Thought I’d add a little cooling system primer.
The coolant system should be full of coolant and without any air within the sealed side of the system (heater, engine block and radiator). The radiator cap is what seals the system but the overflow tank is an integral component and is what allows the system to “breath” as the coolant cycles through hot and cold.
The system breaths in the sence that hot fluid expands and cold fluid contracts. Expanding hot coolant must go somewhere since the system’s sealed coolant volume of fixed. This occurs when the system pressure pops the radiator cap and thus the coolant is pushed into the overflow tank by pressure. When the coolant cools and contracts, this causes a vacuum within the sealed part of the system and the radiator cap opens and sucks coolant from the overflow tank. If this does not occur you can see the top radiator hose being sucked in and squashed. If the hose from the radiator cap to the overflow tank is split or not tight on the fitting, it’s like trying to suck coke through a split straw and in such cases the sealed part of the coolant system will suck air… And just like drinking coke through a straw, the open end of the overflow tube has to sit in the coolant for it to work.
In the normal course, any top ups should be minimal and should occur at the overflow tank and normally you shouldn’t have to remove the radiator cap as it’s part of the sealed system. That’s why the overflow tank cap is coloured yellow, to show the owner it’s “owner serviceable”. Such overflow tank top ups should be rather infrequent and require low volume of coolant to be added because of the system design. In my cars I’d be unlucky if I need to top up with 200mls once a year or two..
So if one is topping up frequently, first check the radiator cap and the sealing surface where it sits. Then check the hose between the radiator and overflow tank for splits or poor fitting and also check the overflow tank bottle for splits and such. These parts are critical to correct coolant system performance.
If no faults are found with the overflow parts, then test the radiator cap for correct vacuum and pressure and test the sealed part of the system for ability to hold pressure when cold and hot.
The radiator cap is used to fill the system during coolant flushed at required intervals and shouldn’t need to be popped off by the owner for any top ups.
Coolant systems aren’t rocket science as they’ve been around for probably over 100 years but they still cause angst for owners though not sure why. Guess mechanical packaging has made it much harder for owners and mechanic to spot small coolant leaks unless they spend time and effort looking and, we’ll, we know how much effort some mechanics put into fault find those sweet smell issues…
As a side, that’s why I like overflow tanks that are visible as you can actually see the “breathing” as the overflow tank level moves up and down between the cold and hot levels and that’s a good indicator of a healthy system. And it’s why I dislike hidden overflow tanks as you loose such clear visual feedback.
