Most common fail is the head gasket. Drain the radiator and the oil. It will be a lot. So get a big container. After it is drained call the shop that did the rebuild and see what they will do. Most likely you will have to pull the engine and return it if the shop is a good one. They should have found any crack in the head, or warp age that would have caused this to happen. Let us know what you find.
