The cap is faulty, replace it.
Because you dont have the engine running
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.
It should be printed on the radiator cap. Most are around 16 psi.It should be printed on the radiator cap. Most are around 16 psi.
Because you dont have the engine running
You likely need to replace your radiator cap - the rubber seal in the cap is worn.
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.
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 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 thread on cap on your engine coolant reservoir is your pressure cap / radiator cap
The radiator cap is located on top of the radiator.