Make sure you go through a warm up cycle with cabin heat on full to circulate the coolant while getting the engine hit enough for the thermostat to open. You will feel the upper hose get hit and full. Then loosen cap so air in the system from when you drained it that may have since been pushed around the system can vent out. Loosening the cap will key it out. The system is self venting so there is no special bleed process like on an E36 with low mount tank. The self venting just gets air into the expansion tank where it naturally goes to the top. You still have to let it out. Typically this happens naturally as you are checking the coolant level after draining and refilling.
https://www.newtis.info/tisv2/a/en/e...-check/Hq5CpsJ