My intake manifold leaks like a sieve

I suspected as much. This seems weird, but aluminum manifolds can be working fine on a running motor. Now you work on the motor and you remove the intake. When you re-install the manifold it leaks- usually in one of the four corners. It is warped! This has happened to me twice. It seems they warp after they are removed. A machine shop can re-surface the manifold (hard to find around here). Or replace the manifold.