Your car’s cooling system isn’t all that hard to understand. The engine coolant keeps your engine from overheating when running and keeps it from freezing in the winter. Without it, your engine would seize up pretty quickly. The thermostat regulates coolant flow throughout the engine, opening and closing as necessary to maintain engine temperature. Hoses direct coolant through the engine and heating system. The heater core provides heat to the passenger compartment by passing fresh air over coils filled with hot coolant. The radiator provides a similar function, but it’s primary purpose is to vent excess heat from the cooling system out to the air. The water pump provides the necessary current to keep the engine coolant flowing through the system and cooling the engine effectively.
The radiator cap keeps the system sealed. But, there’s more to it than just keeping a lid on the system and keeping contaminants out. The radiator cap actually creates a seal which pressurizes the system. This, additionally, raises pressure in the cooling system, which, in turn raises the boiling point of the coolant in the system. This is important, because without raising that boiling point, the coolant would boil, and thus, boil over, at a lower temperature. The radiator pressure cap, as such, allows your engine to run at a higher temperature without boiling over or overheating.
You should replace your radiator cap every time you change your engine coolant. The radiator cap is a wear-and-tear item, and they don’t last forever. Check your owner’s manual or factory-recommended maintenance schedule if you’re not sure of the interval. As this is often an inexpensive item and doesn’t require any special tools, replacing it is a no-brainer. If you can’t find an OEM cap, there are plenty of aftermarket equivalents. Just make sure the one you’re buying is rated for your application.
The only safety item to keep in mind is to wait until your engine has cooled, and never to open the radiator cap when the engine is hot. Remember, your cooling system is under pressure. If you release the cap while the engine is hot, you run the risk of scaling yourself with hot coolant as the pressurized contents of the system escape. If you’re unsure of the cap’s condition, there are testers available. However, in most cases, radiator caps are inexpensive enough that it just makes sense to go ahead and replace it anyway.