Found this, it sounds like cap needs to be closed during this procedure:
https://www.newtis.info/tisv2/a/en/e...check/CMAxpWPX
Venting cooling system: Note:
Do not leave the coolant expansion tank cap open during the venting procedure.
1- Open vent plugs on coolant feed lines to turbocharger.
2- Add coolant up to top edge of expansion tank. Coolant level must not drop further! No drop in coolant for approx. 1 minute. Close vent plugs on turbocharger when coolant emerges.
3- Set heater to maximum temperature. Press Automatic button. Then turn blower down to lowest stage for a minimum period of 5 minutes.
4- After switching off heater pump, add further coolant up to top edge of expansion tank.
5- Check cooling system for leaks.
6- Close expansion tank.
7- Start engine. Allow engine to idle for approx. 1 minute. During this period, press accelerator pedal 3 times (each time lasting 5 secs.) to approx. 3000 rpm.
8- Warm up engine and activate map thermostat via BMW diagnosis system.
9- When engine has cooled down, check coolant level and adjust to max.
Not sure about step 1? what's vent plugs on coolant feed lines to turbocharger?