Originally Posted by 1d10t

If it were the thermostat, it would overheat. Just common sense.
Make sure you have all of the air out of your cooling system.
I raise the front of the car up on Jack stands. Take the rad cap off, fill it up. Start your car. Let it run until the fan cycles twice. Which indicates operating temp. While its running keep an eye on it, and keep it full. It will over spill.
After the fan cycles a second time, and the radiator is full, put. The cap on.
Lower the car. Drive and test after cool down.
The cap controls the pressure. Something may be failing on the purge side.
Could also be a head gasket. There are tests you can buy, that test your coolant for exhaust