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The aux pump has a check valve in it to prevent the heater core loop draining back and creating an air pocket.
You'll see the common thread - I drained my coolant system, refilled it and now it is overheating! It is because some of the coolant drained out of the heater core line. As soon as they turn on the heater core loop it starts circulating air through the coolant system. As soon as they remove the expansion tank cap and turn on the heater...it fixes the problem.
And the pump only turns on when the heater is on. So no, you don't need the aux pump to have adequate flow.
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2018 F30 320iX Melbourne Red
2011 E90 M3 Monte Carlo Blue
2004 E46 M3 Imola Red
2000 E36/7 Z3 Steel Blue
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