The head gasket was blown along the firewall (exhaust) side - gasket just deteriorated after XX amount of years / miles. I'll post a pic later. It's pretty impressive considering how much actual gasket material is missing versus the symptoms of the car.
Car did not smoke white.
Car did not mix coolant / oil.
Leak down test on all cylinders showed no leaks (tested cold)
Compression test was within specifications, no anomalies.
Driving the car under boost would cause the condition (over heat) to appear faster - driving it like a grandma, the car would behave normally.
Car would pressurize the cooling system after it was at operating temperature and under boost - ultimately causing premature failure of the radiator and heater core.
Overflow tank would bubble after shutting the car off: but not every time.
Crappy aftermarket ECT has already failed since the original post. Replaced with OEM Volvo.
It wasn't until the heater core finally blew up under heavy throttle and sprayed coolant into the passenger footwell that I just up and decided to tear the head off. There were just too many coincidental failures within the cooling system itself to attribute the failures to anything other than a bad head gasket, despite the car appearing to run normally.
Lesson learned
Unfortunately it shows how robust the design of the cars is. Even with a breached head gasket, they run and perform for a long time before they send a catastrophic signal to "fix me". And as long as it's heeded before major damage occurs, make it right and press on for another 100k. Thanks for the update, let us know when your motoring again.
