I have a 98 S70 with 327K on it. It has been a fanstastic car with nothing but normal wear/tear problems (excepts rear main seal).

Last week, my coolant level sensor went off. I check and my plastic resivour was empty...that has never happened before. I filled it with a water/coolant mix. A couple days later it was half empty. I checked the radiator and all the hoses (and heater core) and there is no leaking visible. There is nothing under the car either.

I started thinking the head gasket might be leaking coolant into an exhaust port, when I saw that during my 1 hour commute on the expressway, there was lots of steam/exhaust coming out of the back of my car even after it was warmed up. Of course it was -8F here in upstate NY today...so lots of cars has some of this...but mine seemed worse.

How can I diagnose and is it safe to continue driving if it is losing it at this rate? How hard is a head gasket to replace (i've done everything on my car except the RMS). I have lots of tools and am usually pretty good at this kind of thing.