Does stop leak work on a heater core?
On One Hand: Temporary Fix for Small LeaksA leaking heater core is normally caused by rust resulting from a lack of maintenance. If the rust has opened only a small hole in the heater core, adding stop leak to the radiator may plug the leak for some time.On the Other: Ineffective for Large LeaksEven in the best of circumstances, radiator stop leak is meant to be a temporary fix for leaking radiators and heater cores. However, if the hole caused by rust is large or there are other leaks to which the stop leak bonds, you will continue to notice coolant leaking from the broken heater core onto your floor.Bottom LineIf you have a heater core leak that must be fixed quickly, stop leak should provide a temporary fix. While the stop leak may fix the leak, be sure to replace the heater core as soon as possible.