What is antifreeze?

This site is mostly about engine oil, but an engine has one more fluid its life depends on every bit as much: the coolant. Car owners usually remember it in one of two situations: when the engine boils over in a traffic jam, or when something starts dripping from under the car in winter. Let’s work out what antifreeze is, how it differs from Tosol, and how to avoid buying tinted water instead of it.

Why not water?

Coolant has to keep the engine within its working temperature whatever is going on around it: heat, frost, a traffic jam or a motorway. A modern engine runs at 90–100 °C, and since bonnets have got lower, radiators have got more compact, so the fluid needs a margin of at least 10–20 degrees below its boiling point.

Water is poor at this. Even in a sealed system, where the radiator cap holds an overpressure of about 0.5 atm, water boils at roughly 110–112 °C, while a solution of ethylene glycol in water boils at 118–120 °C. Winter is worse still: water expands as it freezes and can crack the block, the radiator or the hoses, and a frozen thermostat all but stops circulation. In frost, a water-glycol mix turns not into ice but into a thick slush and barely expands, so it doesn’t burst the system.

Curiously, pure ethylene glycol (which boils at 197 °C) freezes on its own at around minus 13 °C. It is the mixture with water in the right proportion that gives the low freezing point, which is why concentrate must never go in undiluted, and “more glycol to be on the safe side” is a bad idea.

Besides water and glycol (propylene glycol is sometimes used instead of ethylene glycol), antifreeze contains an additive package: corrosion inhibitors for cast iron, aluminium, copper, brass and solders, alkaline components and an anti-foam agent. The additive package is what sets one brand of antifreeze apart from another, and it is what decides whether an antifreeze is any good.

Tosol and antifreeze

Antifreeze is the general name for low-freezing coolants. Tosol is not a separate substance but a brand of antifreeze, made back in the USSR and still made today. There are three grades of Tosol:

1. TOSOL A-40 is a ready-to-use fluid, good for frosts down to about minus 40 °C. The most common one.

2. TOSOL A-65 is for regions with harsher winters, down to about minus 65 °C.

3. TOSOL-A is a concentrate, used only after diluting it with water.

What should you put in your car? Whatever the car’s manufacturer specifies (more on this in the note on choosing a coolant). If Tosol is allowed, Tosol will do, but from a well-known maker, with a certificate, and not bought from some random seller at a market.

How to check antifreeze when buying it

Sadly, there is no quick test for antifreeze outside a laboratory, but you can weed out an obvious fake by three signs:

1. Density. This is what tells you the glycol concentration and therefore the freezing point. You can check it with a hydrometer or simply by weighing exactly one litre of the fluid. For TOSOL A-40 at 20 °C the density should be 1.067–1.072 g/cm³, so a litre weighs 1067–1072 g.

2. Acidity (pH). This shows how much of the alkaline components that protect metal from corrosion are left in the additives. It is measured with ordinary indicator (litmus) paper. For TOSOL A-40, pH 7.5–8.5 is considered normal. If there is too little alkali, the antifreeze won’t last long.

3. Foam. Shake the can: there should be little foam, and it should settle quickly. If the foam hangs around for a long time, there is no anti-foam agent in the fluid.

And a separate word on the “folk” method. Ethylene glycol really does taste sweetish, hence the name “glycol”. But it is a strong poison, so never, under any circumstances, taste antifreeze. For the same reason, keep it well away from children and pets.

Colour tells you nothing about quality

Red, blue, green, yellow: it’s just a dye added at the factory so that the fluid level can be seen in the translucent expansion tank. Colour says nothing about quality or composition, and “a rich colour means it’s fresh and good” is as much of a myth as the idea that fluids of the same colour can safely be mixed.

How long it lasts and whether you can mix it

Antifreeze, like engine oil, ages: in an hour of running it passes through the cooling system hundreds of times at 90–100 °C and above. The additives gradually wear out, the alkaline reserve drops, and the fluid starts eating into metal and rubber. That is why coolant has its own change interval: for Tosol-type fluids it is usually two to three years or about 60,000 km, and the exact figure for your car is in the service book. If the fluid has turned cloudy or brownish, or has sediment in it, change it without waiting for the due date.

The rules for topping up are simple:

• top up with the same fluid that is already in there, ideally the same brand from the same maker;
• don’t mix different antifreezes, including Tosol with antifreeze if the car’s manufacturer calls for antifreeze (what such experiments lead to is covered in the article on Tosol and antifreeze);
• in a desperate situation you can top up with distilled water, but it is better to replace all the fluid in the system afterwards;
• dilute concentrate only with distilled water, not tap water.

You can read more about how coolant ages and when to replace it in the article ethylene glycol and other achievements protecting your car. And in closing I’ll say the same as I do about oil: skimping on coolant and forgetting when it’s due for a change costs the engine far more than the fluid itself.