My Slant seems a little ill

TylerW said

You almost certainly have a blown head gasket. The factory cap is rated for 16lbs so there has to be some serious pressurization of the cooling system to force it into the overflow.

You can test the coolant for combustion gases but I'd go ahead and pull the head to look inside anyway. If it has a steel shim head gasket they can simply rust away and fail without being overheated. Good luck.

Click to expand...

I have filled many an overflow bottle from just low coolant and over-heating. Steam alone can produce a lot of pressure, that got trains to CA. I would certainly do the gas test first. I still have a kit from CA Customs or such w/ test fluid that is supposed to turn yellow if you suck combustion gases from the radiator top. It never indicated anything on any car I tested. Usually a head gasket leak will put messy oil into the coolant, and I mean a thick brown mousse like the La Brea tar pits, not just a thin oil film.