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Changing the engine coolant...


The following 31 steps are a detailed guide for renewing the coolant:


1. Use a high-quality cooling system flush…

Use a high-quality cooling system flush such as Nulon Radiator Flush & Clean (which is non-acidic) before draining out the old coolant.  This cleans all the accumulated sludge and scale out of the cooling system and ensures that heat transfer between the engine and the new coolant will take place in the most efficient manner possible in the future, which ensures a lower engine operating temperature.

The following steps are a guide for using a cooling system flush:

1.1 Decide on an appropriate place where you’ll work on the vehicle…

Decide on an appropriate place for draining all of the engine coolant in later steps and flushing the entire cooling system with a garden hose once the entire cooling system (including the engine and radiator) has cooled down completely.

It’s useful to decide on this place in advance, because it’s necessary to keep the vehicle immobilised with the engine off in order to cool the engine and radiator down so that you can safely work on the cooling system.

1.2 Drain out some engine coolant to make way for the cooling system flush…

When the engine and radiator are completely cold, in order to avoid being burned by hot coolant, loosen the bottom radiator hose and drain out just enough coolant into a suitable container to make room for the cooling system flush to be added in the prescribed volume to the top of the radiator.  (Alternatively, if the radiator has a drain valve, this can be used to drain the coolant.)

1.3 Refit the bottom radiator hose…

Refit the bottom radiator hose and tighten the hose clamp around it where it meets the radiator.

1.4 Add the cooling system flush to the radiator…

When the engine is completely cold, in order to avoid being burned by hot coolant, remove the radiator cap and add the cooling system flush to the top of the radiator in the volume prescribed by the directions on the container for the flush.

1.5 Top up the engine coolant to the required level…

Top up the engine coolant level in the radiator with pre-mixed engine coolant so that the radiator is completely full again and refit the radiator cap.

1.6 Turn the interior heater on to the hottest setting…

Set the temperature on the air conditioning system inside the vehicle to the hottest setting in order to allow the coolant to circulate through interior heating system inside the vehicle as well and flush the accumulated sludge and scale out of it.

1.7 Run the engine for the minimum time as instructed…

Run the engine for the minimum time as instructed in the directions for the cooling system flush.  Because the flush has a fairly aggressive chemical cleaning effect on the insides of the cooling system, including the insides of the engine where it passes over the cross-section of the head gasket and other seals in the engine, I personally wouldn’t run the engine for any longer than the minimum time stipulated on the directions, in order to minimise the chance of any damage to these gaskets and seals.

If your vehicle has a catalytic converter, it’s important not to run the engine at idle speed (ie. while the vehicle is stationery) for more than 20 minutes.  (The catalytic converter is installed about halfway along the exhaust system.  It looks like a muffler but it actually reduces emissions in the exhaust gas rather than reducing noise.)  Under such prolonged idling, the catalytic converter may overheat drastically because of the large volume of unburnt gas flowing into it from the engine and become damaged.

It’s best to actually go for a drive in the vehicle in order to circulate the cooling system flush for the minimum time required to properly clean the insides of the cooling system, instead of just idling the engine while the vehicle is stationary.

1.8 Raise the front of the vehicle in the place you’ve decided to work on it…

After the minimum time instructed on the directions for the cooling system flush has elapsed, park the vehicle in the place where you decided earlier to work on it.  Turn the engine off; apply the parking brake; hold the rear wheels in place with bricks or purpose-built chocks; jack the front of the vehicle up using the middle of the engine cross-member underneath the engine bay as the jacking point and support the front of the vehicle with two axle stands underneath the two jacking points on the chassis rails directly behind the front wheels.  It’s a good idea to leave the jack in place under the engine cross-member as a third means of support and as an added safety measure.

Further information on raising the front of the vehicle safely so that you can work underneath it safely can be found in the section titled, Working Safely Under a Raised Vehicle.

1.9 Spray penetrant liberally on the coolant drain-plug in the engine…

Spray penetrant liberally on the coolant drain-plug in the engine in order to free it up and make it easier to loosen later on once the engine cools down.  It’s important to spray penetrant on the drain-plug now to give it a few hours to free it up while the engine cools down.

1.10 Allow the engine to cool completely...

Allow the engine to cool completely, which will take several hours.  The engine will cool more quickly if you leave the bonnet up (to allow rising hot air to escape) and if you place an electric fan in front of the vehicle, pointing underneath the front bumper bar at the space underneath the engine bay.  By increasing the flow of air in this way, the cooling process is accelerated and will take less time.

