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.

Last updated 15 January 2020 by Editorial Staff

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/antifreee?

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. 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 accelerate the ‘heat loss’ 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. That said, windchill will not actually reduce the temperature of your engine’s heating/cooling system below ambient temperature, but it will speed up heat loss. So, with a 60:40 mix of coolant/antifreeze:water protection against freezing will extend to -55-degrees C.

Can I use water instead of coolant?

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.

Does it matter what colour coolant/antifreeze I use?

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.

When should I change the coolant/antifreeze?

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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    1. Thanks Richard. If you live somewhere hot or cold, rather than somewhere with a mild climate then you’d probably need to refresh your coolant more regularly than that. But it also depends on the health of the rest of your car’s cooling system. If the radiator cap is losing pressure then this can cause a breakdown in the effectiveness of the cooling system and cause the coolant/water mix to perform poorly. This isn’t a set and forget thing. – Isaac

  1. Hi, it might be a good idea to investigate wind chill a bit further. No matter how fast you drive the air hitting the radiator, in your example, will still be 0 degrees.

    1. Thanks Robs, but I think you’re referring to an inanimate object, so, on that’s stationary. In that example, yes, windchill wouldn’t change the temperature. But if the vehicle is moving, then windchill does lower the temp. Significantly.

      1. Hi Isaac – I think you’ll find that Robs is correct. Wind chill is a “theoretical”/mathematical calculation that estimates how cold the ambient air temperature “feels” to biological organisms (we experience wind chill because we “feel” a faster rate of heat loss when our skin is exposed to cold wind). The only effect wind chill has on “inanimate” objects, such as a car, is to shorten the amount of time it takes for it to cool. A car will not cool below the actual air temperature, no matter what the “wind chill” factor may be as experienced by us as biological organisms. If there are 2 thermometers outside – one in the wind and one shielded from the wind – they will still read the same temperature. The same idea applies to moving vehicles.

        It might be worth updating the article for accuracy on this point?

        1. Hi DWR, I’ll take another look at the article to clarify the explanation, but it stands. Windchill won’t cause a car’s radiator/engine to cool below ambient temperature. I know that. But it will accelerate the cooling effect, meaning the it will take longer for the engine to reach its optimum working temperature. And, so, yes, windchill does have an effect on your car’s heating/cooling system.

  2. I live on a main road in a hilly area and in summer I often have a procession of people with cars that have boiled. These cars a modern cars and it amuses me when the owners say to me “I’ve just had the car serviced”. I answer “who by Dodgy and Sons automotive repairs”. They haven’t had their car serviced for the oil on the bottom of the dipstick is black and smoking hot. One and all of them have no coolant in the expansion tank as the tank is a dark brown colour. They expect me to be able to fix their cars. Well I’m not going to give them any coolant and they get a little angry when I charge them for mains water which I have to pay for so I don’t see why they should get it for nothing. They often just wait for the car to cool down a bit and then they go on their way. I’ve also had new garden taps fitted that require a special key to turn the tap on. People would just go onto my land and take water not only for their cars but also to fill water drinking bottles and water containers. Cyclists were also big offenders for that. As far as I’m concerned if you’re too tight to spend money on having your car serviced at the proper service intervals then you shouldn’t own a car. Ordinary homeowners on main roads are not there to fix your car when it calls time.

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