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Old 10-04-2012, 11:50 PM   #65
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Join Date: Feb 2008
Location: Philadelphia
Posts: 6,919
My Ride: M3 & ZHP
Originally Posted by SeanC View Post
Please pay attention to the bolded statements below:
Sean, I disagree with with your statements about the coolant boiling around areas of higher temperature. Remember, the fluid will be flowing a lot slower around the block - there's a much larger volume for the fluid to fill. Slower fluid = higher pressure = higher boiling point.

There are only a couple ways the cooling system will actually reach 2 or 3 bar. Either A) Coolant starts to boil and the total molecules of gas increase or B) More fluid enters the expansion tank and reduces the volume the gasses can occupy (this will happen when in situations where the heater core closes, thermostat closes, or if the system is overfilled).

If coolant starts boiling, you're in trouble - the engine will overheat drastically anywhere the coolant turns into a gas. So I highly doubt the system is engineered to even allow that possibility. I must therefore conclude that the cap is to allow ventilation in the event the system is overfilled.
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