2007 Chrysler 300 boiling over
 

I have a 2007 Chrysler 300 2.7 rebuilt motor rebuild heads new water pump new tstat. The cars coolant system builds pressure, the over flow boils gets hot and a lot of pressure. The fans are turning on. Today I used a combustion leak tester the test showed no combustion in the coolant how ever the coolant started filling up the tester I'm going to replace the tstat again any ideas on what to check for ?