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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 problem. 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 engine damage. 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 sytem. 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.
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