First let me state I've been an Auto Tech since about 1967, so this is kicking my butt. I've replaced the water pump, radiator, lower hose, thermostat, fan sensor, coolant sender, engine temperature sensor, I've checked it for a head gasket breach (fluid stayed blue), I've bled the air out from every place I could find but feel there is still a pocket of air in the system. This is my girlfriend's car so right now I look like a real idiot.....please someone help me....(I don't look good with egg on my face).....(oh, and I've sprung for all the parts so far....this is getting expensive....HELP!)
Well the new radiator came with a new radiator cap...and it still did it's thing. That being said, I got tired of monkeying with the thing and just tore it apart. I found the cylinder head was warped, the gasket was fine but the multi- layered gasket showed antifreeze between the two center cylinders and when I put a straight edge across it, there was a ,006 deviation in that spot. Not enough to suck fluid into the chamber and cause white smoke, but enough to push a bit of pressure into the cooling system and mimick a blown head gasket. It,however also allowed for the pressure to stay in one place and blow back into the cylinder without pushing into the crankcase to cause milk chocolate oil. It created a hot spot in the water jacket that didn't get pushed away by the water pump.... Yeah, I'm good!...(but this little rice burner gave me a run for my money)!
You've been a auto mechanic a long time. You know you are good. But even the best people at what they do can sometimes look past the easy "stupid" stuff (the stuff, not you). The radiator cap? If it was not replaced with the radiator, that old cap may cause a pressure drop and so it boils over at a lower temperature. We have all done this once in our lifetime. Check to make sure the new thermostat was installed facing the correct direction.