Radiator Cap Service Video

The radiator cap holds your cooling system under pressure, which raises the boiling point of the coolant. It also lets coolant move to and from the overflow tank as the engine heats up and cools down. A cap that no longer holds its rated pressure can make a healthy cooling system boil over.

This video shows what a failing cap looks like and how a pressure test is done. It also shows why such a small part can leave you stranded if it is ignored.

Two spring-loaded valves sit inside the cap. The main valve holds pressure at a set rating, often between 13 and 16 psi, while a second valve opens as the engine cools to pull coolant back from the overflow tank. Cap testing is part of our cooling, AC, and heating repair service, which checks the full cooling loop.

Both valves rely on rubber seals and springs that wear out over time. When either one fails, the system can no longer build or hold the pressure the engine was designed for.

The most common sign is a coolant level that slowly drops with no leak on the ground. Coolant escapes as vapor through the overflow instead of returning to the radiator. A temperature gauge that climbs higher than usual in traffic points the same way.

At Spectra Auto Services, our technicians pressure-test the cap and the cooling system together before recommending anything. A cap that holds its rating stays on the car, and the reading goes into your DVI report so you can see it. When a cap does need replacing, it is matched to the exact pressure rating for your vehicle.

If the pressure test turns up a bigger problem, such as a leaking core or seam, our radiator repair and replacement service is the next step.

In summer stop-and-go traffic around Frederick, the cooling system runs at full pressure for long stretches, and each cycle wears the cap's main seal and spring until it opens below its rating. Drivers from Brunswick and Urbana often have us test the cap when coolant keeps going missing.

If your coolant level keeps dropping or your engine runs hot in traffic, call Spectra Auto Services at (301) 695-5990, or visit your Frederick car care team to have the cap tested.