Cadillac Repair: 96 cadillac deville, vacuum leak, head gaskets

Question
Underneath the norstar cover plate is a large black cover.What is it and how important is it.Also what should my engine idle at. Currently it idles at about625 is this too low too high or ok......P.S.    Bill you saved my life on this deal. I was told repeatedly that I needed a new motor because the head gaskets were blown and replacing them was a no win situation,just better to get another motor or a new one.Also I  would prefer my questions posted along with your anwswers.

Answer
Hi Larry, 625rpm sounds ok to me. You kind of have me confused about the cover though. If you mean when you remove the 4 nuts that hold the silver cover that has the northstar emblem on top of the engine then that is the intake manifold, black nylon 1 piece thing that goes from the front to back of the engine. It is important in that if there is a vacuum leak the engine won't idle right and it also has the fuel rail and injectors inside. If not what you mean then repost and tell me the location of the cover. Hope that helps. Bill