The egr passage is 'cast in' to the manifold & exits behind the t/body via the three holes you mention.the other passage is cast in & runs from the ISC valve...the object of both of these is to distribute idle speed air or egr evenly to both banks, hence the cast in channels from the 'remote' mounted valves...which is the only position they could be mounted for clearence.The egr tube itself has been known to clog in the area of the 'necked' venturi, between the DPFE sample/pressure hose take offs.....
