just to make sure they are clear, I dropped the tank and replaced the screen so I know that is good but maybe the vents are clogged.



There are no visible leaks in the fuel lines, the only ones I didn't replace are the clear plastic ones because Mercedes doesn't have these anymore. Is there any easy ways to test for leaks in the fuel lines?


Any easy ways to test that the lift pump is pumping sufficient fuel? I guess just start the car and take the cap off the secondary filter and see that it's working making sure to have a drain pan underneath it?



Again thanks!

All excellent points.. in short, diesels are simple to keep running with clean air, fuel, and oil but as always the devil is in the details and I’ve yet to see many exceptions when ignoring strict adherence to part tolerance and maintence procedure do not lead to complications. In particular the exact inner, outer, and thickness of each and every seal, hose, clamp, and even those little brass sleeves in the primary filter.

On a related note: I can’t find my MB stem adapter but does anyone have the torque specs for the tire valve stem cap?

Sometimes there is a single simple answer for problems but it more usual for the problem to happen in something that is a system of components and one or more of the components could cause the problem.

Forums like this one can help identify the most likely component that is the issue but that is not always so. After that your knowledge of how the components of the system works is your best friend when troubleshooting.

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84 300D, 82 Volvo 244Gl Diesel