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Pressure isn't only positive. Negative pressure.
The system starts at equal pressure to the environment. As it gets hot, coolant expands and displaces air via the pressure valve in the cap. Pressure becomes higher than atmospheric. When it cools, air doesn't re-enter the system. Pressure becomes lower than atmospheric. Hiss when removing the cap is pressure equalising.
Pressure isn't only positive. Negative pressure.
The system starts at equal pressure to the environment. As it gets hot, coolant expands and displaces air via the pressure valve in the cap. Pressure becomes higher than atmospheric. When it cools, air doesn't re-enter the system. Pressure becomes lower than atmospheric. Hiss when removing the cap is pressure equalising.
I think most people refer to negative pressure as vacuum, and radiator caps have a valve in them to address this. But yes, there is usually a very small amount of vacuum in the system when it's cold.
I suspect that after several cycles there is no more coolant or air being let out of the system. All the excess coolant or air has been expelled and when cold, the pressure really does go back to zero, not negative. Again, I’m surmising.
The valve in the filer cap is a one-way affair, so how does the pressure reduce back to zero? Air (or overfilled coolant) is pushed out as it expands and the system pressurises, but it doesn't/shouldn't reduce again particularly quickly. As Extinct said above, a system with no leaks will hold pressure overnight.
With all due respect, that’s where you’re mistaken. The cap is a two-way affair. As coolant cools, a vacuum valve opens allowing outside air in. An example of a failed cap is also collapsed hoses, which would occur if this function didn’t work.
Found a diagram to help explain. It’s not a perfect match to our caps, but the function is the same. I used to be a BMW tech and the E36’s I worked on the most had the same exact expansion tank cap that the Disco does. Same for those cars, there should be zero hiss when opening an expansion tank after it’s sat overnight. And FWIW, my 2004 doesn’t hiss at all either when opening cold.
Last edited by Brandon318; May 21, 2021 at 10:52 PM.
Ok, so I got it wrong about the cap being a two-way system. Strange then that today I unscrewed the cap on my truck for the first time in months, having not driven it in a week, and heard a small intake of air as the pressure equalised.
I don't have any cooling troubles, but I swapped out the cap for another (known good) one anyway, just to see if there's any difference in another week.
