If your radiator cap is not sealing properly, it can cause a number of problems. The most serious problem is that coolant can leak out, which can lead to engine overheating. A leaking radiator cap can also cause the coolant level in the radiator to drop, which can airlock the cooling system and prevent coolant from circulating. In addition, a loose or damaged radiator cap can allow dirt and debris to enter the cooling system, which can clog up the radiator and cause further problems.