Thought I'd share something I've fought with over the past month, finally solved in my garage.
So I would constantly receive a rear O2 code, rough idle and stalling. Lots of posts I read said to just keep the revs up and it would go away after break-in. It didn't go away.
More importantly, why would I have to keep revs up; shouldn't the EFI be keeping everything in check at all revs? I went investigating...
I had the airbox cover off and was looking down into the TB's while it was idling. I could watch my O2 voltage sit at 0v and my AFV (or whatever they call it) sit at 130%. I noticed a small whistling noise. I tracked the whistling to the rear butterfly. I noticed the front butterfly appeared fully closed but the rear looked barely open. I pushed on it with my finger to close it. The idle got instantly better. I let up with my finger and it got rough again. I pushed again and watched the dash; after a few seconds, rear O2 started switching between .8v and .1v and the AFV came down to ~100-105%. The issue is that the front was closed and the ECM (reading off the TPS on the front) was fueling as such, so the rear cyl was acting like it had a vacuum leak.
I loosened the butterfly stop screw on the rear until it barely closed fully, the O2 operated properly and the idle smoothed. Rode it like this for a week. I noticed that my throttle was iffy just barely off-idle, so I went to investigate further.
It turns out that when I let the rear butterfly close all the way, I fixed one issue but caused another...I now had slack between the actuating of the two butterflies; the rear opened a hair early, reintroducing the issue I previously thought fixed. I knew that damn tie-bar could be adjusted, just needed to figure out how...so I pulled the TB's off. Turns out there is an adjustment screw on the right-hand side of the rear TB to adjust the position of that bar's cam relative to the butterfly. So I loosened that, made sure both butterflies were held tightly closed (helper hands), put that bar under compression (twisted the rear cam to simulate opening pressure on the bar) and tightened that screw.
Reassembled it all. Everything idles smooth, off-idle smooth, no error codes...everything's great.
In short, the butterflies were out of sync and I found the (almost) proper way to fix it. I say almost because a bench-top setup with some dial indicators would be preferred, but I don't have that so I use my eyes.
I'll see about getting some pics soon, to illustrate what to adjust and where.
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