Does anyone know if the 7.4 mercs in the 1992 400 Express Cruiser are full closed cooling include the exhaust manifolds or are they just the blocks? A boat I'm buying has closed cooling and I'm told stainless steel risers. Yet I see rust at the riser/manifold gasket area. I wouldn't think the risers would be rusting like this and if it's complete closed cooling it wouldn't be from the exhaust manifolds.. is it possible to have rust like this from a leaky gasket even with full closed cooling and stainless steel risers?