The coolant won't circulate unless you fill the top radiator hose. The hose runs from the radiator to the thermostat housing. Remove the end attached to the radiator and fill it. Put your hand over the end after it's filled and get it back down and on the radiator outlet as quick as you can to avoid spilling all the coolant.

Fire it up, turn the heater on high and watch the thermostat to see if it's still overheating. If the thermostat doesn't open so the gauge drops when it gets to about 100C then shut the engine down and remove the thermostat for testing.

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84 300SD
85 380SE
83 528e
95 318ic