I have a Mericruiser 305 inboard 228 hp alpha one drive. When I started the boat, with the muffs on, hot water leaked from the side of the exhaust manifold. There is a drilled hole on the left and right side manifolds. However, the hole on the right side, leaked and when reved up it increases. The left side did not leak. The holes are rusty but seem threaded. Can I plug the holes? Or do they have a purpose?