[GrahamBrown1]

Originally Posted by mcfe

Firstly these cars, all of the Volvos, turbo or otherwise often have headgasket issues. Unfortunately they often have warped blocks as well. The turbo petrols also suffer from cracked blocks depending on some things.

Here are a list of possible causes of pressure in the coolant system just from my own observation.

Oil cooler leak
Head gasket(+ inevitably warped block)
Cracked block(thin cylinders in most engines except the 2.3 and 2.0 engines and B5244T5)
Coolant reservoir cap
Thermostat
Failed engine fan(will not cool down car when it reaches a higher temp idling).

Here is some Volvo math since you said temp gauge was at max. The temp gauge of ALL volvos does not move between the range 80-120c, it stays strong vertical.

After 121c or 125c it starts to rapidly climb. Since you drove with less or no coolant and your temp gauge was at max, you likely exceeded 130c or more. At those temps I'd imagine the damage is done. Warped head + warped block.

Time for a block test. If there is head gasket failure, you need to machine both the head and block flat.

Also yes, you tightened it down and the pressure could not escape. Bear in mind that the coolant tank is designed to keep a pressure of 21psi or 1.5 bar. This is because at normal air pressure 100c water boils, but not so under pressure.
If there is excess pressure the cap cannot relieve(or it is broken) it would cause issues.

I’m intrigued how you form the opinion that all Volvos often have head gasket issues. I have owned many as have many others and not had any head gasket issues. The petrols of this era I thought were extremely reliable. The only block issue i have heard about is on the R models but they are a complete different beast. Just out of interest?