I did this mod about a year ago (exactly following the original post) and then recently started throwing P0171 and P0174 codes and she was running like crap. I built my own DIY smoke tester and fired her up and quickly found I had a good leak there I should have had a cap per this original instruction:

"After that you have to plug the port on the opposite end of the air distribution piece, I found a red plastic cap that fit in very tight."

I didn't have anything I could find that would fit great so I had cobbled something together and it had come loose causing this issue.

Well, long story short I found that the following product fit very well for this application: Dorman rubber expansion plug #02608. I bought it at O'Reilly an it was around $4.00. You should be able to search this and find it about anywhere that sells auto parts. There is a small nut on there you tighten. I am highly confident this will never come loose and I am impressed how smooth things are running now compared to what I was used to.

Good luck all!