Winter44

Hey guys,

So last week my car's temp gauge pegged without warning. I pulled over, let the car cool, checked what I could, then got it home.
starts fine, warms up fine, then instantly starts overheating. I replaced the thermostat and what I thought was the fix turned out not to be. I dropped it off at the shop I go to and they are stumped. they tried running the car without a Tstat and the whole system pressurizes and the expansion tank boils over with or without. He did a test for the head gasket twice and results came back ok. The fans wont turn on because there isn't enough flow in the radiator and its not getting hot enough.
They've never seen the system pressurize this much. So now they are unsure of the next step and I was wondering if I could gather any ideas of what to check next?

Thanks