On the drive back from another state, I stopped for gas and when I came back out, there was a green fluid under the car.

I opened the hood and the coolant reservoir level was at the filler cap because it had just boiled over.

I started the engine and the level went down to the low level line which I then refilled to the hot full line.

After that, everything went fine and is still doing well. Oh...the thermostat was replaced a few months ago.

 

I'd flush the coolant, maybe something it stuck in there

 

I drained and refilled it so now it wants to do that (boil over) all the time whereas it has been fine. Oh well.

 

I opened the petcock again and put the hose in the top and filled until there was a steady stream coming out the bottom. Then I refilled the reservoir. All is fine.