Loss of coolant without steam is not good but with low pressure cap you may be venting without realizing.

A recent m54 very similar situation was from the pump impeller failing. It will break free from the axle and not spin. No flow= overheat. Overheat over pressure vent through low pressure cap.

Hopefully that's your issue else it could be the head gasket but you should see lots of white smoke if that's the case.

I'm not sure the easiest way to diagnose. I forget if the pump has houses out front but you may be able to confirm the impeller is intact or not though the hose goesin or goesout without having to remove the pump itself (but of course draining enough coolant).

Good luck.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)