You most likely have a blown head gasket.
The radiator cap might need replacing.
sounds like a leaking head gasket have a pressure chek done on cylinders and cooling system
There could be a number of reasons. Damage to the radiator, damage to the pipes and hoses linking to the radiator. Water pump failure. Head gasket failure.
I would say that you are losing coolant somewhere hoses,water pump,radiator.
first , the thermostat could be stuck. try replacing it. if your radiator OS losing coolant, there may be a leak in the radiator causing it to overheat, or the water pump could be bad. thermostat is the likely cause if it is not losing water.
More then likely your head gasket is gone but it could also be a problem in the coolent tank
This should happen automatically. Are you losing coolant, is the cooling system leaking somewhere? Have you just changed the coolant? Unless there is a leak, the system should purge by itself.
There could be several reasons. Under normal operation, the vehicle will experience this as the coolant heats up and expands into the over flow. A bad water pump, low level of coolant, as well as a clogged radiator could also cause this problem.
either your radiator is leaking,head gaskit is leaking if it's your head gaskit leaking you will have water in your oil or water coming out your exhaust pipe,or you might have a freeze plug going bad.
Check your engine coolant level at the reservoir ( do not take a radiator cap off until the engine is cool )
You may have bad head gaskets when,you are losing engine coolant and you don't see any leaks.you see a white smoke (steam) coming from the exhaust pipe.you have a sweet smell coming from the exhaust pipe.you see air bubbles in the radiator coolant with the radiator cap off and engine running.
is it losing collant? check for oil in coolant or coolant in oil. if so it may have a bad intake or head gasket. if it only idles rough when it gets really hot and the coolant level is okay you probably need a thermostat or possibly a new radiator.