Just went through it, sorry it's not good news but, head gasket. Didn't realize because the symptom's didn't match. It wasn't smoking or showing water. It was a pinhole spot. You'll may need new heads too if the aluminum got too hot. Good Luck
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
Low coolant? Plugged or restricted radiator? Thermostat not opening? Water pump not circulating coolant? Cooling fan not working? Hoses collapsing under pressure?
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
start with checking thermostat
Below the radiator in the middle.
near the thermostat housing
Usually at engine end of upper radiator hose in housing