Esmagamus Guide: Living With You Car's Cooling Circuit

This Esmagamus guide covers basic steps anyone can take to avoid any issues with one of you car's most important systems: the cooling circuit.

Includes:
-Brief introduction to the workings of a closed circuit cooling system;
-Maintenance;
-Troubleshooting;
-Explanation of galvanic corrosion processes and scaling;
-Appropriate coolants;
-Peculiarities;
-Emergency procedures.

Once read, you will have acquired the basic knowledge to keep your cooling circuit in good order. I am available to answer any questions.

The purpose of a cooling system is fairly obvious: the engine produces heat and a certain amount of it must be dissipated. Some cars, such as the VW Beetle and the older Porsches did away with the cooling circuit and used an air-cooling set-up. Most cars use a more convenient water cooling system, one of the reasons being that air-cooled engines work at much higher temperatures, causing an increase in nitrous oxides emissions.


The heat source: an engine has water passages going through the block. The coolant also has to cool the engine head(s) which is usually the hottest part of an engine.

The radiator: usually sits behind the bumper, a know exception being the old Mini that had it mounted to the left side, with a permanent belt driven fan sucking air from the wheel well. Some radiators have a built in expansion tank.

Expansion tank: as the coolant heats up, it expands. The expansion tank has a MAX marking on the side, above which is air. Overfilling of this tank causes excess pressure build-up. This tank usually has a pressure regulating cap. A crack on the cap can lead to loss of coolant or boil-overs. The pressure in the circuit keeps the coolant boiling point higher than 100ºc. This tank may or not be a header tank (the highest point on the circuit, more on that later).

Radiator fan: sucks additional air through the radiator. This may be driven from the engine or, as it shows on this diagram of the Alfa Romeo 164 3.0 V6 circuit, it is electrical and controlled by a thermal switch, the contacts closing as a rated temperature is reached.

Heater core: heats air going to the cabin. In an emergency (the coolant is boiling) it can and should be used as an emergency radiator by setting the cabin heater on max heat and airflow.

Engine thermostat: just like an engine can't run too hot, it also should never run too cold (reasons omitted, trust me). As the engine warms up, the thermostat keeps the coolant from flowing into the radiator. This speeds the warming up of the engine and of yourself in those winter mornings. Usual symptoms of a thermostat stuck open are very long warm-up times, ventilation system blowing cold air (though this can also mean a blocked heater core or one that is filled with air, see peculiarities section), a cold engine at highway speeds, etc. If stuck open, the coolant will simply boil. When replacing a thermostat, consult the technical data for your particular engine.

Sensors and switches: some cars only have a danger light wired to a thermal switch on the pump body. Most have both a thermal switch and a variable resistance temperature sender connected to an indicator. There's still another thermal switch for the fan. As some systems run on two fan speeds, this sender may have two sets of contacts, one activating a lower speed when the AC is on or the engine is getting too hot and a second set of contacts activating a faster "emergency" speed.

Other components:
-Some cars have oil to water heat exchangers. These transfer heat from oil to the coolant. Other designs do away with the heat exchanger and have instead a small oil radiator.
-Belt driven fans are usually mounted on clutches that lock as they heat up. Electric fans have associated  thermal switches, usually closing the power to the coil of a relay that then energizes the fan.
-Throttle bodies are sometimes also connected to the cooling circuit. This also has the function of regulating idle air through a bimetallic valve fitted next to the throttle plate or just prevent icing on the throttle plate; as petrol takes heat as it vaporizes, icing can become an issue on very cold days.
-Turbochargers are frequently water-cooled.

The more heat sources the system has, the more important it gets to keep it dissipating heat at its maximum capacity.

The fact these set-ups are called water cooling systems does not mean it is fine to fill the circuit with tap water.

Antifreeze is usually a greenish solution of ethylene glycol (or a different, pink type, that is completely different and not found on many cars). Note that this is a poisonous chemical and should be handled with care and disposed of properly. Use common sense.

Properties:
-It has a much lower freezing point than plain water;
-It has a higher boiling point than water, thus avoiding loss of coolant as the engine overheats;
-Does not contain dissolved minerals, so it doesn't cause scaling and blocking of cooling passages, radiators or heater cores;
-Inhibits corrosion.

On the corrosion issue, I'll spend some more time as most people don't seem to understand the importance of corrosion prevention.
On systems with different metals (such as iron and aluminium) in electric contact, electrochemical reactions will cause one metal to suffer heavy corrosion while the other suffers no corrosion. Which one gets corroded faster is determined by the electrochemical potential of that metal in relation to the other. All aluminium systems usually suffer more uniform corrosion.

