What are the signs of a plugged radiator? I have a 2000 deville, the upper hose
gets hot and the bottom hose where the thermostat is cooler.
This system flows backwards so the lower hose should get hotter than the upper hose, what your describing is a stuck thermostat, replace it and also check the purge tube that goes to the purge tank, if it is plugged it can also cause overheat cause air can get trapped in the cooling system, you need to make sure you get all the air out of the system on these engines or they will overheat causing dire concsequences.
Just flush and filled and put a new thermostat in.
So either you got a bum thermostat or it is in backwards, the spring side goes towards the engine, it's easy to do this especially if it your first time.
Maybe it's a bad thermostat. Or like you said trying to get the air out. I know it's real hard on these northstar engines.