Is it normal for the level of the coolant in the expansion tank to rise *considerably* when the radiator cap is removed while the coolant is still very hot? I expected it to rise a bit, because is was so hot, but it rose to the very brim of the expansion tank neck. The level was at the max mark prior to that, when the fluid was cool, and went back to the max mark several hours later when it had once again cooled back down. This is on a M50, 1991 525i.

Why does this happen? I allowed the steam to escape before I fully removed the cap, and then the liquid rose rapidly.

I thought I knew how the cooling system worked, but my knowledge does not explain how this would happen. Probably a gap in my learning process.

Thanks,

Dick Schneiders