Cadillac Repair: Overheating, head bolts, head gaskets

Question
I have a 96 Deville that was diagnosed  at a shop (not a caddy dealership) to have a water pump housing issue. That does not seeam to be the issue because the water seems to flow fine. I put a brandnew thermostat in back in july that also did not solve the problems. I had 2 mechanics tell me in the past that there was a head gasket issue but that was ruled out because there is no water in the oil nor is there oil in the water. A cadillac mechanic told me that with head gaskets if its overheating the way i described that i can loosen the cap to relieve pressure causing it to not overheat. This method works fine when there is water in the system but of course the water soon boils out. When I put dex-cool in it overheats even faster and spews the coolant out. why could this be? I am so confused as to what is wrong with this veihicle. do you have any ideas what it could be?

Answer
What you have is the common fault of combustion chamber pressure getting into the coolant passages causing the loss of coolant flow.

This IS a very very common fualt with this engine as the steel head bolts have stripped out the aluminum head bolt threads in the engine block.

You need to drill outr all the head bolts and install steel heli-coils in the engine block and install larger diametr head bolts and of course, Replace the head gaskets.