Cooling system sealants, sold under brand names, such as Bar's Leaks and Stop Leak, work when the product is installed in a cooling system and mixes with the coolant mixture. The sealant, whether in powder or pellet form, creates deposits near a leak and swells to seal the leak, preventing coolant from escaping, which could cause the vehicle to run hot and damage the engine. However, if left in the engine for an extended period of time, the sealant could cause damage by creating large deposits and clogging the radiator.
Prep the cooling system for flushing. Make certain the engine is off and the cooling system is cool. Move the heater controls in the vehicle to the "off" setting. Open the bonnet and disconnect the radiator hoses and radiator cap. Loosen the radiator plug and drain the coolant from the radiator. Turn off the engine and turn the vehicle's heater to the on position. Remove the radiator hoses and the radiator cap and open the radiator plug to drain the radiator.
Flush the cooling system to remove as much of the sealant as possible by inserting a water hose into the top of the radiator and running water through it constantly until any and all debris is loosened and the water runs clear. Repeat this process through the bottom hose connector of the radiator, then repeat the entire process.
Flush the heater core and engine block. Disconnect heater hoses from the engine. Attach the back flush gun to heater and run air and water until the water runs clear. Disconnect the thermostat from the engine. Open the drain cocks of the engine and flush the block until water runs clear.
Clear visible clogs in radiator/hoses. Shine a flashlight into the radiator and, with a screwdriver, loosen any visible clogs the sealant may have caused. Visually inspect the hoses for any sealant deposits and remove if possible. If the sealant deposits cannot be removed, the hoses should be replaced with new hoses.
Reinstall the hoses, close the drain cocks and fill the cooling system with the proper coolant mixture. Run the engine until warm and check the coolant. Reinstall the radiator cap.
Consult an automobile professional who can examine the cooling system and assess it for potential damage caused by sealant deposits and clogs, because even flushing and visually checking the cooling system may not remove and/or prevent cooling system sealant deposits and clogs.