The reason for allowing the engine to cool completely is that engine coolant should strictly only be drained from the cooling system (which includes the engine) when the coolant is cold and the engine is cold in order to prevent burning oneself and also to prevent expensive damage to engine components in the form of warping or cracking.

If the coolant is drained when it’s hot and the engine is therefore hot, colder air moves in to replace the drained coolant and can cause sections of the metal components in the engine to cool more quickly than other sections of those same components, which results in the warping or cracking of those metal components.

Certainly the practice of flushing a hot engine with cold water from a garden hose after draining hot coolant will escalate this kind of damage and absolutely destroy the engine.  An engine empty of coolant must be cold before being flushed with cold water (from a garden hose) in order to prevent catastrophic engine damage.


2. Drain the engine coolant from the radiator…

Once the engine is completely cold so that you can place your hand on it and feel it to be cold, drain the engine coolant out of the radiator into a suitable container.

Coolant is highly toxic, so it’s important to dispose of it in an environmentally responsible manner, such as giving it to a professional mechanic to dispose of with other waste from their workshop.

Most mechanics will be happy to dispose of your used automotive fluids if you pay them an adequate cost per litre (cost per quart ) to compensate them for helping you out and if you present the used automotive fluids to them neatly in suitable containers with the type of used automotive fluid they contain written on them clearly with a thick black marker.


3. Remove the coolant hoses between the radiator and the engine…

Remove the top and bottom radiator hoses between the radiator and the engine.

If either of these rubber coolant hoses doesn’t come off easily, it’s best to avoid using a knife to cut through the rubber because of the risk of scoring the sealing surface of the metal stub over which the hose fits, which can easily result in the coolant later leaking out under pressure through the tiny channel caused by the scoring.

A better method of removing a stubborn hose is to heat the rubber hose with a hair dryer or heat gun while avoiding heating the stub onto which the hose is fitted.  This should cause the rubber hose to expand so that it’s easier to remove.

If you still need to use a knife to cut through the rubber, it’s important to exercise a high degree of caution to avoid cutting through to the metal stub and scoring the sealing surface onto which the hose seals.  One way of minimising the risk of scoring this sealing surface is to use a knife with a retractable blade and only allow the blade to protrude a small distance that’s less than the thickness of the hose that you’re cutting.


4. Disconnect the automatic transmission oil cooler hoses from the radiator if applicable…

If your vehicle has an automatic transmission, disconnect the automatic transmission oil cooler hoses from the radiator (which divert the transmission fluid through the radiator in order to cool it down).  Capture the transmission fluid that spills out of the hoses (and out of the metal tubing in the radiator) in a suitable container for later disposal in an environmentally responsible manner.

Cover the open ends of the automatic transmission oil cooler hoses with doubled-over, clean plastic bags secured tightly with rubber bands to prevent any foreign matter from entering the transmission via the hoses and to prevent fluid from continually dripping out.  Alternatively you can plug these oil cooler hoses with clean ear plugs.


5. Remove the radiator from the vehicle and clean it cautiously…

Remove the radiator from the vehicle and inspect its condition.  Using a garden hose with medium water pressure, flush the radiator thoroughly from top to bottom and bottom to top by putting the hose successively into both of the openings in the radiator where the hoses connect and turning it upside down in between.  It’s important to avoid using excessive water pressure when flushing the radiator because it could it.  Flush the radiator until the water coming out of the radiator is completely clear of any trace of engine coolant or rust, scale or sludge.

It’s also worthwhile to use light water pressure to clean the fins of the radiator from any insects or other debris that have become lodged in them.  The reason for using light water pressure is that the fins are quite delicate.  Strong water pressure could damage the fins.

(During the operation of the engine, the fins of the radiator transfer the heat of the radiator to the air flowing through it at the front of the vehicle as a result of the forward motion of the vehicle.  This is how the heat that’s generated from the operation of the engine is dissipated away from the engine to prevent it from becoming too hot.)

Fill the radiator with water and block its openings with doubled-over plastic bags secured tightly with rubber bands to stop the water from leaking out.  Place the radiator aside in a safe place where it won’t be damaged until it’s time to reinstall it.  By keeping the radiator filled with water, the radiator is prevented from drying out which could see scale in the radiator flaking off and blocking the narrow passages in the radiator through which the coolant flows.


6. Remove the plastic coolant reservoir from the vehicle and clean it thoroughly…

Remove the plastic coolant reservoir from the vehicle and flush it out thoroughly using a garden hose with medium water pressure.  If there is significant dirt lining the walls of the reservoir, it will be necessary to remove this dirt with an old dishwashing brush inserted through the opening to the reservoir.  Flush it again when all the dirt has been removed.

Cleaning the plastic coolant reservoir in this way to remove all traces of dirt is a bit tedious.  However, when the vehicle is back in service and you can view the brightly-coloured coolant in the reservoir without any dirt blocking the view, you’ll be glad that you cleaned it and a sense of satisfaction will sink in.


7. Inspect the heater hoses for signs of wear and replace them if necessary…

Inspect the heater hoses between the engine and the outlets to the heater system inside the vehicle which are mounted on the firewall of the engine bay (ie. the vertical panel that separates the engine bay from the interior of the vehicle).

If any of these heater hoses appear to be close to failing and leaking, it’s important to take the opportunity to replace them with new hoses to ensure that they don’t fail and leak while they’re in service.


8. Carefully loosen the nuts securing the metal water inlet in an alternating fashion…

Carefully loosen the nuts securing the metal water inlet to the thermostat housing on the engine (where one of the coolant hoses from the radiator was attached).  It’s best to loosen the nuts just a little at a time, alternating between them until they’re all loose, so as to avoid damage to the metal water inlet such as warping or cracking.


9. Gently remove the metal water inlet…

Tap the water inlet gently with the wooden handle of a hammer to break the gasket seal between it and the thermostat housing.  Remove the water inlet.


10. Carefully clean the gasket surfaces of the water inlet and the thermostat housing…

Inspect the gasket surfaces of the water inlet and the thermostat housing for any remaining gasket material that’s stuck to these surfaces.  Use a blunt, plastic implement to scrape this gasket material off and a clean rag together with some kerosene to clean the surfaces up.

It’s critically important that you don’t use any metal implement, whether sharp or blunt, to scrape remaining gasket material off of these surfaces, due to the risk of scoring the surfaces so that they won’t seal properly with the new gasket, but rather leak coolant.  This is exactly what you want to avoid.


11. Remove the old thermostat, noting its orientation with pen and paper…

Remove the old thermostat, noting its orientation with pen and paper so that the new thermostat can be installed in the same way.  The thermostat is normally installed spring-first into the thermostat housing.  It must also always be installed so that its jiggle pin is at the highest vertical point that it can be.  This allows air, which is lighter than water and rises above it, to easily pass through the thermostat.

It’s necessary to remove the old thermostat before the engine cooling system can be flushed properly with a garden hose.


12. Loosen the coolant drain-plug on the engine and drain any remaining coolant…

Loosen the coolant drain-plug on the engine and allow any remaining coolant to drain out of the engine into a suitable container.

Hopefully the coolant drain-plug will loosen easily as a result of the penetrant sprayed onto it soon after the vehicle was jacked up in step 1.9 above.  If it still refuses to loosen, don’t use excessive force because you might snap the drain-plug in two.  (The horrible ‘sinking feeling’ in the pit of your stomach that follows this kind of mishap is best avoided.)

Give it another liberal spray with penetrant and let the penetrant sink in for half an hour.  If it still refuses to budge, leave it alone and don’t try to remove it from the engine.  You should be able to remove most of the old coolant, as well as any scale and sludge in the cooling system by flushing the engine in the following steps with this coolant drain plug remaining closed.


13. Flush the cooling system thoroughly with a garden hose...

Flush the engine cooling system thoroughly using a garden house with medium water pressure.  Put the hose alternately in each of the two openings to the engine cooling system where the coolant hoses were attached to the engine.  Also, flush the engine cooling system by putting the hose in the opening to the engine where the heater hose was attached.  It’s important to flush the engine cooling system in this manner until all traces of coolant, scale and sludge, as well as the chemical flush used earlier in step 1. above, are removed so that the water coming out of the engine is completely clear.

Ensure that the temperature on the heater system inside the vehicle is still on the hottest setting in order to allow free circulation of water through the heater system inside the vehicle.  Using a garden hose with only light water pressure being applied to the outlets of the heater system mounted on the firewall, flush the heater system until the water coming out of the outlets is completely clear.


14. Refit the coolant drain-plug to the engine…

Refit the coolant drain-plug to the engine if you were able to remove it in step 12. above and tighten it to the torque specified in the workshop manual for your particular vehicle.  If no torque is specified for the coolant drain-plug, tighten it very securely but be careful not to over-tighten it and damage the thread.


