From TechWiki
Jump to: navigation, search

Coolant is a liquid circulated through the engine and radiator to keep the engine cool. Water is excellent for extracting heat from the engine, but on its own it has a low boiling point, can freeze in cold weather, and causes rapid corrosion of most metals. Therefore it needs an additive to raise the boiling point, lower the freezing point, and prevent corrosion. The additive is generically called coolant, or sometimes just antifreeze. The chemicals used to provide the antifreeze properties are completely independent of those that inhibit corrosion.


Over 90% of concentrated coolant is a polyol, a chemical like an alcohol but with multiple hydroxyl groups instead of the one that alcohols have, and is used to raise the boiling point and lower the freezing point when mixed in to water. Almost all coolants use one or more of three chemicals to provide the antifreeze effect:

  • Ethylene glycol: most modern coolants have only ethylene glycol. It is acutely toxic and sweet, which means it is dangerous for children, pets, and wildlife if spilled in the open, but it is cheap and effective.
  • Propylene glycol: started to become popular as a less toxic alternative to ethylene glycol, but now uncommon. Modern coolants usually use bittering agents to make ethylene glycol unpalatable.
  • Glycerol: sometimes mixed with ethylene glycol, it is a less toxic and more environmentally friendly antifreeze.

Pure ethylene glycol has a freezing point of -12C, which is not low enough for reliable use in the UK. When mixed with water, the freezing point becomes much lower, below -50C at certain concentrations. Most coolants are formulated to be mixed with approximately 50% water for protection to around -37C. Propylene glycol has a lower freezing point. Glycerol is a viscous liquid at room temperature and freeze at 18C, so it must be used in mixtures with other antifreezes and water.

On the whole there is nothing much to worry about. Coolants all have much the same antifreeze regardless of anything else. Choose one with glycerol if you really want to be eco, although it will still be mostly ethylene glycol.

Corrosion inhibitors

There are many different chemicals used as corrosion inhibitors in engine coolants. These make up less than 10% of the coolant, with over 90% being the antifreeze. There are two main classes of corrosion inhibitor, IAT and OAT, but with a bewildering array of combinations, proprietary formulations, names, and colours.

  • IAT, Inorganic Acid Technology. Not actually acids, these are salts of inorganic acids, usually quite alkaline, such as amines, nitrites, phosphates, and silicates. These are the traditional corrosion inhibitors, effective for all metals, and have an engine service life of up to 2 years or 30,000 miles. Some downsides are sludging and rusting from as they quickly lose effectiveness, and water pump damage from silicate precipitation.
  • OAT, Organic Acid Technology. Again, not acids, but salts of organic acids, such as carboxylates, sebacates, and 2-EHA. These give good protection to ferrous metals and aluminium alloys, but poor for "yellow" metals which use solders. They have a longer service life, typically quoted at 5 years or 100,000 miles. These are not safe for use in older engines which contains brass or copper, early versions in the US were dogged by reports of corrosion and engine damage, and one of the inhibitors (2-EHA) has been implicated in damage to silicone-based seals and gaskets. Many modern formulations do not use 2-EHA or have very low levels.
  • HOAT, Hybrid Organic Acid Technology. These are a mixture of OAT corrosion inhibitors with phosphates or silicates, usually silicates in European makes and phosphates in Japanese. They are designed for longer service life, sometimes described as "lifetime". They don't actually last forever, but typically 10 years and 180,000 miles or more.

Other types, such as POAT, exist but not intended for use in small petrol or diesel engines.

In general, coolants with IAT and OAT inhibitors cannot be safely mixed and systems must be flushed when changing from one type to the other. Many HOAT coolants are considered "universal" and can be safely added to systems containing older coolants, although mixing with other types can reduce their effectiveness.



Coolants or antifreezes are usually dyed a strong colour. Originally this was an almost-fluorescent green for easy identification. Later formulations with different corrosion inhibitors were then dyed different colours., but a universal system has never been adopted. Colours vary for the same chemical makeup between brands and in different regions of the world, and the same colours may be used for different chemical makeups.

In the UK in 2018, common colours are blue for IAT and red for OAT, but brand-specific or HOAT coolants may be pink, blue, or yellow, and there is a purple HOAT coolant with glycerol. Green is not usually seen in the UK any more.

Brands and codes

Coolant colour is not a reliable guide to what is actually in the coolant, so it is important both to check what is already in the system and what a new product is. Mixing certain coolant types can produce sludge or thickening, or simply reduce the effectiveness and lifetime. Different manufacturers and car brands use their own or semi-standard codes, but there are only a few basic types in the UK:

  • IAT, VW G11 TL 774C (blue), Comma/Glysantin Classic (blue), Halfords Silicate (blue), ECP Triple QX Blue (blue), Granville Subzero (blue), Bluecol 2yr (blue)
  • OAT, VW G12 TL 774D (red), VW G12+ TL 774F (pink), Comma/Glysantin G30 (red), Glysantin G33 (blue), Glysantin G34 (orange), Halfords OAT (red/pink), ECP Triple QX Red (red), Havoline XLC (orange), Granville Zerocol (red), Honda Type 2 (blue), Bluecol 5yr (red), Renault Type D (green)
  • HOAT, VW G12++ TL 774G (pink), VW G13 TL 774J (purple), Comma/Glysantin G48 (blue/green), ECP Triple QX Purple (purple), Havoline QFC (red), BMW OEM (blue)


Elise S1

The Elise S1 contains a Rover K-series cast aluminium engine. The Elise S1 owner's handbook recommends only using Unipart Superplus, which was green, and a change interval of two years. Unipart Superplus is no longer available, but similar non-OAT coolants are. The "official" Xpart coolant for the K-series engine is now a red OAT coolant, available at EliseParts. Many S1's have been switched to the red OAT type at some stage of their lives, which should last approximately 5 years before needing to be changed.

Either IAT or OAT coolants can be used in the Elise S1. The important thing is not to mix them, and to flush the system if changing from one to the other. HOAT coolants have never been recommended for the K-series engine. Check the levels regularly and refresh at the recommended interval.

Elise S2



The recommended coolant for the Evora is red PETRONAS Paraflu Up (previously orange Havoline XLC), an OAT coolant. The change interval is four years. USA?

See also

Organic Acid Technology

Further reading