I thought our system ran at 1.5 bar but that the cap relieves pressure, if needed, when it builds above 2 bar...the limit of the cap.
It's a sealed system with a thermal expansion tank to buffer changes in pressure but the pressure increases when the car is running. I didn't take physics, but I'm confused a bit since I thought the system, at rest, would be at one atm. My initial thought in reading your post is that the car would relieve pressure when it's running.
How does a cap reduce the pressure at which the system runs without letting either air or coolant out?
Not arguing...just curious.
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