My '04 XC90 T6 2.9L is having a weird issue. As it starts to heat, the coolant tanks boils. When I stop the vehicle, coolant starts leaking from the front passenger side area. I have read some posts about the cap being bad, but I don't know if that's it. It's not overheating, but I need to add coolant that's been lost to the tank. I'm pretty sure it's not a blown head gasket as that would cause it to completely overheat. Any thoughts on this?