Need some opinions please....within a minute or so of stopping my 99 S80 in my driveway this afternoon, the coolant began belching up into the expansion tank. Within another couple of minutes the pressure built up in the tank and the coolant began spewing out from under the tank cap. Oddly, on the way home, the temp gage never indicated overheating, but the low collant warning light did come on. I've been losing coolant for a couple of weeks, but have not noticed a leak until today. What would cause the coolant to run out from under the cap? Shouldn't any expansion of the coolant go back into the radiator?

 

There is a tool to test for "block" leaks at the coolant bottle to see if your head gasket/head has problems. see napa, or other...