The head gasket has no leaks and no oil and antifreeze mixing. It did overheat but as soon as temp gage went up i turned car off and it cooled down instantly. Literally the temp gage went down as soon as I parked it.

Here's the story. My heater core went. We replaced the heater core and thermostat and we let the car sit and idle to get air out of the coolant system. Air seems to leave and it quiets down. We then took it for a ride and it instantly overheats, coolant bubbles and steams out of reservoir. We did th...