Sound advice Izero for the new post that was just added to this 3month old thread today. OP is long gone, new poster says nothing about a mechanic not changing a cylinder whatever that means. Dude, you have to start paying attention to what you are replying to!! You give great advice, and I do appreciate your thoroughness. Then new post does not say what engine he has. I am assuming it's a 4.7. A few gallons a day is an exaggeration, I hope, I don't think your engine would run if it was ingesting that much. I would say you have a bad head gasket due to overheating. Do the tests Izero spoke of to confirm. You say you have overheating and knocking as well. Not good. You are probably looking at a rebuild.