Coolant Change
By: Dcpedit 

Introduction

Toyota recommends a 60,000 mile interval (or 4 years) between changing the engine coolant.  While changing the coolant in an MR2 is more difficult than a traditional front engine car, It is not too difficult for a home mechanic to tackle.  The MR2's front radiator and rear engine mean that there are more coolant lines than on a conventional automobile.  The additional difficulty comes from having to drain the coolant from multiple locations and having to purge air from the system to prevent overheating caused by air pockets traveling around in the cooling system.  The MR2 also requires more coolant than a comparable 4 cylinder vehicle.

Coolant mixture with water   *(data from Toyota brand coolant)
% ethylene glycol / % water Freeze-up Protection Boil-over protection
50% / 50% -34 °F 265 °F
70% / 30% -84 °F 276 °F
*Toyota recommends a 50/50 mixture unless local conditions warrant a higher concentration - but not to exceed 70% coolant.
Capacity
10.4 L (10.9 qts)  A 50/50 mixture will require nearly 1 1/2 gallons (5 1/2 qts) of coolant.  Using 1 1/2 gallons of coolant will yield a 55/45 mixture.

Procedure

View of the front hood area with the cover and tub removed.
Step 1

Remove the spare wheel/trunk box by unscrewing the bolts inside the box. It may seem stuck at first, but just pull on it little by little until it comes loose. Once removed, you'll find the service hoses that are used to drain the coolant and for letting air in during the refilling process.

You'll also need to remove the plastic covering from underneath the engine.

Two service hoses are provided and are tucked behind the spare wheel compartment. One hose should come attached to the heater water valve (towards the rear), and the other one will eventually be attached to the radiator valve (towards the front near the headlight). But I would do that later so that you can use the hose to drain the coolant.

This picture shows where the service hoses will be attached when refilling the coolant later on. They need to be raised up above the valve.

Remove the reservoir tank cap in the back, and start draining the coolant. There are 3 areas to drain from: radiator, radiator pipes, and engine. I started with the radiator.

 
Step 2

Underneath your car, below the radiator, you'll find hole near the the edge of the front bumper. Inside this hole, there's a small spout. Stick a tube over the spout so that it will drain cleanly into your container.

 Step 3

The radiator drain plug can be hard to find. It's a plastic wing nut that you can unscrew with your fingers. Looking down the side of the radiator, you might be able to catch a glimpse of it. That white blotch in the picture is the drain plug. 
View of the tube for draining the radiator.

Tip: Use the provided service hoses to drain the radiator and the engine coolant.
View of the tube for draining the radiator.
 
Step 4

If you stick your head down into the trunk space, you can get a better look at the radiator pepcock.  Start unscrewing the plug until the coolant starts draining. If you unscrew it too much, coolant will start leaking out the side of the plug.  I let it drain for a while before moving on to the radiator pipes and engine.

 Step 5

Locate the radiator pipes underneath the car. The rear of the car is towards the bottom of the picture. Unscrew the plugs and let it drain.

Torque: 12 ft-lb (16.5 N·m, 168 kgf·cm)
Another View of the radiator pepcock.
View of the plugs for draining the pipes.
 
Step 6

Finally, locate the drain plug on the engine. Looking up from the bottom of the car, you should see it above the axle. (Note: The Toyota repair manual is absolutely useless when it comes to locating this plug). Again, you'll want to attach one of the service hoses to the spout to allow for a clean drain.

Tip: The nut on the drain is a bit hard to reach from underneath. I found it much easier to screw/unscrew it from above with an extension.

Torque: 9.6 ft-lb (13 N·m, 133 kgf·cm)

Once the coolant has been drained from all three areas, close all four drain plugs.  Dispose of the used coolant in an environmentally friendly manner.  Ethylene glycol is sweet in taste and will poison animals if ingested.  Clean up any spills immediately.
View of the engine block pepcock looking up from just infront of the axle.
 
Step 7

This is the heater water valve.  One of the service hoses is already attached to this valve.  The other hose will attach to the radiator air valve (next picture).

Attach one end the service hoses to the air valves and hang the other ends up somewhere. Make sure there are no kinks in the hose that will cut off air to the valve.

Tip: I used long pieces of tape and attached the hoses to the front hood.

[Editor's note:  It is safest to pre-mix the coolant just in case the filled capacity is not exactly what the published capacity is.  I mix 1 qt coolant and 1 qt water together in a spare gallon jug and use the entire half gallon before mixing another.  Save any excess from the gallon for topping off at a future date.] 
View of the heater water valve.
 
Step 8

Open both air valves and start pouring the coolant into the reservoir tank in the rear.

When the coolant level starts rising in the reservoir tank, coolant will also start rising up into the service hoses. The absolute-level of the coolant in the tubes and in the tank (measured from the top of the liquid to the ground) should be equal.  If the absolute-level of the coolant is not equal, then you have air trapped inside. This probably means your air valves are not open.

Tip: The tricky part for me was figuring out when the air vales were actually open. In my first attempt, I only gave each valve a 1/4 turn, which is not enough. I realized this when the tank was fill up after 1 gallon, and the tubes had no coolant in them. I had to give the valves another 1-2 full twists before I heard a hiss from the air rushing out. So if you're not sure, close the valve during the refill process, and open them again while listening for the "hiss."
View of the radiator pepcock and hose barb for connecting the spare hose to.
 
Step 9

Once the reservoir tank is full, close the valves, and collect the coolant in the hoses. Replace the tank cap, making sure that it's in the locked position.

Start the engine, and make sure that air is bleeding into the reservoir tank. When the engine is warm, the coolant level should be over the FULL line. When it has cooled down again, the level should be between the LOW and FULL line.

Put everything back where you found it, check for leaks, and you're done!

Note: You should check the coolant level in the reservoir tank once or twice during the first 300 miles on the new coolant. 

[Editor's note: Also, fluctuations in engine temperature usually indicates air pockets somewhere in the system.  If that happens, Bleed the system again (steps 7-9).]
The coolant reservoir in the engine bay.