This paper contrasts the differences between coolant maintenance practices for light duty engines and heavy duty (H.D.) engines. These differences explain why spent coolants from H.D. engines must be recycled using processes capable of removing dissolved contaminants, additives, and glycol degradation products, in addition to removing particulates. Examples and data are included that support this view. Finally, the requirements and recommendations of H.D. engine builders for recycled coolant to be used in their engines are reviewed and summarized.