I noticed my car was overheated today, so I pulled over. It only took maybe a minute (with the car off, hood open, and fan on) for the temperature gauge to start falling, so I figured that I probably hadn't done any major damage. I opened the hood and looked in the coolant reservoir, which was almost overflowing! The next time I looked in the reservoir, it was almost empty.
I waited an hour for the car to cool, then added water to both the radiator and the overflow tank-- over a liter. I started it again, and the temperature was normal, so I drove it to my mechanic.
He took one look at the oily residue around the coolant tank, diagnosed a bad head gasket, said it would be $2-3K to fix, and that the dual overhead cam Subarus tend to have that problem.
He advised me to keep the radiator (not the overflow tank) topped up with water regularly, and drive it until it dies (which will be soon).
1) Why was the overflow tank near overflowing when I first opened it up? Did all the fluid somehow get pumped out of the radiator and into the tank? Could this have been caused by a blockage in the system somewhere, which caused the overheating, which caused the gasket to fail?
2) Why will the coolant no longer circulate properly, just because the head gasket is leaking? Why do I have to refill the radiator directly, and not just keep the overflow tank filled up?
3) Is there anything else besides a blown gasket that could be causing the oil in the coolant? (I don't see any smoke from the tailpipe.)
This is a 97 Subaru Legacy Outback, 125K miles.