The radiator cap is more than a cap for your radiator; it keeps your engine cool by sealing and pressurizing the coolant inside it.Radiator Caps are manufactured using either metal or plastic design depending on the vehicle manufacturer’s specifications to seal the top of the radiator filler neck to prevent coolant loss, pressurize the cooling system and raise the boiling point of the coolant.
In a sealed recovery system the cap (double seal) allows coolant to flow to and from the recovery or expansion tank, maintaining the integrity of the cooling system.
A non recovery system does not have a recovery or expansion tank; therefore a non recovery cap (single seal) only allows the release of coolant and return of air to the system.
here where going to talk about the effects of old bad caps and how it affect overheating problem .
in fact you must open your car radiator cap once in week to check your coolant level inside the radiator before starting your engine instead of checking reservoir tank
one day i found it start to over heat while reservoir tank is ok it drive me crazy then i found the radiator is empty due to bad radiator cap sealing and reservoir tank is plugged so it have constant level after this i had to rebuild engine due to cracked block in few months later
that mean i drive my car while loosing coolant till radiator empty and bad cap sediment plugged the reservoir hose keep showing me great level
it better to change your radiator car once a year and check your coolant level inside the radiator once a week before starting your engine in morning this will will show you quick answers if any overheating issue start in future