Diesel line leaks- why?
 

My 300 D sedan began to run rough and I noticed that fuel was squirting from the #4 fuel line, the prebent metal tube that carries fuel from (I assume) the Diesel pump to the injector. I tried tightening it, with no results, so I removed it.

It was VERY tight, by the way.

There is a tapered tip on both ends and a fairly thick washer on each end and a fitting that screws down and pushes the tapered tip into the Diesel pump fitting on one end and the injector atop the motor on the other end.

There is no sign of why this might be leaking: the end that does not leak looks exactly like the end that does: there is no break, hole, rough place or other sign of why this thing might be leaking.

As yet there it was, merrily squirting little droplets of Diesel all over the place.

The last work done on this engine was a replacement of the flexible return lines, which I had done at a shop, where the mechanic had replaced the crush washer on the #5 cylinder. None of these repairs would require any touching of the fuel lines, which are below the turbo.

I am supposing that the problem can be solved by replacing the fuel line. They are ordering this and I should have it tomorrow.

I cannot figure out how Diesel fuel under pressure could cause a leak in these metal fittings.

Previously, the #1 line was broken off at the tip where it enters the Diesel pump, and I replaced it. I have no clue as what could have broken this, either.

First you need to check and see if all those little metal holders which connect across the lines are in place...
Some were missing on my wagon...and I did not realize it until someone pointed it out....
Your metal lines go through BILLIONS of cycles of hydraulic pressure pulses.... in the life of your car...
that alone can cause them to metal fatigue and break or spring a leak....so be sure they are supported properly...

Check all IP mounting bolts too!
 

I once had a nefarious Rabbit Diesel pick-up. It literally ATE number 2 cyl fuel delivery lines...it wasn't until I'd bought the third one I discovered that the front support for the IP was missing, allowing the IP to vibrate.

You should check all the IP bolts (there's one that's terribly hard to get to, underneath the IP near the oil filter housing) for tightness.

I was unaware that there are or were supposed to be any connectors or holders on these lines. Some PO's mechanic must have left them off.

Thanks!

But I still don't see how this thing could leak now, because there is no sign of a hole or rupture or anything changed on the end that leaks.

And yet it undeniably does.

Richard,

With fuel pressure at ~1500 PSI, it doesn't take much of a gap or pit to leak fuel.

P E H

I actually ended up with a pin hole leak right in the middle of the steel line. Had stopped to take pics in the boonies of northern Wyoming a number of years ago. When I got back in the car odor of diesel was very strong. Could not get to Billings before the Dealership closed, so went back south and a small shop operator brazed the hole. Bought the lines the next day, but the brazed one stayed in place ten more years.

Diesel injector lines are known to leak on OM601, 602 and 603 engines on cylinders 1-4, leaks on cylinder 5 or 6 (602 or 603) are unusual.

There is a modified part availible from MB. Make sure all the plastic clips holding the fuel lines to the manifold/together and all attached and intact.

The line had an invisible leak
 

I finally found out where the leak was. The pump side had a very small, invisible crack there at the base of the tapered head.

The new replacement line has a slightly different tapered head and has no washer between the head and the tube. Apparently the Trolls of Unterturheim have redesigned this part.

I plugged the end and blew on the engine side of the line with the pump side under water and got a stream of bubbles. So I guess the cause of the failure was vibration.

I had a previous problem with line 2,3 or 4 about four years ago. At that time, the line was broken completely off where it joins the pump. I assumed at the time that the previous owner had disturbed it when he washed the engine a month before.

There is a plastic clip that holds the other lines, but Line 4 does not have a clip and I can't see where you could put one. The pump seems very firmly attached. I don't think there is more vibration than is expected.

It runs just fine now that I have replaced the line. I couldn't figutre out how to torque the fitting, but I used a line wrench and a stubby wrench to tighten it down.

Thanks for your advice!