Is the oil loss on the front of the engine or the back? Check the back of the engine on the underside of the cylinder head. Use a mirror and see if the back passenger side corner has oil spots or drips. M103's leak oil from the head gasket back there! if you smell burning oil while sitting idling in traffic that is a clue as well.

My 103 swings temp like yours does. Its what they do. After purchasing a Behr fan clutch and having it be locked up nearly all summer once the engine temp was high enough. I went the all electric cooling fan route. Mine works well but is not a cost effective route. Performance will depend on where you live too. Mine now takes much longer to swing up since airflow is better at idle. Your using MB coolant?

I agree with the injectors needing to be changed. They are not at all serviceable and you should change all the seals at the same time. My car just ran better after the swap. I still have a lumpy idle but its less with the injectors. Mine were changed because of stalling when coming to a stop.

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Current
1989 300SE - Siegfried
2010 Toyota Tundra Crewmax
2005 Honda CRV

Gone but not forgotten
2001 F150 Supercrew - The Screw
1966 Ford LTD 390 - The Big G!
1995 Mercury sable
1986 Subaru DL
1982 Ford EXP - my first car