Radiator Caps
A radiator cap seals the cooling system and holds it at a set pressure, which raises the boiling point of the coolant well above 100 degrees. It is a small part with a large influence on overheating.
What it does
A spring-loaded valve holds pressure until it reaches the cap’s rating, then vents to the expansion tank. A second valve lets coolant return as the system cools.
Signs a radiator cap is failing
- Overheating with no visible leak and a healthy radiator
- Coolant loss into the expansion tank with none returning
- Boiling or bubbling in the expansion tank
- A collapsed radiator hose after the engine cools
When to replace them
Inexpensive and easily overlooked. Worth replacing whenever the cooling system is opened, and always when investigating unexplained overheating.
What to confirm before ordering
Have these ready and the counter can usually confirm the part in under a minute.
- The correct pressure rating, which is stamped on the original cap
- Cap type and neck size, since they are not universal
- Whether the cap sits on the radiator or on the expansion tank
Radiator Caps questions
Can a faulty radiator cap cause overheating?
Yes, and it is one of the most commonly missed causes. Without correct system pressure the coolant boils at a temperature the engine would otherwise handle without difficulty.
Does the pressure rating matter?
Very much. Too low and the coolant boils early. Too high and you risk stressing hoses, the radiator core and the heater matrix. Always match the original rating.