Yes good idea , but i would think if the thermostat is closed then it will interrupt the flow of coolant through the system . I am interested and i will use this method just need to know as the thermostat is closed, and the water is cold in the system, will it still circulate .

Yes. The thermostat only prevents engine block to radiator, you're filling from the reservoir, the first thing you will fill up is the radiator.

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1990 190E 3.0L