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Originally Posted by Sobeus

Why do this bleeding procedure. It's a self bleeding system. Or does the electric water pump live its own life going on and off when you drive the car?
Start the car and let it run on idle with cap to the expansion tank off. Look at the stream in the expansion tank you will see a clear stream just after a few minutes.

This is the completely wrong process and should not be followed for a BMW with an electric water pump. First off the E9X doesn't have an "expansion tank"; it has a coolant reservoir. Second, the head can get air trapped in it and block coolant flow while the engine is warming up. The head on any engine warms up very quickly. The electric pump and cold bleeding procedure prevents this from happening.

OP. Fill the system with coolant until the bleed screw bubbles stop. Close the coolant reservoir cap and bleed screw. Insert the key fob into the dash slot. Keep your foot off the brake pedal and push the start button, the car now is in ignition mode. Turn the heater fan to low and set the temperature dials to max (84 deg. F). Hold the gas pedal down for 10 seconds and release. The water pump will run for 12 minutes and then stop. Top off the coolant as necessary once it bleeds.

You absolutely don't need to use a supplemental 12V power source for the bleeding procedure. The battery if fully charged has enough reserve power to run the pump for 12 minutes. However, just for good measure, what I do is take a spare battery and jump it to the terminals under the hood. If you have some type of charger on the battery terminals in the trunk, the IBS may be turning off the system.