What is the reason for water leakage caused by the expansion tank cap not releasing pressure?
4 Answers
Failure of the radiator cap to release pressure can lead to abnormal operation of the radiator, affecting the engine's normal working performance. The radiator cap, also known as the pressure radiator cap, is installed on the car's radiator and is an important component of the vehicle's cooling system. The radiator cap is equipped with a valve that generates pressure, helping to maintain the engine at an effective temperature. Below is relevant information: Working principle: When the car is running, the temperature of the radiator increases, raising the pressure inside. When the pressure reaches a certain level, the pressure valve is pushed open by the pressure, allowing the coolant to flow into the adjacent overflow tank. When the car stops running, the cooling system draws the water back from the overflow tank. Precautions: Never fill the radiator with tap water, as prolonged use of tap water can cause rust or scale buildup in the radiator. This is a very common issue, as rust and scale can restrict the flow of antifreeze in the cooling system, reducing the radiator's heat dissipation performance and, in severe cases, damaging the engine.
Even a seasoned driver like me with over 20 years of experience has encountered this issue. The coolant reservoir cap failing to release pressure and causing leaks is primarily due to cap aging problems—such as weakened internal springs or deformed rubber seals blocking the vent hole, preventing pressure release. When the cooling system heats up, fluid expansion causes pressure to spike, forcing coolant to leak from radiator seams or rubber hoses. I remember one severe leak in my old car where the temperature gauge skyrocketed mid-drive, nearly causing a breakdown; replacing the cap finally solved it. I recommend regularly checking the cap’s condition by gently pressing to see if it vents properly, and cleaning the vent holes to prevent clogging. A new cap costs little—don’t wait until leaks damage the engine or fry the water pump, which isn’t worth the risk. Adopting this habit saves major repair costs and ensures peace of mind on the road.
With over a decade of car repair experience, I can say that pressure cap relief failure is a common issue. The root cause is usually a stuck or inoperative pressure valve inside the cap, often due to dust buildup blocking the orifice or a rusted/deformed spring. Normally, the cap should automatically release pressure at a specific threshold; if the valve fails to open, excessive cooling system pressure forces coolant to leak from weak points—like radiator connections or overflow tank cracks. Simple checks include using a hand-held pressure tester to verify valve operation or listening for venting sounds during routine inspections. Preventive measures include regular coolant replacement (every 2 years) and cap replacement for added safety. Sustained high pressure also accelerates seal wear, worsening leaks and potentially causing hose bursts that damage the engine.
I just bought a car not long ago, and the leakage really scared me. After checking, I found the expansion tank cap was the culprit. If the cap doesn't release pressure, the accumulated pressure forces coolant to leak from the edges. Possible causes include an aging gasket sticking too tightly or a blocked/rusted valve hole. A simple self-check is to twist the cap when the engine is cold to feel resistance - replace it immediately if abnormal. Untreated coolant leaks can lead to engine overheating and failure, which is too risky. I recommend going straight to the shop for repairs without delay.
