Here's what I was taught in my youth...
Radiator cap is simple seal. Expansion cap has a spring loaded seal. This loading can vary with engine type but the usual load is 15psi. Check your engine spec for type needed. This raises the system pressure slightly resulting in a corresponding boiling point rise to supplement the coolant additive effect. The expansion tank collects some coolant as the engine warms it up and when things cool it gets sucked back. If the system pressure gets too high ( greater than the cap rating) the cap spring gets pushed up to allow excess pressure to release. It's a pressure relief valve rather than thermostat. The actual thermostat is elsewhere and restricts coolant flow on cold start so that running temp is reached sooner. Fuel efficiency and emissions improved when cold. The expansion tank level when cold is what you check for coolant volume. Most will have a tab inside to show correct level BUT the Plus CX don't. There is a label on the side so a dipstick is used to check. I use my middle finger as it reaches the correct level. GRRR. Old school tab would be easy.
Bit of a minor rant.....Same for oil level...no dipstick and the on-board " computer" needs a warmed engine running. Stop the car on the level. Press a button. Wait as the system checks. Watch the screen whilst it runs. Progress eh!
Last edited by sospan; 10/07/24 08:44 AM. Reason: Typo
