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What is coolant or antifreeze and why does your car need it?

Practical Motoring logo Practical Motoring 28-12-2016 Isaac Bober

Pouring coolant © Pav_1007/iStock/Getty Images Pouring coolant Coolant or antifreeze is one of the most important fluids in your car. We explain what it is and why it’s essential to the good health of your car’s engine.

AS ITS NAME SUGGESTS, antifreeze is designed to keep the water in your car’s engine from freezing.

But it’s also designed, rather confusingly, to keep your engine cool by reducing the chances of the water in the engine from overheating – just about every single engine on-sale today (petrol and diesel) is water cooled. Confused? Don’t be.

Call it what you will, but in Australia antifreeze is generally referred to as coolant which makes sense given its dual purpose and our hot climate.

And by calling it coolant here, you sidestep the ‘who needs antifreeze in Australia’ argument.

What is coolant/antifreeze?

Basically, coolant or antifreeze, which mixes with water in an engine’s cooling system is designed to both lower the freezing point and raise the boiling point of the system.

This means, by adding coolant/antifreeze to your car’s cooling system you’re able to increase the temperature a vehicle can run at before the cooling system will boil.

Coolant or antifreeze is an alcohol-based additive that’s usually green in colour (although it can also be red, blue, and orange) and made up of Ethylene Glycol.

Why do I need coolant/antifreeze?

As the name suggests, coolant/antifreeze is necessary to reduce the freezing point of your engine’s cooling system and raise the temperature at which the system will boil when compared to water alone.

The boiling point of water is 100-degrees C but inside a car’s radiator where the system is pressurised this boiling point increases to ??-degrees C.

The accepted pressure range for a car’s radiator is between 75-100kPa (or 11-15psi) and with a mix of 33% coolant/antifreeze to water the boiling point is raised to 121-125-degrees C.

Change the ratio to 55% and this lifts to 125-129-degrees C.

If you live in a cold climate where the temperatures regularly (in winter) drop below freezing then the accepted ratio of coolant/antifreeze to water is 60:40.

Oddly, increasing the amount of antifreeze in the system will actually reduce its protective qualities.

So, with 60% coolant/antifreeze in the system the temperature at which the first ice crystals will begin to form in the cooling system is -55-degress C.

With no antifreeze/coolant in the system (and just straight water) ice crystals will form at 0-degrees C.

Letting the engine’s cooling system freeze will see the fluid expand and crack engine blocks and blow the radiator apart.

But, even in Australia the use of coolant/antifreeze to protect against freezing is vital. See, wind chill can also cause freezing of your car engine’s cooling system.

For instance, say the ambient temperature is 0-degrees C (a regular winter temperature in the Blue Mountains where I live) driving at just 24km/h will cause wind chill to reduce temperature of the air hitting the radiator to -21-degrees C and at 60km/h the temperature would end up being -30-degrees C.

Beyond 60km/h the wind chill factor effect becomes minimal. So, with a 60:40 mix of coolant/antifreeze:water protection against freezing will extend to -55-degrees C.

Can I just use water?

No. Well, maybe not. The idea of running coolant in your car’s engine is to increase the boiling point of the water which is being pumped around your car’s engine and through the radiator where the air flowing through a moving car’s radiator cools down the water as it continues on its way back through the engine.

Depending on where you live in Australia, your water will either be harder or softer than somewhere else.

All water contains dissolved salts and minerals which if running straight water in your engine (without coolant) can reduce the life of some components.

So the argument is one that’s less about cooling properties and one about the potential damage water can do to your car’s cooling system.

In colder climates (see paragraph above), using just water means the car engine’s cooling system will freeze at 0-degrees C, and when water freezes solid it expands which can lead to cracked engine blocks or burst radiators.

Why does coolant/antifreeze come in different colours?

Coolant/antifreeze can range from greed, to orange, red and even blue and it all refers to the type of inhibitor technology.

Most bottles of coolant/antifreeze are inorganic and (are green) include rust inhibitors and protectants to stop corrosion of componentry in the cooling system.

Then, in the 1990s General Motors pioneered organic coolant/antifreeze which is designed for newer cars with aluminium radiators rather than old-school copper and brass.

The colour of organic coolant/antifreeze can be orange or red, or blue depending on the make of your vehicle and whether it’s from the US, Japan or Europe.

And, just to make things more confusing the type of coolant/antifreeze used in modern cars are generally a hybrid of the two which is referred to as Organic Acid Technology (with phosphates) which offers better protection to aluminium components.

Can I mix different types of coolant/antifreeze?

No. Mixing two different types of coolant/antifreeze can cause damage to your engine’s cooling system. Mixing two incompatible coolant types will usually cause them to gel in the cooling system.

And you don’t want that. The best way to work out which one you need is to look inside your owner’s manual. It should also suggest the optimum ratio mix as this is sometimes recommended as 50:50 (water to antifreeze/coolant) or 60:40. 

You can’t use newer coolant/antifreeze mixes in older cars and vice versa and sometimes you can’t use the organic hybrid coolant that Japanese cars run in US-made vehicles. So, check your manual or ask your dealer.

Can I just use straight coolant/antifreeze?

No. And the reason is that coolant/antifreeze on its own isn’t actually very good at protecting against freezing or at dissipating heat while running through a cooling system.

So, stick with a mix of coolant and water and stick with the rough ratio of either 50:50 (water to coolant/antifreeze) or 60:40 depending on where you live and what the manufacturer recommends.

How to check if the coolant/antifreeze is healthy?

You’ll need to have the coolant/antifreeze in your car’s engine cooling system flushed and replaced from time to time.

And because of its importance, it’s vital you keep a regular check on the condition of the fluid.

So, pop the bonnet, locate your coolant bottle and remove the lid. Take a sniff and if the fluid smells hot and burnt then it’s time to have it flushed and filled with new coolant/antifreeze.

More than that, you’ll need to check and find out what caused your system to overheat in the first place, so take your car in to your mechanic.

Similarly, if you look at the fluid and it looks a little milky then it’s possible that oil has got into the system via a leaking head gasket or transmission cooler or the intake manifold.

And this can kill your car’s engine. Get the car to a mechanic ASAP.

Ideally, the coolant/antifreeze should look a vivid green (if the fluid in your car is green, of course) and it should feel slippery, like oil. If it feels gritty to the touch, then you should get the cooling system flushed and replace the fluid mix.

You don’t want to risk scale or rust being washed around your cooling system which could cause the water pump to seize and the engine to overheat.

Sometimes it can be hard to check the coolant in your car because the while the head and neck will be up at the top of the engine bay and easily accessible, the reservoir can sometimes be buried deep down in the engine bay. And that makes it hard to touch and even get a proper sniff of the stuff. That said, you should at least be able to see whether the coolant level is high or low.

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