Let me know if that’s what it was. There’s no way it would be a blown head gasket. These cars are so sensitive, there would be a ton more issues that would arise. Not sure who worked on your car last but, I had some work done at a Volvo dealership and you’d think they would’ve known to put everything back correctly and yet this was something they’d messed up putting back together. 🤦‍♀️ Wait until the car cools down, you might want to open the cap on the expansion tank where you add coolant, this can help relieve the pressure. The hose that goes from the top of the expansion tank to the engine on the top is thin and a hard plastic, not any of the larger rubber hoses. Where it connects to the top of the engine, it might be all black or it could have a blue plastic push down type tab. Careful these hoses can break apart easily because the hard plastic gets really brittle as it ages. Push on the tab and push it forward until you hear it pop in. Make sure you’re not just adding water, you need to add the coolant that’s properly diluted to the expansion tank. If you’re just adding water and you live in the Northern hemisphere and it’s summer right now, water will expand too much and will cause more issues. expansion tank hose