I used prestone flush a few time unsuccesfully i even left it in the car for a whole week. Sure i drained the nastiest sludge ever and flushed till clear water came out but one week later it was back to same bs. The radiator cap that had just been clean was back to thick build ul in the inside just a week later. I even got oxalic acid(main ingredient in older prestone flush) that helped but nothing ever took it all out