Tags:FAQMR2 Mk1 Cooling System
by Nick Challoner
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.
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.
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
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
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.
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.
Toyota themselves say any good quality ethylene-glycol based coolant can be
used. Up to you, but the second option is half the price! Finally, the
correct way to bleed/refill the Mk1s cooling system is described in the
The MKI Overheat FAQ
MR2 cooling systems failures are almost always associated with air entering
the cooling system. The typical symptom is regular, erratic idling. The
tachometer will bounce rhythmically (especially when the engine is cold).
This is usually coupled with random overheating either at highway speeds or
immediately after slowing down from highway speeds.
If you are caught in traffic while overheating, turning the heat up
full-blast and holding the engine at about 2000 RPM will help a little bit.
Directing heat away from the engine and into the passenger compartment seems
to work towards keeping the temperature gauge out of the red.
Regularly overheating your engine for extended periods will almost
inevitably lead to a blown head gasket (which will set you back about $1500
US). So, if you are having problems with your cooling system, it\rquote s
very important to make the necessary repairs. Unfortunately, mechanics love
to diagnose an overheating MR2 as having a blown head gasket. This is
usually not the case. It\rquote s important to pursue the other alternatives
before turning your hard-earned cash over to the dealership. If it turns out
that the head gasket must be replaced, a more economical alternative may be
to replace the entire engine with a used low-mileage engine from Japan.
The most common reason for the cooling system to fail is because the gasket
on the radiator cap has failed. This allows air to enter the cooling system.
The radiator cap is a typical 60,000 mile repair item. If you have any
doubts at all about the age or condition of your cap, you should replace it.
This is a simple $5, five minute operation. If the cap is not new, you
should replace it just to be certain that it is not the source of the
The second most common failure is the thermostat. The thermostat is also a
60,000 mile replacement item. A failed thermostat will typically freeze in
the closed position. This will close the cooling lines that connect the
engine with the radiator. The engine will warm up and then quickly proceed
towards the redline of the temperature gauge. If you are inclined, replacing
the thermostat is a fairly straight-forward do-it-yourself weekend project.
If you have a manual, it can be done in a couple hours. It is wise to
replace your thermostat with a genuine Toyota part from the dealership
instead of an after market version. Toyota has several thermostats for the
MkI. The part number for the thermostat is dependent on the build year AND
MONTH for your car. I have had unpleasant experiences with installing and
removing after market thermostats that completely failed to do the job.
If air has entered the cooling system because of a failed radiator cap or a
thermostat replacement operation, you will need to bleed the cooling system.
The bleeding process will flush the air from the system. Toyota has an
elaborate method for how this is supposed to be handled. The Owner's Manual
and the Repair Manual both outline this process. In the general case, the
air can usually be removed from the system by running the engine for several
minutes without the radiator cap. Continually add water/anti-freeze as the
water level drops. You can see the air bubbles coming to the surface. Once
the bubbling stops and the water level remains constant, the system is
probably clear. Failure to completely bleed the system may lead to serious
Toyota recommends replacing the coolant every three years. Some MR2 owners
believe that this should be done every year. If rust and debris builds up in
the system, the lines from the engine to the radiator can become restricted.
This leads to very expensive repairs. Other MR2 owners believe that the
coolant should never be replaced. There are those that believe that flushing
the system can actually do more harm than good. Flushing the system could
introduce contaminants that would lead to a blown head gasket. One possible
theory is that high-mileage cars that have not been flushed regularly,
should not be tampered with. The quality of the water that is used to flush
the system may also be relevant. Some people claim that using distilled
water instead of tap water will minimize the contaminants that enter the
Flushing the coolant is a moderately complex process that should not be
taken lightly. Consult the manuals. NEVER ALLOW AN UNDER-PAYED OIL CHANGE
FLUNKY TO FLUSH THE SYSTEM. The MR2s cooling system is unusual. The engine
is at the back and the radiator is at the front. The system must be treated
with care. Either flush the system yourself, take it to the dealership, or
use a competent mechanic that has experience with the MR2. If you flush it
yourself, please be careful with the anti-freeze. Anti-freeze is extremely
toxic (especially to pets and small children).
A more obscure source of overheating is a failed air bleeder valve. The air
bleeder valves are used to allow air to escape when the cooling system is
being filled. If one of the two O-rings on a valve fails, air will be able
to enter the system. When the valve is closed, air should not be able to
enter. If the rear air bleeder valve fails, coolant will be able to travel
from the thermostat housing directly to the coolant overflow bottle. Under
normal circumstances, coolant should never pass from the overflow bottle to
the thermostat housing. Replacing a damaged air bleeder valve is a trivial
and inexpensive operation.
Visit Ron's pbase page at http://www.pbase.com/aw11mr2/mki_cooling_system for detailed pictures of an entire AW11 cooling system assembled and set up outside of the car. The cooling system is very complicated but very easy to make sense of when you see it outside the car.