sgoodhart,

From your description of the source of leakage, you should probably see some gasses escaping there too when the car is either cranking on the starter, or running. There is a seal ring to effect a seal between the prechamber and the head, but there is no seal other than the injector nozzle face to the prechamber "nozzle plate" or "seal plate" at the bottom of the injector bore in the prechamber.

To have fuel leak out the spot you have identified there are only a few possible causes and most of them are unlikely to leak fuel alone. First, assuming the injector bodies on two of your injectors are not flawed, liquid fuel must leak out of the nozzle and go radially, past the nozzle plate or nozzle seal, then up and around the threads that hold the injector in the prechamber. This is feasible if there is no actual sealing occuring at the nozzle to prechamber nozzle plate/seal, and there is a lot of extra fuel coming out that path. That condition can arise if either the wrong seal exists there (there are two different kinds, and while I am certain the old style can be replaced with the new style, it is not clear if a new injector body and nozzle will work with the old style plate/seal), two seals have been put in on top of each other (introducing another leak path), the seal is in backwards (either type but only likely on the new seals) or there is no nozzle plate/seal there at all. Any of those conditions will also likely damage the injector nozzle and could lead to extra fuel coming out. Enough extra that the gasses from the combustion chamber and prechamber don't make the trip? I don't what quantity would be required and how that would affect normal running of the engine, but find it unlikely without some other apparent indications of something being amiss.

Another, but unlikely, scenario is that you have a ruptured injector body. Having two of these would be really unlikely, while a mistake on the nozzle plate/seal is likely to be repeated by a mechanic not knowing any better.

To get to the bottom of this, next time you take the injectors out, look into the area at the bottom of the prechamber bore that the injector body goes into, and remove any loose parts. The new seal plates are about the size of a quarter or smaller in diameter and about the same thickness, have a smooth chamfered edge surface on one side and distinct grooved surface on the other. These stay loose in the prechamber when the injector body is removed.

The old nozzle plate/seal parts are much smaller (half the diameter of the injector bore in the prechamber or so), and are not flat. They look a little like a pipe flange or top hat in cross section, with the flat top removed to provide a bore through the top hat. They are a goldish color when new, and stay that way or turn a darker color/black as they get old. The prechamber inside in the area of the injector bore is a silver metal color and should stay that way. So the old style seals look like a dark, flat donut around the hole in the prechamber bore leading to the actual prechamber. They get the top hat section jammed into that hole to the prechamber proper and they can be a bit of a chore to get out. None of the nozzle seals can be reused so you have to get them out. Others have reported here that they have gasses leaking at the location you have identified and it turned out to be a problem with these seals, usually having both installed because the mechanic did not know the old style ones had to be removed.

After all that I do not believe your problem is leakage coming from the location you have described, but leakage collecting there. I find it much more likely the return leakage connectors, which are subject to some abuse in the removal and reinstallation operations, are damaged. This can cause a leak between the hose and the nipple, or the nipple and the injector body, which would run down the injector body, possibly behind the line of sight, to the recess with the locking ring in it that holds the prechamber into the head. Another possibility is the injector body joint between the nozzle top and bottom, which is outside the prechamber and between the two large sets of flats on the injector body, is leaking. This would indicate a problem sealing between the nozzle body, the holder insert or the injector nozzle top. All more likely than a rupture, or the other scenarios without any gasses coming out.

I hope all this helps, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)