Which cap ? The radiator cap or the coolant top up expansion container.
How do you build a pressurized system for a container system?
Higher pressure raises the boiling temperature of the coolant.
recovers refrigerant in a non-pressurized container (Terry Gohring)
recovers refrigerant in a non-pressurized container (Terry Gohring)
ALSO, Keep in mind that your coolant system is designed to be pressurized. When it is under pressure the coolant can work very efficiently to keep your engine at operating temp. If you have a small leak, even a pinhole (or head gasket leak etc.) your engine will easyily overheat because the "boiling temperature" of the coolant is significantly decreased when its not pressurized. For example when the coolant is pressurized it can withstand about 270 degrees, but when not pressurized it can maybe withstand 230 degrees.
the cap controls the pressure in the coolant system. by allowing the system to be pressurized, it raises the boiling point of the coolant. it also controls the amount of fluid in the system, as the pressure increases it allows coolant to be diverted to the overflow to keep the pressure from blowing the hoses off.
The radiator does not have a cap, instead, the coolant system reservoir is also pressurized and the pressure cap is on the reservoir.
the cooling system is a pressurized system with the radiator cap designed to release pressure and expanded coolant into the overflow can. there is a drain however, look at the bottom of the radiator on the passenger side for a wingnut, loosen it and the coolant flows out.
either radiator or pressurized reserve tank. or a coolant hose high on engine will bleed the system for you too.
You should add coolant with the engine off and cool, top off the radiator and fill the coolant reservoir to the cool or cold level. When the system is hot or running it will be pressurized and will be dangerous to open. You can however add coolant to the overflow reservoir at any time.
I believe the 1990 year still had the sealed system. So there is no rad cap. You can add fluid by pourig it into the overflow bottle with the vehicle running the coolant will be sucked into the system. No radiator cap exist in the 90, 91 model. You add the coolant to the coolant container (The coolant container cap is the equivalent of the radiator cap, (pressure device))and the palstic container is under pressure all the time. Be carefull when you want to open it after the car has been running. After those year models, Jeep discontinued the use of the pressure container and went back to tha radiator cap and remove the pressure coolant container.
An pressurized automotive cooling system containing a 50/50 coolant/water mixture will boil at 236 degrees F.