A similar situation I have...I have not installed my 255 yet, but at idle I'm running lean (fluctuating from 15.9-17.0). I have an evo 8 and I've got my MBC turned all the way down (for safety) until I can go get my tune fixed to account for my manifold, o2 housing, and AEM intake I've added. If I install the 255, will it help my AFR? I'm going to install it prior to tuning anyways, but maybe I'll do it tonight to try to fix this and allow the car to be "driveable." Seems like it will help richen it up a bit from what I've read in this thread.