98 Silverado Z71 5.7l Vortec

I've got a 98 with a 350 Vortec that is running really rough. I noticed a hissing coming from the top of the intake manifold and used some carb cleaner to narrow down the trouble. There is an area where the fuel inlet / outlet come into the manifold. One of these was loose. I got them seated back in and it didn't solve my problem. That's when I noticed that the plastic piece where the fuel lines go in would physically move when pressure was applied. I can't believe this would be normal... Has anyone else run into this problem? Any help in fixing it up or advice on whether or not i need to replace the whole manifold would be greatly appreciated. Thanks!

Sounds like the throttle body is loose? Need to be more descriptive.

You may just need the upper intake gasket set. I think the movement is somewhat normal because of the bracket that holds the CSFI unit. The gaskets in the upper intake are just o-ring type rubber material
http://gsi.xw.gm.com/image_en_us/gif/000/000/066/66929.gif

In the picture, you are saying part #12 is loose, #11 is the gasket I mentioned. #12 just snaps into a bracket on the lower intake manifold, its not the greatest setup and I think I shimmed the bracket up when I switched from the CSFI to the GM upgrade because I couldn't get it to seal

Yeah... part #12 is what appears to be loose. I haven't had much time to dig into this more, but plan on being out there this week to get it finished.

What does GM offer for an upgrade?

The upgrade is an assembly that replaces the CSFI poppets with injectors. The CSFI unit has injectors in the piece that you say is loose and hoses run to poppet valves that are inserted into the lower intake manifold, the updated setup has the injectors inserted into the lower intake manifold, I think the cost is around $400 now, plus labor if you dont do it yourself