Im also having trouble with my Fluid level on my 2000 323Ci.
A week or so ago I was driving and my Radiator Expansion Tank cracked. I drained what was left and changed the fluid and put in a new expansion tank.
Everything was Fine, no leaks with the new fluid. Then I was at a long light in this hot southern weather. I noticed my Air quit blowing cold and temp Gauge go up, so I turned into a parking lot and let it cool off.
I waited for traffic to die down and then limped it home, shutting of the engine at lights and keeping the heat on high.
I thought it then might be the thermostat which might have caused the initial trouble, so i changed that. Then refilled the coolant with the key in second position and the heat on high, warming it up a while then letting it cool and then check the level again.
Im still having trouble overheating. Thoughts of what this might be?
I'd appreciate it