Just wanted to close up the thread with how I ended up fixing the sealing issue. I removed the cold start injector/valve completely. I then plugged the inlet hole with a piece of dried up glue from a hot glue gun. Then, I used a green brillo pad, held the conical surface against the pad and rotated the valve a few times. I cut a small square of brillo pad and rotated it against the hard line which goes into the top of the injector.

I removed the glue piece from the cold start valve and wrapped the threads in about 2 layers of tightly pulled PTFE tape. I slipped the injector back into the hole in the hose that the injector sprays into, but did not tighten the two allen screws that hold it in place. I first threaded on and tightened the hard line (12MM open end wrench) as much as I could, then used an old brake pad pin I had to align the holes for the two allen screws (5MM). Got 1 in, then the other. Tightened those and went for a final tightening of the 12MM hard line.

Primed the fuel pumps a few times, checked for leaks, then started the engine for about 10 seconds, turned off and checked for leaks again. Replaced air cleaner, started car and with a flash light, monitored the cold start injector for leaks. (You can still see it from the side, between the valve cover and the air cleaner.)

Problem solved, case closed!

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2006 BMW M5 "Heidi" @ 109,000 miles
2005 MBZ C55 AMG "Lorelai" @ 165,000 miles
1991 MBZ 300E "Benzachino II" @ 165,000 miles
1990 MBZ 500SL "Shoshanna" @ 118,000 miles

(On the hunt for a good used M103 engine as of 6/10/23, PM me if you have one to sell!)