My S80's coollant is bubbling up into my overflow resevoir tank, of coure only after driving awhile. It just started doing this though. I can even rear it bubbling when I'm sitting in my car after shutting the engine off. It's not hurting anything right now, but what would make my coollant start boiling/bubbling back up into my resevoir? If I squezze onto the upper radiator hose (from the rad to tank) the bubbling will stop in the tank. Does it need a good flushing or what??

 

When is the last time you've had a coolant flush?

 

Thermostat migth also be bad.