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Old 01-06-2013, 08:31 PM   #13
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Join Date: Mar 2006
Location: Southern California
Posts: 26,192
My Ride: Lexus
Originally Posted by 7xWDC-MSC View Post
The only thing that happens with the heat on is the heater valve opens allowing hot coolant into the heater core

You need the car running to allow the water pump to circulate coolant and force air to the bleeder screw and cap.
Per BMW TIS, the bleed process is done with the engine off. The engine is not airlocked. Air will naturally find its way out with the engine off and thermostat closed.

Running the engine introduces the risk of an overheat while you're bleeding, heats up the expansion tank which increases the chance you'll damage the threads or the bleed screw when reinstalling it since the parts will have expanded. It also introduces the risk of injury due to hot coolant.
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