96 cadillac deville overheat

Most likely a head gasket failure. I would pressurize it first!

Check for coolant flow at the Purge line,this is a common problem on the Northstar.
There is a small diameter hose coming from near the top of the coolant tank,this is the purge line.
With the engine idling,pull the purge line from the tank,if there is no coolant flow from this small hose,it is blocked and this will cause overheating.If the hose is clear, check for blockage at the nipple on the tank.

you cant feel the difference unless you try to drive in a long drive,first you might check you thermostat they tend to get stuck which restricting the flow on your cooling system easy way to do that is by squishing the hose on your radiator and try to feel if there is a flow or when you squish it it should feel soft which indicate that your coolant are circulating also check your coolant temp. if it work check for resistance i cant give you spec but easy way to find out where after changing your thermostat it should be work fine but if its not you just eliminate the problem goodluck...