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The demands on coolant for heavy duty engines are different from those for coolant for automotive engine and other light duty applications. The heavy duty engine coolant often runs 16 to 32,000 km/month (10 to 20,000 miles/month) at engine loads of 70%. Under these conditions the engine coolant's inhibitor package is stressed. The heavy duty engine manufacturers currently recommend the use of supplemental coolant additives (SCA) to extend engine coolant life and to provide liner pitting and hot surface scaling protection. Care must be taken to ensure that the correct SCA levels are maintained. A survey was conducted to characterize engine coolant chemistry for a fleet of heavy duty vehicles. Analyses of the coolant, coolant filter, SCA, make-up coolant and water showed many variables are involved in maintaining the correct balance of inhibitors in the cooling system.

The cooling system in passenger cars and other light duty vehicles is maintained by changing engine coolant periodically. On the other hand, in heavy duty vehicles, the coolant life is extended by using periodic additions of supplemental coolant additives (SCA's). Today, both light duty and heavy duty fleet-service managers are faced with ever-increasing environmental pressures concerning the handling and disposal of used engine coolant. Coolant recycling is one alternative which significantly reduces the amount of waste disposed, while at the same time conserves valuable natural resources.