It is a old remedy. Black pepper will not degrade and will flow thru the system for a long time plugging other leaks without harming the cooling system.

 

Do not use stop leak!! I hate that crap. If you could see what it does to your motor you would never waste your money on it. Heater core is not that hard to replace. Todays vehicles have a pan under the heater core that allows leaking fluid to run outside the cab and not dribble on your carpet. That's why you don't see the foggy windows or coolant on the inside.