What you are seeing when the t-stats do that, is the car IS overheating, the coolant thats going over the ECT sensor is coming from the radiator so its cool, keeps the temp gauge normal, but there is not enough cool, coolant going into the head to keep it cool, so that coolant is boiling by time it gets the coolant resivoire, and its boiling over.

I wonder if the coolant comes out of the jug, on a cold start, or if the car has to be warm.. That will be the tell all.

 

The coolant only comes out of the reservoir when the car is warm and at highway speeds.

 

if it only does it warm then its probably not a headgasket issue. The thermostat could be broken like tech said. But its not due to pressure, The car is actually overheating and the coolant is boiling out of the jug.

 

There's a really inexpensive and easy way of checking this!
Take the thermostat out, put it in a pot of water and bring it to the boil. You will be able to see if if it's operating correctly and all it will cost you is time..

 

or you could just pay 8$ for a new one. Also his bolt is stripped, so its not as easy for him to remove the T-stat as it should be. But i good mechanic can probably replace that and wont charge you much, Id go order a new bolt tho.. maybe even 2, so that you can replace the existing ones once their out, with the correct ones..

 

I think that's what I'll do. thanks for all the advice.
I will report back with results.