An extended-service engine coolant filter has been developed, using conventional supplemental coolant additive (SCA) technology. This has been achieved by the use of a proprietary polymer coating, which delays the release of active engine coolant chemistry. Initial field data show a gradual release pattern for introducing SCA chemistry into the coolant, with complete release typically occurring in the range of 70 000 to 140 000 miles (112 651-225 302 km).