I need some Cooling System 101 answers

[Dick Schneiders]

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

[Bill R.]

around 20psi just for grins.. the boiling point is raised considerably but when you release that pressure all the sudden that coolant attempts to boil and gas bubbles are generated... just like boiling water on the stove.. Even if its not boiling over there are enough bubbles generated that the level increases.. That plus the fact that the coolant hoses will actually swell some under 20 psi of pressure and contract when you release it raising the level of the fluid too.... Also the hot spots in the engine , say around the combustion chamber and surfaces in the head will be much hotter and the coolant will flash there boiling along the areas of contact

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

[Dick Schneiders]

when the coolant was so hot. I usually let it cool down first.

I actually have boiled water on the stove before :-). I know why that causes the water to rise, but I didn't equate that to this situation. Since the coolant didn't appear to be boiling, I didn't think about it. I forgot my grade school science about the boiling point of fluids being raised when under pressure.

I was fairly certain that the situation had to be normal. The car doesn't overheat and there aren't any leaks. Dumb follow-up question. Since the coolant went from the normal level when cool to right at the brim of the expansion tank when I removed the cap, is that how the maximum cold fill point line is established? Do they determine that line so that dummies that try and remove the cap when the fluid is very hot don't get scalded by the hot fluid raising over and out of the expansion tank? Would they put that much thought into the design of the expansion tank, etc?

Dick Schneiders

around 20psi just for grins.. the boiling point is raised considerably but when you release that pressure all the sudden that coolant attempts to boil and gas bubbles are generated... just like boiling water on the stove.. Even if its not boiling over there are enough bubbles generated that the level increases.. That plus the fact that the coolant hoses will actually swell some under 20 psi of pressure and contract when you release it raising the level of the fluid too.... Also the hot spots in the engine , say around the combustion chamber and surfaces in the head will be much hotter and the coolant will flash there boiling along the areas of contact