Experimental formulations of automotive supplemental coolant additives (SCA) were evaluated for their effectiveness in reducing corrosion in cooling systems with over-extended or additive depleted coolants. The evaluation involved both laboratory glassware and simulated service tests that were modified to produce novel “simulated additive depletion”, which provided a unique approach for evaluation of the SCA's. The study centered on two basic corrosion inhibitor packages. The base coolant used in the evaluation represents a contemporary, high silicate formulation which is commercially available and was purchased directly off the shelf. This unique testing documents that automotive SCA's can provide effective corrosion protection to an automotive cooling system operating on a weak or depleted coolant.