Hey guys, I'm a kinda expert on this thang now! Had overheating for about a month and changed thermsotat etc. Wanna say some important things here, cos I've been there, okay? Hopefully all sorted now, but some of this is CRITICAL if you get overheating!
1. Change the therm. Can't do any harm
2. BLEED PROPERLY. I take up to an hour bleeding, and it's a critical process. Don't rush it and don't think it's done until you've done it again! I'll go into detail below!
3. If your heater isn't working well, it's a sure sign on the MR2 that it hasn't been bled properly, or there's air in the system.
Now, bleeding goes something like this:
Car on level ground. Put the heater control on the HOT position. Fill the coolant to the top of the filler. Attach clear bleed hoses to the front heater bleed point and the radiator bleed point. Attach the other end of hoses to the hood, so they're wayyyyyy above the height of the engine. Open each bleed valve (two remember) at LEAST 3 TURNS. You'll see the coolant creep up the hoses, but don't take any notice of this at present. What you're looking for is coolant creeping up BOTH hoses, equally, ALMOST TO THE TOP and STAYING THERE, without falling back, and without air bubbles rising...
Put the cap back on the filler and half turn it to it's first position. Start the engine and run at fast idle (3000 rpm) for tow minutes. You can see the hoses from the driver seat and will see the coolant rise and fall as you work the accelerator pedal. After two minutes, switch off and refill the filler to the top. Replace the cap again and run at fast idle for another two minutes. Repeat this for about an hour (which is what I did) until the coolant gets to almost the top of the hoses and stays there without falling back. It takes some work, and you'll see air bubbles in the tubes. You don't want air bubbles, so this is an important process. Once the coolant rises in the tubes, stays there without receding, and no further air bubbles are rising, you SHOULD be okay. Also, while sitting in the car, you should be able to hear the system burbling and bubbling, as air is forced through the system and out.
It also helps at some stage during this, if you raise the back of the car, so the hood points slightly downwards. This helps the liquid get from the back to the front.
Once all is okay, you can tighten the filler cap fully and the bleed valves, remove the tubes and run the car for 10 minutes or so, to check the level in the filler doesn't fall.
One noticeable sign that the car is bled properly is the fact the radiator gets VERY hot throughout its mass. The radiator fan should cut in somewhere along the way, and the fan will ONLY work when the radiator gets hot. So if your fan doesn't cut in (and it should) the radiator isn't bled properly or is blocked otherwise. The Stainless pipes (two) that run into and out of the radiator should also be VERY hot - another sign it's bled properly. Your heater should be throwing out hot air too when the engine is running - yet another sign.
The main point here is the bleed tubes and the height of the coolant in the tubes. It needs to stabilize right up towards the top of the tubes and no air bubbles. This is the best pointer I can give - and I spend an hour doing this process, as I say. Don't cut corners, but some people will bleed it quicker - if they rev the car more, for example.
As I say, critical critical process, and most MR2's will overheat if not bled properly. Hope this helps, guys, but let me know if I can help further. Good luck