I picked up the 4runner from a tire rotation and wheel alignment. All normal. After sitting in my driveway for most of the day, I went out to idle her for about 10min to get the engine warm in order to perform an oil change. I went back inside after starting, when I went back out, I noticed a lot of coolant on the ground and the overflow tank was bubbling/boiling. I turned the car off. I went ahead with an oil change. I didn't see any coolant in the oil, vice versa. After the oil change I started the car and a lot of white smoke or steam came out the pipe. I let it idle for a little while longer to see if the temp. gauge would rise, it was normal. After a few minutes the tail pipe went back to normal and no more white smoke. No heat was coming out of the vents when placed on High Heat. Lot's of pressure was building under the radiator cap.

This morning I replaced the thermostat and this seemed to fix the problem. Heater started working immediately. No more boiling over in the expansion tank. While I did lose quite a bit of coolant throughout my diagnosis cycles, I was repeatedly filling it back up directly in the radiator cap. I was also keeping the expansion tank topped off. Since allowing the engine to cool down after the replacement, most of the coolant within the expansion tank has been sucked backed into the engine. This is a good sign. Do you think I should refill the expansion tank back to the full line with distilled water or more coolant?

I couldn't get the exact OEM thermostat because the autoparts store could not cross-reference to the 2L-T diesel, only the gas version. The thermostat I used looks somewhat different, same diameter size and gasket, same temperature setting 88degC, but different length and missing a spring unit on the bottom. I plan on ordering the OEM today and using the truck for the next week as is. Question is, beyond just filling the coolant back to normal, do you think I can roll like this until the new OEM thermostat arrives? This is the part:
(90916-03099 - Genuine Toyota THERMOSTAT)

The temporary replacement is:
http://www.amazon.com/Motorad-240-19.../dp/B000C5I82U

When I do get the OEM and install it, do I need to do a full coolant flush/refill?

Thanks for your thoughts!
- nick