On the M103 a trick that people have mentioned using is to squeeze and release (over and over) the radiator hose going from the thermostat housing to the top entry point of the radiator. This is done while the system is being refilled, but maybe some variant of this trick will work for an already filled car.

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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!)