This little cap basically maintains pressure in your coolant system and keeps everything sealed up tight. It prevents coolant from escaping, and also allows a little bit of expansion as things heat up. It’s got a pressure relief valve built in, so if pressure does get too high – like if your engine is really working hard – it’ll vent a little bit of coolant to keep things safe. It also keeps dirt and debris from getting into your cooling system, acting like a shield for everything inside. It sits right on top of the expansion tank, which is where extra coolant goes when things heat up. That tank’s connected to the radiator, the engine, everywhere the coolant flows. The cap is the final point of sealing the whole system. Without it, you’re basically open to the elements, and potentially losing coolant. Losing coolant is obviously bad news – that leads to overheating, and overheating can cause serious engine damage. It's a small part, but it plays a big role in keeping your engine running at the right temperature. You’ll notice if this cap fails, usually by seeing coolant leaking around the cap itself, or maybe your temperature gauge creeping up. They can get brittle over time, or the seal can just wear out. Fixparts Online has a pretty good selection of these, if you’re looking to replace it. It's definitely a good idea to have a spare, or replace it as part of routine maintenance. Secure your purchase: Sealing Cap (MPN 1J0121321B) includes a 12-month warranty!

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