your radiator clogged up.cheaper to get a new radiator. nothing to do with the radiator cap.
Will a 99 Cadillac deville radiator work in a 2000 Seville
Bottom of the radiator.
By the thermostat
Either at the bottom or rear of the radiator
Disconnect the lower hose
check radiator for small leaks or cooling lines which would most definitely cause it 2 steam when running after awhile
Don't know what year you have, but the price for a radiator for a year 2000 Deville ranges from $200 to $300 on advanceautoparts.com
it is the black tank on the right side of the engine mounted on the fender in front of the strut
Below the radiator in the middle.
near the thermostat housing
Usually at engine end of upper radiator hose in housing
Is it leaking from the water pump or radiator? Is the system properly pressurized?