The cap releases coolant to the overflow bottle, at a certain pressure, then draws it back as the engine cools, to maintain the correct coolant level.
A weak radiator cap (or the wrong one for the application) can allow excessive amounts of coolant to escape into the overflow reservoir when the engine gets hot. Which then prevents it from being siphoned back into the radiator as the engine cools leading to a low coolant level. So always pressure test the cap to make sure it holds pressure and meets the specifications for the application.
|Coolant leaking from radiator. Pressure not maintained. Over pressure caused by fluid not being able to force over flow bottle.|
When checking the coolant, also check the radiator cap for worn or cracked rubber gaskets. If the cap does not seal properly, fluid will be lost and the engine will overheat.
A pressure test should be performed on your radiator cap to ensure it is working properly.
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