Is there some kind of trick to getting the hard lines to seal? I've tried 3 different junkyard sets $70 and all but 1 pipe leaks badly. I press the pipe down into the socket and tighten the nut all the way down by hand before tightening it with the wrench no tighter than I would a sparkplug.
I have tried loosening and tightening a number of times and removal and installation a number of times with no success. Also tried scotch brite on the ends but that didn't help either. Any help would be greatly appreciated.

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1984 300D Turbo 200K

Inspect the seats/balls for scratches? A rather strange issue

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1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD

Are they leaking at both ends or only at the IP? If only at the at the IP it might be the delivery valve holders leaking.

Sixto
83 300SD

Never had that issue with four 617 powered vehicles and removing hardlines dozens of times. Odd for sure.

One of the original lines was scratched but I didn't find anything else scratched. The original lines will still seal enough for the injectors to fire with some dripping. I can't get the junkyard lines to seal at all except for 1 line. The leak shoots out of the top of the injector line nut so I am sure it isn't a delivery valve seal or the injector's return lines.

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1984 300D Turbo 200K

You also need to inspect the part of the Injector that the line seats on. If they are scratched, rusted or split/cracked they are not going to seal.

It is the some on the Fuel Injection Pump end of the lines but with the delivery valve holders.

This is just a wild thought but I have seen complete made in china Injectors for Mercedes on sale. If the Injectors look new perhaps they are the made in china ones.

See if the Nuts on the Fuel Injection hard Lines are bottoming out on the Injector or delivery valve holders.

I believe I haver read that the MW Fuel Injection Pump Hard Lines are different from the M type Fuel Injection Pump hard lines.

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84 300D, 82 Volvo 244Gl Diesel

I was able to use my MW lines on my M-based SuperPump with no issue - other than I had to bend each one to fit. But no issues as noted by the OP - sorry I'm not much help here.

Dan

The seats look nice and clean too. I looked under the nuts after tightening and can see that they are not bottoming out. The injectors appear to be old rebuilds with the rubber bands on them. The injector lines I pulled were from turbo engines with MW injection pumps, which I have. I tried again just threading the nuts down without putting pressure on the pipe and that didn't work either.

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1984 300D Turbo 200K

I did have to bend 1 set of lines to fit because the IP was tilted in more, the other set did not require bending but the problem is the same with both of them.

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1984 300D Turbo 200K

There was a post in the last 6 months about metal a "soft seal" (copper or aluminum) you can insert between the injector and tube. I used one once, a "Voishon washer", common in aerospace testing.

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1984 & 1985 CA 300D's
1964 & 65 Mopar's - Valiant, Dart, Newport
1996 & 2002 Chrysler minivans

Recently had this problem myself - with only one of my lines.

These are lines original to the car, and I've removed/re-fitted them myself many times in the past with no issue.

This time, however, I'd slightly deformed line #5; bent it out of alignment ever so slightly that it didn't sit quite squarely. A little bit of tender loving care (bending) to get it right did the trick. Hint: the line ends should seat squarely; they should drop in easily, into their respective fittings with not even the slightest force to fit them. Do, this, and you'll create a good seal.

All this assuming no damage to the seat faces on either side, as others here have discussed.

I'll take this opportunity to again plug greazzer's very good work on injectors. He recently re-did mine, and the car runs beautifully.

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'83 300 SD
'68 Triumph TR 250 - The only car I ever loved more than the Mercedes; who needs electricity, anyway? - Damn, why did I sell it?!
'59 Jaguar 3.4 'Le Chat Noir' - Damn, why did I sell it?!

It's difficult to make predictions, especially about the future.
- Niels Bohr

I will look into the washers before I look into trying to bend in a new straight line. I had to bend the junkyard lines so that they go on without any strain but they still leak worse than the original.

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1984 300D Turbo 200K

I can't find any information on the soft washer seals. Does anyone know where to buy them?

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1984 300D Turbo 200K

I looked up cup washers but most look like bottomless dog dishes. The rest are Starbucks job listings . Fastenall has a listing for #4 brass cup washers but no pictures.

Sixto
83 300SD

If the line has a crack, this won't work: 400 grit wet dry paper dampened with WD-40 and twist back and forth on the line tip to clean up the groove that is usually worn there.

I have not had to do this yet but if the wet dry paper did not smooth it enough, I'd cut the mating part off an old injector, chuck it in a drill, valve grinding compound and lap it smooth.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked