Cooling System

Most of the energy in petrol is converted into heat, and it is the primary role of the cooling system to keep the engine from overheating by transferring this heat to the air, but the cooling system has several other important functions as well. When the engine is cold, components wear out faster, the engine is less efficient and produces more pollution, that's a fact. The system therefore needs to allow the engine to heat up as quickly as possible, and then to keep the engine at a constant temperature and this is accomplished by the thermostat. The cooling system circulates water through the passage ways in the engine and in doing so the water absorbs the heat that is generated by friction and the combustion process. After the water leaves the engine, it passes through the radiator which transfers the heat from the water to the air blowing through the cores of the radiator and then the cooled water is pumped back into the engine.

Pressure Cap

A 15 PSI radiator cap actually increases the boiling point of the water by about 48 F (27 C). How does it do this? In the same way a pressure cooker increases the boiling temperature of water. The cap in essence is a pressure release valve, and on cars it is usually set to 15 psi. The boiling point of water increases when the water is placed under pressure. In fact for every one PSI the boiling point increases 3.25F / 1.8C. To check the cap is holding pressure is as simple as sqeezing the top radiator hose when the engine is hot. It should be difficult to squeeze the hose, this indicates that the system has pressure, the radiator cap is working and there are no leaks in the system.

WARNING. NEVER EVER REMOVE the radiator cap when the engine is hot & under pressure. The water may instantly boil depending on the temp and cause serious injury.

When the water in the cooling system heats up, it expands, causing the pressure to build up. The cap is the only place where this pressure can escape, so when the pressure reaches the caps rated pressure, the pressure pushes the valve open, allowing water to escape from the cooling system into a reservior. This type of system is called a closed system as illustrated below. In the early days the expanding water was just dumped to the ground, so in this type of system it is normal for the water level to be a little low in the radiator (about 1" from the top) when it is cold. The closed loop set-up keeps air out of the system and when the radiator cools back down, a vacuum is created in the cooling system that pulls open another valve, sucking water back in from the bottom of the overflow tank to replace the water that was expelled. All that is required to check the water level in a closed system is to check the water level remains between the low and max marks on the overflow bottle, according to whether the engine is hot or cold, provided the radiator cap is working correctly.


So then, what is the correct method of checking the water level in a closed system? The last sentence in the previous paragraph is in theory all you have to do, when the cap is new. However just like any automotive component, they don't last forever. If the cap fails to allow the water to re-enter the radiator during the cool down phase then air may enter the cooling system through the cap or the contraction of the water may create a partial vacuum in the system. This may cause the radiator hoses to collapse and will be easily seen if you happen to take a look at the right time. On some vehicles air in the system will cause localised overheating of the engine where an air lock can take place. Not good. How then can we avoid this? Let's assume that the cap is functioning correctly. This is what should be observed in the overflow bottle. With the engine cold the water level will be approx on the MIN mark. When the engine heats up and is under pressure the level should be close to the MAX mark. This is how the cycle continues between cold and hot. Now if the water level suddenly appears below the low mark when cold and is lower than normal when hot, then you have a leak somewhere. What is happening is that the water leaks out under pressure so when the engine cools back down it draws the water back out of the bottle and needs to replace the water lost, so that is why the level in the bottle drops. If left undetected, eventually it will suck all the water out of the bottle and then start to draw air into the system. Consequences already mentioned.

Now lets assume the cap has failed and the system has a leak. Most people just check that there is water in the bottle, big mistake. Lets see why. The water expands to fill the bottle to the MAX mark when hot. The engine cools but does not suck this water back into the radiator. Already we have a problem if you did not notice that the water level was on the MAX mark when the engine was cold. The engine keeps leaking water without you noticing because you were told not to remove the cap to check the water as it was not necessary. All you had to do was make sure there was water in the overflow bottle. Well before to long there won't be enough water in the system to keep the engine cool and so you will cook the engine and fork out a thousand or so to fix the damage caused. In summary. I highly recommend to check your cooling system at the radiator cap rather than the overflow bottle at least weekly. This must only be done when the engine is stone COLD, generally first thing in the morning before you start the engine. Simply remove the radiator cap from the radiator. The coolant should be full right to the overflow pipe. If it is not and you still have water in the overflow bottle then your cap is faulty and needs to be replaced. I have made a diagnostic matrix for easy fault finding at the end of this article.


The thermostat's main job is to allow the engine to heat up quickly, and then to keep the engine at a constant temperature. It does this by regulating the amount of water that goes through the radiator. At low temperatures, the outlet to the radiator is completely blocked and all of the coolant is recirculated back through the engine.

Once the temperature of the coolant rises to between 180 and 195 F (82 - 91 C), the thermostat starts to open, allowing fluid to flow through to the radiator. By the time the coolant reaches 200 to 218 F (93 - 103 C), the thermostat is fully open.


Taking out the thermostat or modifying it as the right picture shows is a bad idea, unless you have serious overheating issues. Fix the problem and then refit the thermostat. Fitting a 160deg (71C) one is also not desirable as the engine needs to operate at the correct temperature to be most efficient. An engine running at 71C wears out quicker. If either of these conditions exist on an EFI vehicle then the mixtures and timing advance will be affected by the sensors reading low temperatures. The result will be richer mixtures.



A radiator is a type of heat exchanger. It is designed to transfer heat from the hot coolant that flows through it to the air blown through it by the fan and by road speed. Radiators are made by brazing thin aluminum fins to flattened aluminum tubes. The coolant flows from the inlet to the outlet through many tubes mounted in a parallel arrangement. The fins conduct the heat from the tubes and transfer it to the air flowing through the radiator. Radiators usually have a tank on each side or top and bottom. The device below is used to simulate the pressure in the system which allows a mechanic to check the system for leaks. You can check your system yourself if your radiator cap is working correctly. Start the engine and leave it on fast idle for approx 15 minutes or until the top radiator hose is stiff to compress. Have a look around and underneath the engine bay for signs of water leakage. The caution here is that you don't know if the cap actually holds 15 psi or not. It may only reach 12psi and may be inadequate pressure to generate a leak that you are having trouble in finding. The device pictured below left, is the only surefire way to find coolant leaks in the system.

The photo above right shows a radiator that has a lot of road kill & debris in the fins. In front of the radiator there is another radiator type device called a condenser which is for the air conditioning system and uses the same principles of a cooling system radiator to cool the hot refrigerant gas as it passes through to the other end. It also was full of similar material. On top of that there is also a turbo inter-cooler placed in front of the condenser. You guessed it, the inter-cooler was blocked as well.

My point here is to say that the passage of air passing through so many radiators must be un-restrictive if any of the devices are to accomplish their job in cooling the medium passing through them. In this particular vehicle, there was overheating problem. The heat could not be dissipated through to the air because hardly any air was passing through the fins and cores. So checking your radiator for blockages in the fins should be one of your checks while looking for possible problems.


Like the thermostat, the cooling fan has to be controlled so that it allows the engine to maintain a constant temperature. Front-wheel drive cars have electric fans because the engine is usually mounted transversely. The fans are controlled either with a thermostatic switch or by the engine computer, and they turn on when the temperature of the coolant goes above a set point. They turn back off when the temperature drops below another set point. Rear-wheel drive cars usually have engine-driven cooling fans. These fans have a thermostatically controlled viscous clutch. This clutch is positioned on the hub of the fan mounted to the water pump. In older cars the fan was solidly mounted on the water pump and the amount of air drawn through the radiator was engine revs dependant.


Cars operate in a wide variety of temperatures, from well below freezing to well over 100 F (38 C). So whatever fluid is used to cool the engine has to have a very low freezing point, a high boiling point, and it has to have the capacity to hold a lot of heat. Water is one of the most effective fluids for holding heat. The fluid that most cars use is a mixture of water and ethylene glycol, also known as antifreeze. By adding ethylene glycol to water, the boiling and freezing points are improved significantly. The temperature of the coolant can sometimes reach 250 to 275 F (121 to 135 C). Even with ethylene glycol added, these temperatures would boil the coolant, so in comes the pressure cap which raises the boiling point of the coolant when it is under pressure. Most cars have a pressure limit of 14 to 15 pounds per square inch (psi), which raises the boiling point another 45 F (25 C) so the coolant can withstand the high temperatures without vapourizing, hence why the cap should never be removed when the engine is hot. Please observe the WARNING at the start of this article regarding this situation. Antifreeze also contains additives to resist corrosion and corrosion is a common fault in todays engines if no inhibitor is added. The following chart shows comparisons of the fluid under normal atmospheric pressure.

I should add that if you live in a cold climate, you must use anti-freeze otherwise you will likely crack the engine block, cylinder head, radiator or other components. Water has a peculiar characteristic, it expands when it heats up and also expands when it freezes. This is ok when it is a liquid and will not harm anything but when it turns into a solid, then it will expand and break stuff. If you are familiar with caravan/motorhomer hot water systems, you may have heard that the system must be drained to avoid damage in cold weather. The reason is the same.

Diagnostic Chart - This chart is a guide only. Depending on the circumstances of when the engine overheats will dictate where the fault may lie.



Engine Overheats (general)

coolant level low (a leak somewhere)

Faulty temperature gauge

Defective water pump

Low system pressure

Radiator grille is dirty

Blocked radiator cores

Improperly adjusted fan belt or belt broken

Faulty thermostat (won't open) 

Inadequately functioning fan

Incorrect ignition timing - initial timing

                                    - centrifugal advance

                                    - vacuum advance

Head gasket leak

Engine runs cold

Defective or no thermostat

Low temp thermostat fitted

Temperature gauge or sender is faulty

System is corroded water contain impurities or has no antifreeze / inhibitor

The following diagnostic matrix is only for closed systems with the radiator cap fitted to the radiator.



Engine Cold - Remove cap to inspect radiator level

radiator is full, bottle on MIN No Problem
radiator is low, bottle on MAX cap faulty -  replace cap, drive the vehicle as normal & recheck again the following day
radiator is low, bottle on MIN cap faulty & a possible leak, requires immediate attention
radiator low,bottle empty a definite leak and requires immediate attention


bottle on MAX normal
Bottle on MIN or empty a definite leak, requires immediate attention
Bottle on min or empty, radiator luke warm or cold NO water in system and engine is cooked

Now for the consequences of having your engine overheat. I define two types of overheating. The first one still has coolant in the radiator, however it has boiled and can be heard bubbling in the overflow bottle, this is a standard overheat. The second I call rissoled. That is when it has gone beyond the point of overheating and all the coolant has been lost from the system because of a leak. There is no water to boil, so there is no sound of bubbling not even steam hissing from the overflow. The radiator is generally cool to touch and the engine has ceased to run and if not it was about to. When you get to this stage you have rissoled your engine and it's all bad from here. Take a look at my other article on Overheated Aluminium Heads in the Engine section. You will then want to take extra care of your cooling system.

Here is just one thing you can do next time you are working on the cooling system. If your car is fitted with factory spring loaded hose clamps, like the photos below, get rid of them. They are reasonable clamps when the car is new but should NEVER EVER be re-used. Use good quality hose clamps that do not bite into the hose when tightened.


Check your system on a regular basis and replace faulty or dodgy looking components when you find them. Don't put it off till next week, it may be too late. Definately use corrosion inhibitor to ensure the system stays clean and free from corrosion. The coolant needs to be replaced generally every 2 years. Keep the radiator fins clean from bugs and insects that accumulate there. Some vehicles require special procedures for refilling the cooling system. Make sure you follow them to the letter otherwise an air lock may develop causing localized overheating and consequent damage may result. Keep at least two to four litres of water in the boot. Make sure you glance at your instrumentation panel temperature gauge regularly. The gauge usually creeps up to overheat slowly, if you regularly check it, you might be able to pull over and rectify it. Here is another trap for the unwary. If you missed your temp gauge going to the red zone, it is not uncommon for the gauge to come back down to near normal after all the water has been lost. There is no water, so the gauge can't measure the temperature, there is no steam either because steam is the vapour from the water boiling. The gauge can't measure the air temperature very well and so in some cases it actually comes back down. If you glance at it, all would appear to be normal without you realising it until, you see smoke from under the bonnet or the car starts running poorly. I am afraid it is rissoled.

