MR2 Mk1 Cooling System
by Nick Challoner
Introduction
The MR2, in common with many other mid-engined cars, has a more complex coolant system than front-engined cars. This is primarily because
the engine is in the middle and the radiator is at the front. Hence there is quite a lot of piping, and the system needs to be bled
correctly to prevent airlocks and subsequent overheating. Airlocks manifest themselves as not only a drop in coolant (the level should be
right up the filler neck and not drop significantly at all over time), but often as a "bouncing" idle (especially when cold) as the air
passing the coolant temperature sensors give a false reading and the ECU alters the fuelling etc. accordingly.
Additionally, the level in the expansion tank should be kept above the minimum level at all times to prevent air being sucked back into the
system as it cools down. Ideally it should be topped up to maximum when cold, as the level does not vary much even when the system is up to
temperature.
In the following descriptions "right" and "left" side of the vehicle refer to the right and left as seen by somebody sitting in the car
facing forward.
Draining
To drain the system you will need access to the underside of the car. There are four drain valves: bottom corner of the radiator on the
right side, one each near the front of the steel pipes that run from front to back through the middle of the car, and one on the back of the
engine (below the distributor and oil filter). Slacken all the drain plugs, let the coolant drain, then tighten them all. The coolant pipe
and engine drain valves should be tightened to a torque of 17Nm (12ft-lb). No torque setting is given for the radiator drain valve, so
tighten it a reasonable amount. If it leaks after refilling tighten it some more! If it does not stop leaking then the o-ring seal may have
failed, replacements can be bought and fitted.
Refilling and Bleeding
The first task is to slacken the bleed valves approximately three turns. The bleed valves are on top left of the radiator, on the heater
piping in the middle of the front bulkhead (behind the spare wheel), and on top of the thermostat housing on the left side of the engine.
Attach lengths of clear plastic hose to the valves on the radiator and the heater. Originally these clear plastic hoses were supplied with
the car and are stowed next to the heater piping, but over time they can go missing, in which case you can buy suitable piping from car
accessory or DIY stores. Suspend them from the underside of the front boot. Remove the pressure cap (on top of coolant pipe in the right
side of the engine bay) and fill with coolant until coolant flows out of the engine bleed valve. Close the engine bleed valve and continue
filling until the levels in the two plastic hoses are level with the filler neck. Close the bleed valves and replace the pressure cap, but
only to its first stop. This is important, do not close it fully to its second stop at this point. Fill the expansion tank to its maximum
level mark.
Run the engine at fast idle for 3 minutes. Check the level at the filler neck - it probably will have dropped. Open the heater and radiator
bleed valves again and fill until the levels in the plastic hoses are level with the filler neck once again. Close the valves and run the
engine again. Repeat this loop until the level does not drop at the pressure cap.
Now replace the pressure cap to its normal second stop position. Check all the valves are closed and take the car for a run. Check for leaks
from the bleed and drain valves. Check the coolant level in the filler neck when the engine has cooled fully.
Coolant
You will need approximately 12 litres of coolant to carry out a coolant change (the full capacity of the system is 12.8 litres, but you are
unlikely to drain absolutely all of it out). Toyota specify that a "good brand of ethylene-glycol based coolant" should be used. Many
coolants available fit this description. Toyota sell their own coolant called Forlife in two different flavours, the red one is
recommended for the Mk1 MR2. This is somewhat expensive, but it does have the advantage of being pre-mixed to the correct concentration. If
you are mixing your own coolant do not use hard tap water. Use strained rainwater instead, to prevent the build-up of lime scale.
Temperature Gauge and Thermostat
The temperature gauge should rise gradually to the around half-way mark, and not move significantly from the half-way mark when the engine
is up to temperature. Any fluctuations (particularly during warm-up) may indicate a faulty or weak thermostat. If this is accompanied by
overheating and no leaks or airlocks can be found, then the thermostat may need checking/replacing. It is important that the thermostat is
installed with the small bleed valve aligned with the mark on the housing, otherwise it may cause overheating and other problems. Some
people report problems with non-Toyota thermostats, so it is probably worth paying a little extra for a Toyota thermostat.
The basic opening and closing action of the thermostat can be checked by dunking it into a pan of hot water that is being heated. At around
80 degrees Celsius the thermostat will open. As it cools again it should close at around the same temperature. When doing this make sure
the thermostat does not touch the hot surface of the pan, and obviously a thermometer is useful for measuring the water temperature. If
the thermostat does not open and/or close as described, replace it. Note this test only indicates whether or not the thermostat opens
and closes correctly, it does not indicate how good a condition the mechanism is - some users suspect that their problems have been due to
the mechanism not being able to hold the thermostat open or closed effectively. This is reported in some non-Toyota thermostats, old thermostats,
or even ones clogged with a foreign body.
Expansion Tank Cap
From the factory the expansion tank was fitted with a white plastic cap. This hardens with age and cracks, which can lead to loss
of coolant from the tank and piping. A replacement flexible black cap can be obtained from Toyota, complete with piping for the extortionate
sum of around £15.00.
Credits
As well as my own experiences, this article is written with material provided by Alan Head and Tim Morton on the IMOC-UK mailing list, the
Toyota 4A-GE engine repair manual, and countless individuals that have posted on the IMOC-UK list over the years.