Another 2 cents:
 
http://www.aa1car.com/library/poor_fuel_economy.htm
 
"If the thermostat does not close tightly or does not close at all, coolant will be circulating while the engine is trying to warm up. This will prevent the engine from warming up quickly, and it may never reach normal operating temperature. This can delay the powertrain control module from going into closed loop operation, causing a rich fuel mixture and poor fuel economy."

May have found a replacement from Autobarn. Price isn't listed, but it looks right - 2 pin, green..

https://www.autobarn.com....rature-sensor-sp112730

This is the thing that's jammed:
 
http://www.ebay.com.au/itm/Genuine-OEM-Toyota-Thermostat-MR2-SW20-3SGTE-GEN-III-Engine-/141833901003?hash=item2105f593cb:g:DLEAAOSwMmBV4vQL
 
It's just a little valve that opens when it's hot, and sits somewhere just above the water pump usually where there's a bend in the pipe. I have no idea if genuine is better, but you can get cheaper ones.
 
Edit: they are a bit more complex than that actually, they are more of a heat valve that varies the flow of coolant, it's not like an on/off switch. Also when they are cold they allow a tiny amount of coolant through a little hole, I think that's to reduce wear on the water pump when it's cold.
 
Edit2: it's kind of an important piece of the engine, it has to be designed exactly right to keep the engine at the right temp. So I would assume genuine is always better. It's one of those things that COULD be made electronically but that would probably be a disadvantage with reliability and cost.

I was wondering why it was so expensive on ebay, and yeah you usually need a gasket I forgot that, they go hard and usually crack when you open it up.
 
Also I remembered why they have a small hole in them, it's to let the coolant flow a small amount so it doesn't get heat locked inside the engine and boil.

Also finally found the table.. sucks my PDF version of the manual doesn't have a search function, had to scroll page after page to find it haha. So I'm not the brightest tool in the shed - I want to test the resistance from cold, make sure it meets 10 - 20 kiloOhms then as the car warms up, the resistance should diminish? 

Also how would I test it? Just the positive probe into the back of the positive pin/negastive into the back of the negative pin?

You have to be careful testing resistance, you can't do it while it's connected. If you unplug it while it's running I think it will throw up an ECU fault code and possibly put it into "limp" mode, so I probably wouldn't do that. Nothing to stop you from running the engine until warm, then switching it off and quickly unplugging the cable to it and then measuring the resistance. Just don't measure resistance where there's 12v power.
 
The other way I mentioned is to set your multimeter to volts mode (usually 20 volts for a car) and get a voltage from it. Don't stress about where you stick a digital volt meter, you can practically put the two probes anywhere without upsetting anything as digital voltmeters have a huge resistance, they don't really appear as anything to a circuit.
 
To get some sort of information out of it though, you'll need to figure out the reference voltage MR2s send to the sensor. I would guess they send 12v, but don't quote me. Then apply ohms law and you can figure out the resistance.
 
Does it have two wires going to it or one wire? If it's got two wires then hopefully one wire will read 12v and the other wire will read the output of the sensor. It's kind of hard to explain it all, to measure the voltage of each wire you put one lead on the terminal and the other lead on ground somewhere, usually right where the sensor screws in.
 
Hope that makes sense.
 
 
 

So I went and took a look at my gen 2 JDM 3S-GTE engine, and the ECU temp sensor is indeed a two wire. I have no chance of fitting my hands and the multimeter probes in there so next best thing is to check it from the ECU terminals. Supposedly it's terminals C4 and C9, brown and red wires. Now I just had a look and it seems each different generation had different wiring to the ECU so I'll give it a go on mine supposedly a gen 2.

Mine was 741 ohms which was about 50 degrees according to your chart. About right since it was cooled off from driving a coupe of hours ago. I tried getting voltages but failed, I'll have to play with that some other time.
 

Hoonsy
Also how would I test it? Just the positive probe into the back of the positive pin/negastive into the back of the negative pin?

 
I couldn't fit my hands in there so I unplugged the C connector of the ECU (the middle one) and stuck the multimeter across red and brown according to my pinout here: http://imgur.com/6hGuEOc
 
Edit: forgot to mention my ECU was getting a battery in voltage of 10v :/ might have to check that out some time, I might have some dodgy wiring as mentioned in another post I read where the early gen wiring harnesses had corrosion to the ECU.