The corrosion inhibitor chemicals on antifreeze "get spent" as they react with free ions causing corrosion of the system. Eventually it will still keep its anti-freezing properties but will no longer protect the system from corrosion. As corrosion particles build up, the risk of blowing head gaskets, heater cores, hoses, etc. increases exponentially.

Changing coolant is a simple operation, it should be done every two years or less if the manufacturer recommends so. If the coolant looks rusty, replacement should be considered.


In emergency situations (stranded in the middle of nowhere), distilled water is acceptable as temporary coolant. It should be used instead of plain water, if available. When replacing, I recommend filling with pure antifreeze (it usually comes diluted anyway) so the anti corrosion effect lasts longer. Note you should never cool an overheated block with cold water, it will crack. Instead, water should be slowly added with the engine running to avoid thermal shock.

On antifreeze concentration

On some markets coolant will only be available pure (for example, 1L bottles of concentrated ethylene glycol for dilution with equal parts of water) and in others it will usually be found in jugs of various concentrations that sometimes are not written on the label. In terms of freeze protection, the more concentrated (up to 50% dilution) solutions are superior. On rust protection, it all depends of the additives added by the manufacturer so do try and buy a reputable brand.
For mild climates with higher minimum temperatures, low concentration solutions are acceptable and serve more to protect the system from corrosion than freezing. If you get the pre-mixed (diluted) coolant, do not add more water.

If you buy the concentrated, dilute only with distilled water in equal parts. Do not exceed 50% ethylene glycol concentration as more concentrated solutions have lower heat transfer.

Things I omitted and were promptly reminded by other users:

- Different coolants should never be mixed as they are chemically incompatible and might even gel. So, never mix coolants of different colour and check if your vehicle requires any sort of special coolant;
- If there is a need to change the type of coolant (suppose the specified coolant is not available in your area), you have to get every bit of the old coolant out by flushing several times with the block drains open before refilling with new coolant (thanks to Phoenix17).

While having a shop manual with schematics for every electric circuit is extremely useful, the sort of circuits that power cooling fans are usually very simple, relying on basic, tried and true, electromechanical components.

Usually, a small control current runs through a thermal switch. When that switch reaches a set temperature, the coil of the fan relay is energized and the relay powers the fan until the contacts in the switch open again.

To test the good functioning of such a circuit, one has to find the fan thermal switch and bridge the two contacts. A relay should then be heard clicking and the fan should run. If the fan has two speed settings, it might have a three wire connection on the motor, one ground and one +12V for each speed or it will have a two wire connector feeding a variable voltage (for example, +9V for a slower speed and +12V for a higher one, plus a ground connection). These two speed systems usually work by putting a resistor in series with the fan motor to give the lower speed. On some cars, these resistances are known to crack or otherwise failing over time.

If the fan fails to run with the thermal switch contacts closed, check the wires for +12V. A fuse might be blown in the relay energizing circuit, the relay itself might have failed, etc.

It is also advisable to test the thermal switch.  A simple setup involves heating the switch with warm water and checking at what temperature the contacts close. Use a good thermometer strapped right to the switch for more accurate results.

As explained earlier, the thermostat guarantees the engine gets up to running temperature faster. This will save fuel, avoid excessive engine wear, assure the catalytic converter gets up to operating temperature faster, guarantee cabin heating in cold weather, etc. It is a vital part of the circuit. Failure will either stop the engine from getting hot enough or from cooling down enough.

To find the thermostat on your engine, follow the large hose that takes hot coolant to the top of the radiator. The fitting where this hose is connected on the engine is usually the thermostat housing, placed over the coolant pump. Removing the thermostat is usually a matter of unscrewing two or three bolts, being careful as these are often corroded in place and might break easily.. When refitting the housing over the thermostat, assure any gaskets are in good condition. If in doubt, seal with a smear of engine silicone. Use common sense and do things right the first time to avoid leaks.


Testing is done by immersing the thermostat in hot water with a thermometer strapped to it. Refer to the technical data book to know at what temperature the thermostat is supposed to open and at what temperature is it supposed to be fully open. Replace if it won't close or open as it should.

When a car leaks coolant from the cap without reaching high temperatures, either there is a leak of compression due to a blown head gasket that pushes coolant out of the engine or the cap just isn't holding enough pressure to keep the coolant from boiling prematurely. In either case, a new cap is cheap enough to replace if there is any reason to suspect it isn't good.

