If you look at how the injector fits into the prechamber, there is no heat transfer path for the injector nozzle body to the head. Thus, the washer with the pattern cut into it, which is to ensure the nozzle body surface at the bottom of the injector contacts the raised ring at the inside diameter and then is allowed to conduct heat away and out to the prechamber body.

I am not certain but I also think this part helps keep combustion junk out of the injector to prechamber threaded bore. Since the injector bottoms on these seals/heat shields and there are no other sealing rings or sealing devices it seems they must also perform that function as well.

Overall I think it is not wise to run without all the parts the engineers included in the engine design. I would put them back in and just figure I might have been lucky nothing bad happened that I have noticed, so far. Good Luck, 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)