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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Technical Forums > Mechanical Maintenance: Break-in / Oil & Fluids / Servicing / Warranty > Boiling & Pressurized Coolant Fix



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      12-10-2014, 02:39 PM   #1
oogabooga
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Wanted to share my experience as this might happen to someone else and this solution - although simple, doesn't seem to be out there already.

Had my water pump and thermostat replaced. Coolant was was boiling after driving the car and the entire system was extremely pressurized when opening the expansion tank cap. First suspected a thermostat failure as many people with similar issues have found a failed thermostat to be the problem.

Solution: Cap to the expansion tank needed replacing. The thing regulates pressure in the whole system.

Last edited by oogabooga; 12-10-2014 at 08:09 PM.
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      12-10-2014, 07:01 PM   #2
Sgop335
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I replaced my cap few days ago as well. The vent screw was busted and is a menace. Replaced.
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      12-10-2014, 10:04 PM   #3
Efthreeoh
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Just a bit backwards. As you increase the pressure in a closed system, it lowers the boiling point of water, not increase it. The failed coolant reservoir cap is not holding pressure, which is causing the boil-over.

BTW the tank is the coolant reservoir it is not an expansion tank. Expansion tanks are not under pressure.
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      12-11-2014, 02:05 PM   #4
oogabooga
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Thank you for the clarification. So a coolant cap that leaks will cause coolant to boil but how do the physics work to explain this?:

Tank was overflowing with coolant even after the car was sitting for two days. When the car was ran, it would boil the coolant. Once the cap was replaced, the coolant level dropped to normal and coolant no longer boils.

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      12-11-2014, 02:15 PM   #5
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I thought increasing pressure resulted in a higher boiling temperature for water and vice versa. The cooling systems are pressurized when the car is on. So if the cap is bad and not holding pressure, the system would boil at a lower temperature.



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Originally Posted by Efthreeoh View Post
Just a bit backwards. As you increase the pressure in a closed system, it lowers the boiling point of water, not increase it. The failed coolant reservoir cap is not holding pressure, which is causing the boil-over.

BTW the tank is the coolant reservoir it is not an expansion tank. Expansion tanks are not under pressure.
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      12-11-2014, 02:23 PM   #6
shadow191
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Water expands as it heats up. That's why cars have expansion tanks or reservoirs. On most cars, the radiator cap relieves pressure when the system gets hot. There is a spring in there that allows coolant to flow to the expansion tank when it's hot (expansion tank isn't pressurized). On BMW's, we have no radiator cap, just the reservoir and it's part of the pressurized system. So if the cap isn't sealing, it will boil over.

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Originally Posted by oogabooga View Post
Thank you for the clarification. So a coolant cap that leaks will cause coolant to boil but how do the physics work to explain this?:

Tank was overflowing with coolant even after the car was sitting for two days. When the car was ran, it would boil the coolant. Once the cap was replaced, the coolant level dropped to normal and coolant no longer boils.

Thanks
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      12-11-2014, 03:26 PM   #7
Sgop335
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Quote:
Originally Posted by shadow191 View Post
I thought increasing pressure resulted in a higher boiling temperature for water and vice versa. The cooling systems are pressurized when the car is on. So if the cap is bad and not holding pressure, the system would boil at a lower temperature.
You are correct about higher pressures resulting in higher boiling point temperatures (or higher saturation temp).

Some people...o well
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      12-11-2014, 08:32 PM   #8
Efthreeoh
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Quote:
Originally Posted by shadow191 View Post
I thought increasing pressure resulted in a higher boiling temperature for water and vice versa. The cooling systems are pressurized when the car is on. So if the cap is bad and not holding pressure, the system would boil at a lower temperature.
Yeah, closing out the night too quick. LOL. My second statement is clear, the cap is not holding pressure, thus lowering the pressure of the system, which then lowers the boiling point of the coolant mix (water).
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