On an I6 exaust manifold, it usually cracks where the pipes join together on the collector area. It would be best to remove the Exaust manifold to weld the cracks but this would only be a bandaid, Not a permanent fix. If you have the manifold that does NOT have the flex tubes on both ends of the manifold(#1 and #6 cylinder) ,then you have the old design that's notorious for cracking. The new Design has this flex tubes to accomodate the heat flexing the pipes as they heat up..