Cooling system additives cover the products used to clean, seal and protect a radiator and cooling system: radiator flushes, stop leak products, corrosion inhibitors and water pump lubricants.

A radiator flush is worth running before a coolant change on any system that has been neglected, since it lifts scale, rust and old inhibitor residue that would otherwise contaminate the fresh coolant and keep restricting heat transfer.

Stop leak has its place as a temporary measure for a weeping core or a small seep, particularly to get a vehicle home or to a workshop. It is not a repair, and using it as one risks blocking the radiator core, heater matrix or thermostat, which turns a small job into a large one. Find and fix the actual leak, then flush the system properly afterwards.