15. Install a new thermostat into the thermostat housing…

Install a new, high-quality thermostat into the thermostat housing with the same orientation as the thermostat that was removed from the thermostat housing.  Be sure to install the new thermostat so that its jiggle pin is at the highest vertical point that it can be.  This allows air, which is lighter than water and rises above it, to easily pass through the thermostat.

It’s good policy to fit a new thermostat as well as new coolant hoses, new hose clamps and a new radiator cap when renewing the engine coolant in order to avoid the scenarios where the thermostat fails, a hose leaks or the radiator cap malfunctions while in service.  All of these scenarios can lead to engine overheating and expensive engine damage such as a warped cylinder head and leaking head gasket.

The objective is to avoid the situation where parts fail while in service which requires unscheduled, inconvenient, time-consuming, annoying repairs and the loss of use of the vehicle while the repairs are carried out.  Almost all parts will fail eventually.  It costs less in time and money to replace parts during scheduled maintenance before they fail than to replace them after they fail while in service.


16. Install the metal water inlet over the newly-installed thermostat...

Install the water inlet over the newly installed thermostat and back onto the thermostat housing together with a new gasket in between the water inlet and the thermostat housing.

Before tightening the nuts that secure the water inlet to the thermostat housing, be sure to check the workshop manual for your particular vehicle for the correct tightening torque for these nuts.  Over-tightening these nuts can easily lead to the metal water inlet cracking under pressure which would require a new one to be purchased.  This is obviously something to avoid.

It’s equally important to tighten these nuts in stages and work your way up to the specified tightening torque.  For example, if the recommended tightening torque is 20 foot pounds (27 Newton metres), I’d tighten the nuts alternately until they were firmly hand-tight and then torque them alternately to 10 foot pounds (13.5 Newton metres), 15 foot pounds (20 Newton metres) and 20 foot pounds (27 Newton metres) in three separate stages.

This avoids the scenario of one side of the water inlet being fastened down tightly onto the adjacent surface of the thermostat housing while the other side isn’t bedded down tightly at all.  When you go to tighten the other side under this scenario, the metal water inlet can easily snap in two because it’s essentially bending in the middle.  The metal water inlet obviously isn’t designed to bend.  It snaps instead.  I know because I’ve snapped one.

A healthy dose of paranoia when securing the water inlet to the thermostat housing is good insurance against breaking it and having to invest the time and money in obtaining a replacement (while the vehicle is still jacked up and out of action).


17. Install new heater hoses if necessary…

Install new heater hoses, between the engine and the outlets to the heater system inside the vehicle which are mounted on the firewall, if you decide that the existing heater hoses are in need of replacement.

Smearing a light coat of light-weight grease (such as Vaseline) over the metal stub on the engine and on the outlets on the firewall over which the heater hoses fit will go a long way to making sure that the hoses are easy to remove the next time they’re replaced.  This light coat of light-weight grease will minimise the likelihood of high-temperature, chemical bonding occurring between the rubber heater hoses and the metal stubs, which often makes them very difficult to remove.

When you do eventually come to removing the heater hoses again, you’ll be grateful to yourself for the easy manner in which the hoses slide off the metal stubs after only a little force is applied, as a result of the preventative measure of using light-weight grease on the stubs.


18. Install the plastic coolant reservoir…

Drain any residual water out of the plastic coolant reservoir and install it back onto the vehicle.


19. Install the radiator onto the vehicle…

Install the radiator onto the vehicle, keeping it full of water with its two openings still blocked with doubled-over plastic bags secured tightly with rubber bands.


20. Reconnect the automatic transmission oil cooler hoses…

If the vehicle has an automatic transmission, remove the coverings or plugs from the ends of the automatic transmission oil cooler hoses and reconnect them to the radiator.


21. Drain the water out of the radiator…

Remove the doubled-over plastic bags secured around the two openings to the radiator and allow the water inside the radiator to drain out.


22. Install new coolant hoses between the radiator and the engine...

Install new coolant hoses between the radiator and the engine as well as new hose clamps.

It’s important to tighten the hose clamps very securely so that they cause the coolant hoses to seal properly against the metal stubs over which they fit, but not so tight that the hose clamps pinch the hoses too much.  Any time that you spend making sure that the hose clamps are properly installed is well worth it.

