I have a 2000 Disco II. Previous owner had valve job done and the milled the heads as part of rebuilding the heads. I have tracked down a vacuum leak to lower manifold gasket. It does not look like they milled the intake. Question should I have the intake milled before I put it back together?
I have never heard of anyone having an intake manifold machined without having reason to suspect its warped.
When you remove it to fix the leak bring it to a machine shop and they can check if the mating surfaces are true. If it's warped they can then mill it. Watch how much they charge because you can probably get a really good used one cheap.
Before I did that I would simply try replacing the gasket. Just to clarify, are you referring to steel gasket between the upper and lower intake manifolds?
Sometimes people cheap out and try to re-use these. Even if its the valley gasket just try replacing it first paying close attention to the torque procedure.
