Cleaning, Flushing & Filling the Supra Cooling System

By John Cribb

Materials Required:

Optional Parts:

Tools Required:


This is an environmentally sensitive job. You should recover old coolant and dispose of it properly and responsibly. Flushing everything down your driveway into a storm drain in your neighborhood is not a responsible way to dispose of the coolant, and may earn you the wrath of your neighbors, a fine from your city, or worse….

Keep small children and pets away from any spilled coolant as it is toxic and can cause blindness or death in small animals when ingested. Minimize its contact with your skin, and flush spills on the car’s paintwork immediately.

Removing a radiator cap from a hot engine, or draining hot coolant is very dangerous. You can be seriously scalded or burned. Always allow the engine to cool down an hour or so before removing the cap. Never stand directly over the radiator cap when removing it, and take the necessary precautions to shield your hand/wrist/arm. Always use thick rags over the radiator cap to contain the release of vapors or liquid coolant under pressure.


Why the Optional Parts?
Cooling system flushing and refilling is an ideal time to replace old radiator hoses. If you have a high mileage, older Supra and the service history is unknown, you should consider replacing these hoses. Lower hoses are known to collapse internally, causing pump cavitation and strange overheating problems, while the older style upper hoses are known to sag too close to the serpentine belt, and may eventually be rubbed through by it. The 1998 model upper hose is molded differently to avoid this problem and it is highly recommended as a replacement for the earlier hoses.

The thermostat and cap replacement are a good idea if you’ve owned the car more than a few years and you’ve never replaced them. For reasons to consider a higher pressure cap, or a lower temperature thermostat than the OEM models, see this area of the FAQ.


Why Toyota Coolant?
Toyota brand coolant is recommended for the Supra, and it’s not significantly more expensive than the discount store brands. If you insist on saving a few dollars and buying coolant that’s on sale at Wally World, just make sure it contains no silicates or other additives. Also, make sure you are purchasing an ethylene glycol based coolant, not propylene glycol.


Why Use Distilled Water, when the Toyota Dealer Said Normal Tap Water is OK?
Why are you doing your own flush instead of paying the Toyota Dealer to do it? You want it done right, right? Tap water contains minerals, heavy metals, chlorine, and possibly either very high or very low pH. All of which is potentially harmful to the soft metals in the Supra engine (the aluminum head & water pump for example). Distilled water has had all the minerals, ions and harmful materials removed, and its pH is neutral. For a more complete discussion on this see this area of the FAQ. In addition to all the logical reasons to use distilled water, it’s also recommended in the Factory Service Manual for the Supra and other models with aluminum components.


First thing to do is fit the Prestone flushing tee in place of the factory installed copper nipple in the heater supply circuit. This work should be done with the engine cold. The heater hose that you want is the odd looking one that makes a 180 degree loop over the rear of the coil cover. Look closely at this hose and you will see a set of banding strap type clamps which are held together with cotter keys. Remove these cotter keys, remove the clamps, and hold one hose still, while trying to rotate the other hose to break the grip of the rubber against the metal nipple inside. It’s best to cover the hoses with rags, and use channel-lock type pliers to do this. Once you have one hose free of the nipple, repeat the process by holding the nipple and trying to rotate the other hose. Eventually it will come free – don’t worry about bending or deforming this nipple, you’re going to throw it away, anyway. Try not to damage the hoses, as they are difficult to replace.

With the nipple removed from both hoses, slide a worm clamp over each hose, then determine which of the Prestone flushing tee’s will be a good tight fit in these hoses. Insert the tee into each hose, and twist it to fit snugly all the way down. Try to keep the tee pointing “up”. Tighten each worm clamp down snugly (not so tight that it cuts into the rubber hose), cap off the tee with the provided cap in the flush kit, and you’re done with this part. See photo:


Cleaning the System
With the engine cold, open the radiator cap and inspect the coolant level to see if the BG Flush can be added. Normally, the coolant level is right at the cap, so you will probably have to remove some coolant. An easy way to do this is “gently” squeeze the upper or lower radiator hoses, which will cause the coolant to overflow the top of the radiator. Be sure to have your catch pan underneath the car to catch this overflow. 

Pour in the coolant flush, put the cap back on the radiator, start the engine and go for a drive. Have the heater turned on and set to its warmest setting during this time. Different flushes require different times, however 3-4 hours driving or running time should be sufficient for the flush to do its work.

Bring the car back to your work area, put it up on ramps or jack stands, and remove the forward engine cover which will give you access to the underside of the radiator.

Get the car off the ramps or jack stands and, if you have an inclined driveway, park it on the incline so the front of the car is lower than the rear.

Wait an hour or two for the engine to cool off before proceeding.

After an hour or so, feel the top tank of the radiator. If you can touch it, and it’s not uncomfortably hot, you can proceed “cautiously”.

