The reason that disconnecting the vacuum diaphragm does not solve the vacuum leak is that the butterfly valve shaft extends through the forward outer wall of the plenum. It connects to the diaphragm actuator here. The bushing where the shaft comes through the plenum wall is one of the ones that gets sloppy, increasing clearance between bushing and shaft. This is essentially a hole that allows unfiltered air into the plenum. Since this air is also not metered, it can cause problems for the ECM and fuel mixture. Its been awhile since I've had a faulty plenum in hand, but it seems like there was more than one bushing for the valve shaft that was exposed to external air, but for sure the front one.

It is the front bushing, right by the vacuum diaphragm, that can be heard leaking at idle if the bushings are really sloppy. Also, if one or more of the bushings have worked their way out of the aluminum plenum and slid down the shaft to one of the valve blades, you may be able to hear it rattling on the shaft. Disconnecting this thing is NOT a fix, like I said above. Replace the lower plenum or your very expensive four cam motor will be a not so good boat anchor (not heavy enough)!