Those trims are not unreasonable, especially for a big injector and a tune that probably didn't focus strongly on tight fuel trims. The hole in the MBC can, and should have been, accounted for in the tune, however, check valve isn't a bad idea as long as you use a quality one that has a very low opening pressure. Otherwise, I'd say just get a retune on closed loop.

 

MBC leak by design, they bleed off air to the WGA.

 

I understand, I'm just surprised blocking it off at idle had that much affect on the trims so quickly. Since it serves no function when the manifold is full of vacuum, I think it would be a more "pure" solution to put a check valve inline the control hose and only allow boost pressure in, right?

I understand completely, but rather than compensate in the tune I do like the check valve idea - any suggestions on a product?

I would think in an ideal world the MAF would account for all of the fuel determination.

 

I've been happy with my Boomba Racing check valve that I use for a different purpose. The one in the link below is their smallest. Should work for you.

http://www.boombaracing.com/check-valve-5-mm-3-16/

 

Thought the more consistent fuel helped, but the code came back eventually. I have subsequently found a cracked injector o-ring. No code as yet, since I replaced it.