Question about the Diverter in intake plenum

[maytag]

Hey y'all:
I'm getting ready to trouble shoot a minor problem on this new/used 3.2 I just installed in my '03 S. I could use a crash course on a couple items.

I've only got about 200 miles in the car since the motor was installed. It's a 3.2 from an '04 with 86k on it. It's popping a code that suggests vacuum leak (I think it was P0440?). The symptom I'm FEELING though, seems more specific. When accelerating, especially at full throttle, it feels like there's a flat spot where power falls off around 4000rpm or so.

I'd like to understand the "flap" that's in the intake crossover plenum, which is vacuum controlled. What's the intended operation there? I assume it opens under certain vacuum conditions, and allows more volume for cylinder fill? Is it possible this is my culprit?

TIA

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