There's no easy way to test a thermostat. A former coworker was warned by three of us when he put one in a soup can, then heated it with a torch to see at what temperature it opened. The new thermostat had already solved the no-heat complaint. That stunt was the last straw and it got him fired for wasting so much time. (This was not an isolated incident).
Thermostats can fail by sticking open or closed, but they rarely close after the engine is warmed up and has been running. I won't say you can't have a bad thermostat, but I'd be very surprised.
Coolant in the oil is not how head gaskets usually leak. 90 percent of the time you'll get exhaust gas in the coolant and a loss of coolant out the tail pipe. When you DO lose coolant and can't tell where it's going, you can add a small bottle of dark purple dye to it, then search a day or two later with a black light. The dye will show up as a bright yellow stain that you can follow back to the source. If you find it inside the tail pipe, the head gasket is leaking.
Thursday, February 12th, 2015 AT 8:16 PM