What do you do, if the engine overheats? Once you have pulled over, you need to determine if it is overheated or rissoled. If it is rissoled, the decision is easy, TURN THE ENGINE OFF, call a tow truck.

A little additional information on switching off the engine. If there is still water in the system, then once the engine is switched off, heat soak occurs where all engine components will actually increase in temperature causing the water to raise in temperature even higher. This is mostly evident when suddenly you can hear the water boiling into the overflow bottle, probably a lot harsher than it was when the engine was running.

Now, the do or die effort to save the engine from a major overheat. If you can hear the violent bubbling noises at the overflow bottle, then just switch off. It would be rather risky to attemp to refill the radiator. If it has not reached this stage, then keep the engine running and grap your spare water from the boot. All of this needs to be done ASAP. What you are going to do is to top up the water in the radiator while the engine is running, so as to avoid the heat soak situation. Now this can be very dangerous, because the radiator cap has to be removed in order to do this. But wait isn't one of the rules 'NEVER REMOVE THE CAP when the engine is hot'. YES it is, but what choice do you have. In the case of hearing bubbling noises, trying to remove the cap is even more risky. In both cases the water will instantly vapourise and if contact is made with the skin severe burns will result.

If you want to take the chance to save your engine then read on to see how I would do it and have done a couple of times in my life and just only a few days ago of writing this extra addition to my article. Firstly, you need a towel or something that is close to the thickness of one. Place the towel over the radiator cap, keeping it away from the moving fan. Locate the radiator cap under the towel and press on it. Turn it slowly to undo it, while still pressing down. Once it is unlocked, let go and jump back. You will know when to let go because the cap will want to launch itself through the towel. You must be quick, treat it with respect because this instant vapour will scold you. This is why the towel is important. Once the towel stops jumping around, slowly remove it so you can add the water to the radiator, if the cap needs to be removed, grab it with the towel, because it is HOT. Lots of steam may be coming from the opening, try to avoid it while filling through the opening. Pour the water in very slowly. About the pace you normally drink at will be good. DO NOT pour it in quickly, you will crack the block or cylinder head- GOODBYE ENGINE. Stop when you think about 500ml has gone in. If it has accepted that amount without spitting it back out, then pour some more in slowly and again stop at 500ml. Now check the temp gauge and see if it has come down slightly. If it has, your on the right track, keep going repeating this cycle until the radiator is full. Refit the cap. The whole process from starting to fill the radiator to actual full mark should take a few minutes. Hopefully the engine has come down in temperature by now. If there is a leak, you may not get very far before you have to pull over again. The chances of repeating this procedure increases the risk of engine damage each time you do it, so you are probably better off leaving the engine running for another five minutes to let the pressure build back up, then shutting down the engine and look for obvious leaks. Another cause of action may be that the radiator cap is only loosely fitted to the first lock. In this position the system will not build up pressure and this may reduce the speed of water leakage out of the system. What action you take from there depends on what you find and the risk you are willing to take to get to your destination. of course if you have already blown a head gasket, then adding water will be fultile. It won't be accepted and will just vapourize out shortly after putting it in, in this case it is a lost cause, you can only switch off and wait, then try and add water after several hours and start the engine & see if it is running Ok.

Remember I said I just had to do this a few days ago. The lady rang me asking what she should do. I explained the above procedure to her which she carried out. Well done actually, it is a scary thing to attempt for the first time. She managed to refill it and drove it to my place only about three kilometres from where she was. When she arrived the temperature was well over normal but this time I had to do the procedure. During the fill, I had a look around the hoses and spotted where her trouble was. I continued to refill untill full and refitted the cap, loosely. The temperature came down to normal and then I shut down the engine. The heater was disconnected from the engine and these plugs were used to block off the water outlet on the engine. Take note of the left photo. The top of the tubing is bulging a little like a balloon. This is not normal and is a sign of trouble to come. If you have these on your engine and they are balloooned, then replace them.


Let's go back a step to where we are adding water a little bit at a time. If the water starts to discharge back out of the radiator, this is not a good sign. Try adding some more but if it does it again, shut the engine down. What you have here is a blown head gasket at the very least and you will require a tow truck.

I hope this article will be of benefit to you should bad luck strike you unexpectedly.