WARNING: Never remove the radiator cap when system is hot. System under pressure!
Check and test radiator for blockage or leaks:
Pressure test radiator with appropriate pressure tester, looking for leaks and cracks.
Remove top and bottom radiator hoses and flush with fresh water through system, check the flow of water out of the lower section of the radiator
Check radiator cap
Check radiator cap seal, if damaged replace.
Check radiator filler neck and the cap sealing surface for marks or damage.