Can't find a vacuum leak and need ideas...
 

I installed a plenum spacer (AAM). Since it's been installed, the car idles a little higher than it did pre-spacer (maybe 900ish); however, if I cut the air off the idle drops to 650ish. It doesn't drop enough to throw a code, but it sputters a little bit and definitely drops a noticeable amount. As soon as I cut the air back on the idle jumps back up right below the 1k mark.

Everyone keeps telling me there is a leak somewhere. I've broken the plenum down numerous times and put it back together in search of a leak, but can't find one, but don't really have any other explanation for the big difference in idle. I'll try and get video of it tomorrow, but I'm looking for ideas on how to test for a leak or thoughts on where it may be leaking from. There's no audible hissing sound or anything, all bolts are torqued to spec, gaskets are new, etc...