درپوش فشار مایع خنک‌کننده خراب

The pressure cap no longer holding its rated pressure. Pressurising a cooling system raises the boiling point of the coolant — that is the cap's entire purpose — so a failed one lets the system boil at a temperature it should handle easily.

The symptom is coolant loss with no visible leak, since it boils off through the cap, together with overheating that appears only under sustained load. It is the cheapest part in the cooling system and the first to eliminate. Test it on a pressure tester against its stamped rating rather than by eye; the seal hardens and the spring weakens invisibly. A cap that vents early also draws air in as the system cools, which is how an air lock forms in a system nobody opened.