Anyway, a test can be performed with a small foot pump by compressing the circuit until the cap rated pressure is reached. If it is working properly, it won't release pressure until the working pressure is exceeded.

When buying a new cap make sure it's rated for the exact same pressure as the old one. If it's too high, something might blow and if it's too low, coolant will boil and the trapped steam bubbles might displace coolant from the engine block.

Testing is done with a multimeter. The shop manual to your car should include a table of resistance values for each temperature value. Testing should be done with accurate instruments, the procedure is about the same as for thermal switches.

Wiring usually has higher failure rates than senders. Corroded contacts can cause erratic temperature readings. When reassembling any electric contacts in the engine bay, it is good practice to apply some dielectric grease to keep moisture away. It is also vital that the spring holders on connectors are doing it's job. Some engines will stop if the signal from the temperature sensor is cut, as this signal is used by the ECU to regulate the fuel-air mixture.

Whenever coolant looks too rusty or every two years, the whole circuit should be thoroughly flushed. This also applies whenever the cabin heater seems to be heating up too little even if the temperature gauge shows the engine is warm or the circuit just doesn't cool the engine enough and everything else seems OK.

You'll need:
-a hose;
-a place to work with good ventilation;
-pliers or spanners, according to need;
-new antifreeze;
-read peculiarities section.

I will describe the way I do it on the Citroen ZX, other vehicles have characteristics that make this method impractical but the same basic procedure still applies.

-Take the cap out of the fill tank and put a hose with running water in it.
-Undo the block drain bolt if you can reach it. All water drains (there is usually one in the bottom of the radiator) and purge holes (one on top of the radiator or thermostat housing, there might be another one on the heater core hoses) should be open with water running out of them.
-Turn the engine on, more rusty water should come out. Assure the circuit is never depleted of running water as this would damage the engine.  Rev it up a few times until all water comes out clean.
-Close the drains and purge holes one by one, from the lowest to the highest. Close the water flow when you're done, shut down the engine and let it cool.
-Once cooled, drain all the water out an fill with antifreeze.

As mentioned earlier, the expansion tank isn't always a header tank. The Citroen ZX has one of these cases, where the heater core is the highest point on the system. To bleed the system, a "header tank" consisting of a bottle with the bottom cut out has to be fitted on the expansion tank. It then has to be filled with enough antifreeze to have its lever higher than that of the heater core. With the engine running antifreeze is added to the said bottle until air bubbles stop coming from the heater core bleeder hole.

When the heater core has a significant amount of air in it, it won't blow hot air. Check if the air comes out when the heater is on the max setting. If in doubt, bleed the heater core again.

Antifreeze leaves crystal deposits where it leaks. If there is a leak that can't be seen, the whole circuit can be pressure tested with the help of a small pump. The radiator too can be checked as in the annexed image.

A good alternative to pressure testing is smoke testing. While it will be easier to trace a plume of smoke to its source than tracing a drip to a hole, professional smoke testing equipment is expensive. It will, however, be a good alternative if you already have a smoke machine or can improvise one from a clothes iron steam boiler or a portable steam gun. This will require you to drain all coolant from the circuit.

Any oily deposits on the coolant tank mean trouble. Usually there will be a leak on the head gasket that is letting oil into the cooling circuit. Re-torquing head bolts sometimes solves it. When changing oil, also make sure there is no water in it.

Leaks to the outside are easier to fix. A hose may leak because of a loose clamp or the radiator might have a pinhole leak. Replacement radiators are usually available from breakers yards for as little as 30€. When replacing a radiator you might consider fixing a bigger one. There are also products that will plug leaks, not all of them work.

Some cars such as the Citroen ZX have two different radiators: a big one for the diesels and high displacement petrol models and a smaller for cars with less displacement. Putting the bigger one in place of the smaller is just a matter of getting a bigger fan holder. The stock radiator is fine, however, and should do it's job unless there is something wrong with the rest of the circuit.


There are of course hundreds of miracle products to stop small leaks in the market. Some work, some don't and some might fail when you're out in the middle of nowhere. Consider replacing the leaking component: a hose with a hole in the end might just be cut and refitted, a replacement radiator might be obtained almost for free and a leaking gasket can always be fixed.

In today's world, information is free. Being self taught was never so easy.

In today's hard times, saving money becomes more and more expensive As fuel, maintenance and parts costs keep increasing, one might be short of money to pay a professional mechanic.

The reason why one is a professional and another person is an amateur is just this: we don't all do it for a living.