The coolant expansion tank cap maintains the necessary pressure in your cooling system, ensuring everything runs effectively. But over time the seal can weaken or deform, gradually leaking coolant and leading to overheating issues. Thousands of heat cycles can also cause the caps to form to the unique shape of the old tank, and we often see customers replace their expansion tank only to experience issues with their old cap not sealing properly. With this in mind, we always recommend ordering a replacement cap when replacing your expansion tank. These caps are manufactured by OEM supplier Reutter, and are identical to those fitted by BMW.