Look at the bottom of your radiator cap. There are two seals. One that seals at the top neck and one that seals at the bottom of the filler neck. As the fluid heats up it expands. It eventually over powers the primary spring in the radiator cap.
The bottom seal is broken as the pressure exceeds the spring force of the cap and the cap rises. Fluid then enters the overflow tank until enough pressure is relieved to allow the spring to force the bottom seal back into place.
You'll also find on the bottom center of the cap that there's a brass or stainless circle plate. That circle plate actually creates a one way check valve. When the engine cools back down the coolant shrinks in volume. As that happens the pressure in the coolant system becomes a vacuum pulling down on the bottom circle plate and thus opens the check valve.
Once the valve opens the fluid is drawn in through the tube that leads to the bottom of the reservoir and refills the system. The system is actually self burping in that it evacuates air on each temperature cycle. Air is pumped out when the system gets hot and fluid is then drawn in when it cools.
The cap is faulty, replace it.
With a radiator cap pressure tester. The tool usually comes along with a cooling system pressure test kit.
The PSI (Pounds per Square Inch) rating is what pressure the radiator cap is designed to hold up to but not exceed in the cooling system.
Radiator caps are pressure relief valves. They are rated at "pounds per square inch" and are designed to release some pressure from the radiator if it gets too high. If an engine overheats it can boil the coolant and this will cause the radiator cap to let out steam. Radiator caps DO lose their ability to regulate pressure and can release too soon. If the engine has not overheated, have the radiator cap tested and replaced as necessary.
It sounds like there's a leak somewhere. If it's not in the cap, it could be in the hose, or the connection, or the radiator itself. If you can't figure this out on your own, take it to a mechanic.
The spring on the bottom of a radiator cap will cause it to fail. If the spring does not work properly it can lead to pressure escaping from the radiator which can overheat the vehicle.
If you have a threaded cap on your coolant reservoir that is your " radiator cap "
No , the " radiator " cap / pressure cap is the thread on cap on the engine coolant reservoir
The radiator cap is on top of the radiator.
A hot radiator releases excess coolant through the spring-loaded cap in to the reservoir, and when cooled the partial vacuum created by cooled radiator pulls it back in. A reservoir should be only about 1/4 full cold on a system that uses a radiator cap, some cars have a pressurized cap on reservoir and no cap on radiator
the radiator cap is on top of the radiator you idiots
A lot of vehicles don't have the radiator cap on the radiator , they use a thread on style cap on the engine coolant reservoir ( that is the pressure cap / radiator cap )
The thread on cap on your engine coolant reservoir is your pressure cap / radiator cap
The thread on cap on the engine coolant reservoir is the pressure cap / radiator cap
The thread on cap on your engine coolant reservoir is your pressure cap / radiator cap
The radiator cap is located on top of the radiator.
On a 2004 Ford Freestar : The thread on cap on the engine coolant reservoir is the pressure cap / radiator cap