A new supplemental-coolant additive (SCA) has been developed suitable for heavy-duty diesel engines. The formulation stands apart from the commercially available SCAs in that, it is free of nitrite, phosphate, and all heavy metals. Its corrosion-inhibition performance in ASTM D-1384, ASTM-4340, and ASTM D-2570 have been measured. When compared to two commercially available SCAs, test results indicated better or equivalent protection for: copper, brass, low lead and silver solders, carbon steel, gray cast iron, cast aluminum and heat-rejecting aluminum surface. Its maximum hard-water stability at 480 ppm hardness and, its ability to prevent liner-pitting, as measured by the bench-top cavitation-erosion test (ASTM G32), were superior to the nitrite containing SCAs. Its scale inhibition and antifreeze compatibility was equivalent to the commercial SCAs.