Use a thick rag or old towel, folded over several times, put it over the radiator cap; slowly and carefully open the cap a quarter turn. This will allow pressure to escape. Make sure the rag is 3-4 layers thick, and it covers the radiator cap well. Do not stand directly over the cap. As soon as you hear pressure escaping let go of the cap, and move back as your hand/wrist/arm may be scalded by the hot vapor escaping.

After the system has depressurized, wiggle the cap to confirm there is no trapped pressure. Pressing down firmly on the cap through the rag, unscrew it another quarter turn and remove your hand again quickly, in case there is any trapped pressure left. Now remove the cap fully.

Get under the car and locate the drain cock for the radiator (lower driver’s side). Fit the 3/8” fuel hose on it and direct the other end of the hose to your drain pan. Open the drain cock, being careful to guard your face & eyes in case the fuel hose should fall off, or leak.

Open the Prestone flushing tee that you installed earlier. The combination of the open radiator cap, and the open flushing tee will allow the system to drain faster.


Clear Water Backflush
Once the system has drained fully, now is the time to either 1) back flush with clean water, or 2) refill the system with plain water and add neutralizer if your flush requires to be neutralized. Assuming you purchased a one-step flush product, you can now fit the special garden hose adaptor that came with your Prestone kit, to the flushing tee, connect the hose to the adaptor, and turn the garden hose. Leave both the radiator uncapped, and the drain open. Don’t try to catch the overflow of this operation as it will be more than a few gallons of dirty water. Let it flow into your storm drains, but flush the area where it drains thoroughly afterward.

With the water steadily overflowing the radiator, start the car, and turn on the heater to its highest setting. Since you are constantly adding cold tap water to the system, the engine may not warm up quickly and the thermostat may not want to open fully – you can help the process along by blocking the front of the radiator intake with some cardboard. Watch your temperature gauge, and ensure that it gets into the correct operating range, and the heater is putting out plenty of heat, but do not allow it to overheat with an open radiator cap! It’s not necessary to “race” the engine during this process – running it at normal idle is fine.

When you see that the flushing water is running clean, turn the engine off, but allow the the flushing water to continue running another 15-30 minutes to ensure an even cool down of the engine.


Optional 2nd Flush (Vinegar Douche)

This step is optional, but some MKIV members report good success with it.

Turn off the hose and remove it from the flushing tee, but keep the tee and the radiator drain open to allow the system to drain thoroughly. The next step will require you to start fitting new parts, and may require the car to be on jack stands, so you may wish to keep the clear water in the system long enough to start the engine and move the car to your normal work area if you intend to put it on jack stands. Don’t run the engine without some type of coolant in the system.


Fitting the New Parts
Once the system has drained, and you have good access under the car, you can begin taking off the old parts and putting on the new.


Refilling the System
Now is the time to decide how much coolant you need. See <this area> of the FAQ document for reasons why you should run the minimum amount of coolant required for your climate. Here is a table giving the boil & freeze points for different percentage mixes of coolant and water:

% Coolant Freeze Boil
20% 16 253
33% 0 256
40% -12 260
50% -34 265
70% -90 277

  Here’s the table expressed in fractions of the gallon of coolant that you will add, based on the 10 quart capacity of the OEM cooling system. These are approximate percentages, so leave yourself some reserve protection. If you had a 23 degree freeze last year, don’t just put ¼ gallon of antifreeze in!:

Amt of Coolant Freeze Boil
¼ gallon 22 245
½ gallon 16 253
¾ gallon 2 255
1 gallon -1 260

First put about a half gallon of the distilled water, and the pint of Redline Water Wetter, then add the amount of coolant that you need for freeze protection. Continue filling the radiator with distilled water until it either overflows the radiator cap, or the flushing tee. At this point, cap the flushing tee, and prepare to start the car. Check over the engine bay to be sure no tools are in the way, check under the car for leaks and to be sure the new hoses are snug. Now start the engine – the level in the radiator should drop, so add distilled water until it is at the filler neck, then cap the radiator. Inspect all the new hose connections, radiator drain cock, etc. for leaks. Let the car run with the heater on until the engine comes up to operating temperature. Add distilled water or coolant to the overflow tank until the level reaches the high level “Hot” mark.

Let the car cool down for an hour or two, then carefully open the radiator cap, using the same precautions as before, and inspect the coolant level. If it is not at the radiator filler neck, add distilled water to bring it up to the neck. Replace the engine undercover, and remove the cardboard in front of the radiator if you used any.

It shouldn’t be necessary to reopen the system and add coolant or water again. Any entrained air in the system should purge itself out over the next few warm-ups and cool downs. Monitor the overflow tank and keep it topped up.

Clean up any coolant spills, and bring your old coolant to an automotive shop to be disposed of properly.


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