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How to test the cap? You know those pumps you install in place of the cap to pump up the cooling system and check for leaks? there are adaptors to put the cap straight onto the pump to test it.
Alternatively, if the cap has been on it from cold and the engine is now warm and upon squeezing the upper rad hose you find that it is not hard, you know your cap isn't holding pressure.
Alternatively alternatively, a rad cap is considered a maintenance item and is pretty cheap, so it's not a waste to just change it. it's considered appropriate to change it any time you do mainenance on the cooling syste, like changing coolant, thermostat, water pump, rad, etc, because by the time the car is old enough to need those maintenance items, the rad cap is probably done too.
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