Cadillac Repair: Overheating, coolant temp, radiator cap

Question
I have a 99 Seville, NorthStar. I have replaced the water pump and thermostat, due to overheating and water loss? I noticed now, the water is coming out of the radiator cap. The sensor does not show the car getting too hot but we loose most
of the water after about 20 min of driving.
I am going to replace the cap, but I am curious to
what might be causing the preasure build up.

Answer
Hi Joe, Sounds to me like you need to check for bad head gaskets or a cracked head. If within 2 minuits running time the antifreeze starts comming out of the radiator opening that is a sign that a gasket or head problem exists. Of course you could have a faulty coolant temp sensor that isn't allowing the fans to turn on soon enough. The best way to check them is with a scan tool that can see all the temp sensors and if the coolant fan one isn't near the same as the others before the car is started first thing in the morning after sitting overnight then then replacing it might fix the problem. You could have a weak pressure relief spring in the radiator cap but combustion pressures will overcome even a new cap spring. Sorry to give you such bad news. Bill