This little cap seals the expansion tank. The main job is to let the cooling system breathe—to relieve pressure buildup as your engine heats up and coolant expands, and to allow coolant to be drawn back in as things cool down. It’s also designed to maintain the proper pressure in the system, which raises the boiling point of the coolant. Think of it like a pressure relief valve. It’s a small part, but it’s part of your engine’s cooling system. The cooling system keeps your engine from overheating, which could cause serious damage. The expansion tank collects excess coolant and the screw cap keeps everything sealed up tight. It’s a direct connection to the radiator and overall coolant circulation. If this cap fails, you’ll likely see coolant leaks. Even a small leak can lead to overheating. You’ll notice if it’s gone bad – usually you'll see coolant pooling somewhere in the engine bay, or the temperature gauge will start to climb higher than normal. It’s a relatively inexpensive part to replace, and doing it proactively can save you from much bigger problems down the road. We carry a good selection here, and you can also find them pretty easily on a site like Fixparts Online if you prefer to do the ordering yourself. It’s a pretty simple fix most folks can handle. Your trust matters: Screw cap (MPN 17117521071) comes with a 12-month warranty!

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