I noticed evidence of copper Stop Leak in the previa I just bought. So I decided I'd flush it out today.

Drained the radiator. Then tried the block plug. It was easy to remove, but nothing came out. So I loosened the thermostat cover, and the rest of the coolant drained.

Filled it with water to rinse out the system. But it was overheating. Bottom hose off the radiator was cold. Thinking it might be an air bubble, I tried to refill and bleed a few times. But no luck. Cold lower hose.

Decided to remove the t-stat and replace the cover. Filled it with water, and ran it for a bit. Good coolant flow; lower hose got warm.

Popped the t-stat into a pot of water and turned on the heat. It opened just a little bit when the water was boiling--maybe 1/4 inch? It was also coated in stop leak.

I Am hoping that the t-stat was sticking closed/not opening enough. My wife did say the temp gauge rose when she was driving it (we've only gone about 150 miles so far).

Flushed the cooling system till the water was almost crystal clear by disconnecting the upper hose only.

Going to to get a new t-stat tomorrow.