Smearing a light coat of light-weight grease (such as Vaseline) over the metal stubs on the radiator and the engine over which the coolant hoses fit will go a long way to making sure that the hoses are easy to remove the next time they’re replaced.  This light coat of light-weight grease minimises the likelihood of high-temperature, chemical bonding occurring between the rubber coolant hoses and the metal stubs, which often makes them very difficult to remove.

When you do eventually come to removing the coolant hoses again, you’ll be grateful to yourself for the easy manner in which the hoses slide off the metal stubs after only a little force is applied, as a result of the preventative measure of using light-weight grease on the stubs.


23. Ensure the heater system is on the hottest setting…

Ensure that the temperature on the heater system inside the vehicle is still on the hottest setting in order to allow circulation of coolant through the heater system inside the vehicle.


24. Lower the vehicle to the ground...

Jack the front of the vehicle up a little higher than it currently is using the middle of the engine cross-member as the jacking point.  Remove the axle stands that were supporting the front of the vehicle and lower the vehicle to the ground.  Remove the bricks or purpose-built chocks that are holding the rear wheels in place.


25. Add fresh engine coolant to the radiator and the coolant reservoir…

Add the fresh engine coolant that you’ve already mixed in the correct ratio with distilled water to the radiator until the radiator is full, as well as to the plastic coolant reservoir up to the full mark.

When mixing the coolant with the distilled water, it’s very important to use the highest allowable concentration of coolant that the owner’s manual for your particular vehicle and the manufacturer of the coolant specifies for your vehicle.  This will ensure maximum protection of your engine against corrosion and overheating.

For example, if you find that the correct percentage of coolant for your particular vehicle is between 33% and 50%, mixed with between 67% and 50% distilled water, then it’s best to use the highest concentration of coolant of 50% with 50% distilled water for maximum protection of your engine.


26. Run the engine at idle speed with the radiator cap off…

With the radiator cap off, run the engine at idle speed until the cooling system heats up which will cause the thermostat to open and coolant to circulate throughout the engine and radiator.  It will also result in the coolant level in the radiator falling as coolant flows completely through the engine and removes any pockets of air that exist inside the engine.  The reason for running the engine at idle speed with the radiator cap off is to purge these pockets of air from the engine.


27. Stop the engine and allow the cooling system to cool completely…

Stop the engine and allow the cooling system to cool completely.  Since it will take several hours for it to cool completely, it’s best to leave it overnight.  Put the radiator cap on to cover the opening at the top of the radiator.

Be sure to leave a note on the driver’s seat of the vehicle reminding you not to drive the vehicle until the remaining steps below have been completed, since there is still more coolant that you’ll need to add to the vehicle in the next step and the automatic transmission fluid level must also be checked before driving the vehicle, if your vehicle has an automatic transmission.


28. Add coolant to the radiator until it’s full…

Once the cooling system has cooled down, add coolant that has already been mixed in the correct ratio with distilled water to the radiator until the radiator is full.  Squeeze the top radiator hose to purge any air inside it and add more coolant if the coolant level in the radiator falls as a result of the air being purged.

Install a new radiator cap onto the radiator.


29. Run the engine at idle speed and check for coolant leaks…

Run the engine at idle speed with the radiator cap firmly on until its highest, normal operating temperature is reached.  This should take about ten minutes.  Thoroughly check for coolant leaks from the coolant hoses and heater hoses and ensure that the engine temperature gauge on your vehicle’s dashboard shows that the engine is operating at an acceptable temperature.

Don’t drive the vehicle yet if it has an automatic transmission because you still need to check the automatic transmission fluid level in the next step as a result of the loss of fluid from disconnecting the automatic transmission oil cooler hoses from the radiator in step 4. above.


30. Check the automatic transmission fluid level…

If the vehicle has an automatic transmission, check the automatic transmission fluid level according to the instructions in the owner’s manual for the vehicle.  If the fluid level is found to be low at the automatic transmission dipstick as a result of the fluid lost when the transmission oil cooler hoses were disconnected from the radiator in step 4. above, fluid should be added through the transmission filler tube where the dipstick normally resides to make up for this loss of fluid.


31. Continue to monitor the coolant level and check for coolant leaks over the next few days of driving…

You can now drive the vehicle.  Over the next few days as you drive the vehicle, continue to check for coolant leaks and check the coolant level over this time to ensure that it’s correct.  It’s well worth having a healthy dose of caution and even paranoia to ensure that the vehicle continues to operate normally after this significant work of changing the engine coolant has been completed.



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Copyright 2016 Andrew Mackinnon.  All rights reserved.