if it isnt smoking out of the tailpipe then the smoke isnt coming from a cracked manifold....otherwise it would be smoking all the time out the back, not just a little out the crack, does that make sense?? there must be an oil leak, and its most likely out of the valve cover gasket, or the distributer...my friends 87 zuk leaked out of both when he still had the 1.3L, it would smoke a little when it would hit the manifold....cracked manifolds are common on any vehicle, and if its cracked then definitely replace it, but i would look hard for an oil leak before i spent any money on a new manifold....