I have a mechanic friend with a blown head gasket on his aerostar. He thought he would give that stuff a try for the $50 or so it cost. He tried it, it didn't work. He called customer support, They asked if he followed the directions EXACTLY. He followed them pretty close, but not exactly. They sent him out another can and said that it was very important to follow the directions EXACTLY. He tried again and followed the directions exactly. It worked! That's been about two years ago. He keeps driving it but knows that it needs to be fixed correctly. I think a valve seat or something got taken out when the gasket blew. He keeps running it with one dead cylinder.