Changed out the lower manifold on a 4.9, Now the engine is idling very fast - over 4000 rpm. Gaskets appear to be in place and vacumn lines. Any suggestions? If I cover the ports on the throttle body the engine keeps running.

 

The engine should still run with the throttle body blocked off, because the IAC is allowing a small amount of air in so it will idle. The IAC could be your problem with the high idle. I would think a vacuum leak will make it surge.

 

Is the IAC located on the trottle body? Is there a way to test it?