Coolant is the generic term for the cooling liquid in the cooling system. Coolant protects against frost, corrosion, overheating and lubricates. Its task is to absorb the engine heat and dissipate it via the cooler.

The coolant is a mixture of tap-water and anti-freezing compound (glycol/ethanol) mixed with various additives (bitter substance, silicate, antioxidant agents, foam inhibitors) and colored. Bitter substances are used to prevent the coolant from being drunk inadvertently. Silicates form a protective layer on the metal surfaces and prevent e.g. limescale deposits. Antioxidant agents prevent corrosion of components. Foam inhibitors suppress the foaming of the coolant. Glycol keeps hoses and seals smooth and raises the coolant’s boiling point.

The mixing ratio of water and antifreeze should lie between 60:40 and 50:50. This usually corresponds to antifreeze protection from -25°C to -40°C. The minimal mixing ratio should be 70:30 and the maximal 40:60. Further increasing the proportion of antifreeze (e.g. 30:70) does not lower the freezing point any further. On the contrary, undiluted antifreeze freezes at around -13°C and does not dissipate sufficient engine heat at temperatures above 0°C. The engine would overheat. As the boiling point of glycol is very high, the boiling point of the coolant can be raised to up to 135 °C by using the right mixing ratio. Therefore, a sufficient antifreeze share is important even in warm countries. Always follow the manufacturer’s instructions. A typical composition could be 40%/60% or 50%/50% with the use of inhibited water (drinking water quality). 


The coolant and its additives are subject to a certain wear, i.e. part of the additives will be used up in the course of some years. If, for example, the corrosion protection additives are exhausted, the coolant turns brown. Therefore, some manufacturers specify a coolant replacement interval. 


However, the cooling systems of newer cars are increasingly filled with so-called long-life coolants (e.g. VW G12++ / G13). Under normal circumstances (if no contamination occurs), the coolant need not be changed (VW) or only after 15 years or 250,000 km (newer Mercedes models). As a rule, the coolant should be changed if contamination (oil, corrosion) has occurred and in the case of vehicles which are not equipped with long-life coolant. The vehicle manufacturer’s instructions must be followed in terms of the specifications, replacement interval, mixing ratio and the miscibility of the antifreeze. 


Coolant must not get into the groundwater or be discharged via the oil separator. Coolant must be collected and disposed of separately.

Additives / change intervals

The coolant and its additives are subject to a certain wear, i.e. part of the additives will be "used up" over the years. For example, if corrosion protection additives are removed, it may result in brown colouring of the coolant. 

Therefore, some vehicle manufacturers specify a fixed coolant replacement interval (e.g. Opel Sintra: Every 5 years). However, the cooling systems of newer vehicles are increasingly filled with so-called "long-life" coolants. Under normal circumstances, if there is no contamination then it is not necessary to replace the coolant (VW). For some Mercedes models, specifications state that the coolant only needs to be replaced after 15 years or 250,000 km. Differing statements are provided depending on the vehicle manufacturer.

Generally speaking, the coolant must be changed in the event of contamination (oil, corrosion) and the cooling system must be flushed. For vehicles that are not filled with "long-life" coolants, it is recommended to replace the coolant every 3 years. With regard to specifications, change intervals, mixing ratios, and miscibility of antifreeze/coolants, the instructions of the vehicle manufacturer and coolant manufacturer must be observed.