Cadillac Repair: northstar overheating, seville sts, radiator cap

Question
My 95 seville sts 4.6l northstar is overheating. I have replaced the heatercore, thermostat, waterpump, coolent Temp. sensor, lower radiator hose, the water pump belt, flushed the system, added DEX-COOL, and drive with the heat on 90, in northern MI, I also changed the 4 water jacket gaskets. I have compression tested all 8 cylinders and run a pressure test on the coolent system, both checked out OK. The car runs much cooler without coolent, only water. What is this cars deal?

Answer
Did you remove the radiator cap, fill the engine with water, Remove all the spark plugs and put approx 90 lbs of air pressure into each spark plug hole for a few seconds and see if the water comes pouring out of the radiator cap opening.

If it does, Replace the head gaskets according to the manufacturers specifications.