I've tried several commercial stop-leak products over the years and none of them out performed black pepper. Use a combination of coarse ground and very fine ground pepper. It will migrate to the leak, lodge itself into place and then swell to make a permanent seal. My present car developed a small leak at the head gasket 4 years ago. Water was both dripping down the outside of the motor and getting into one of the cylinders.
Within one day of adding the pepper the drip stopped forever and all signs of water vapor disappeared from the exhaust.