Coolant expansion cap...exposed

Having recently changed my expansion cap I decided to see what they are made of. A bit of junior hacksaw action later I revealed that there are two pressure relief valves within.



Firstly a large pressure relief valve comprising a valve disc and spring which maintains the cooling system pressure at the set point, around 1.2bar iirc, and no more so as to protect the engine from overpressure but also maintain the system pressure slightly above atmospheric pressure to increase the boiling point of the coolant.



The second valve is a tiny vacuum relief valve that ensures the cooling system does not get into negative pressure (vacuum) during cool down. This valves sits within the large valve disc and is the small brass plate that can be seen through the hole in the underside of the cap. This valve has a very light spring and a small seat. I suspect that it is this valve that causes cap failure due to its probable frequent operation on engine cooling. Any dust or dirt that gets sucked across the valve seat during cool down will cause the valve to not seat properly again with resultant leakage and then failing to maintain the necessary system pressure.



It's a shame that such a small delicate valve is in this highly vulnerable position and has probably been the originator of so many over temperature events and HGFs!

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