AFR(M) is occasionally out of range from AFR(C)

I have a 2007 infiniti G35x sedan, VQ35HR motor. It has Long tube intakes, carless downpipes, full stillen exhaust, and tune. Recently, I did the water pump, timing chain, tensioner, guides, and gallery gaskets. However I was experiencing this issue before I did the repair, so I dont believe it is related. 

The issue goes, lets say im just cruising doing maybe 35-40 mph and hit a yellow light at an awkward distance, and decide to go through it. If im in the regular drive mode, and mash the gas a bit, the car stutters. It's almost like it doesn't know what to do for a second. After about 1 or two seconds, it finds its power and you're moving. When I watch the live data, for that second or so the AFR is leaning out to upwards of 17-18:1. It then fixes itself. My long term fuel trims are usually anywhere from -1 to 9 on both banks, and my short term fuel trims sit fine at idle, so I dont suspect a vacuum leak. Something weird about this is that if im in the manual mode, and downshift manually instead of just mashing the gas and letting the car do its thing, I have all the power I need, and the AFR stays in check. I did some testing of my own, and if I manually select 5th gear around 40mph and hit the gas, I see lean AFRs. Dont know if this is related. Another condition where the issue occurs is when im literally just cruising. No gas, no mashing the throttle, and I look top at my values, and for some reason im running at a measured value of 16.5:1, with a commanded value around 14.7:1. If I let off the gas and go back on, a lot of the time it just resets.

On my way to school today, I decided to take a log. For whatever reason, im missing a few PIDS, but you can definitely see the issue in both of the cases I described. I also decided to do a pull from a stop in manual mode, where the issue did not happen. Not sure if this indicates a bad sensor, either upstream or downstream, a possible fuel issue, dirty MAF sensor, or what. Theres definitely something going wrong though, and I find it really weird that the car hasn't thrown me a lean code yet. I included the log if you could be so kind to take a peek for me. If you need more data, I can re log and hope the other PIDS come through, there was quite a few of them.


For some reason I can't upload the file, so hopefully you can view and download it here:
drive.google.com/file/d/1xrNmSGfi_kSEyMO...36p/view?usp=sharing
 

Thanks for adding the log! I can definitely see the measured AFR going lean from around frame #53 to 60.

Frame #54 might be worth a closer look at. Measured AFR is at 17:1-ish, total fuel trim on bank one is past +30%, bank two is around +4%. That would suggest a problem specific to bank one. The stuttering you noted sounds like a misfire, and would also explain the lean fuel trims on one bank.

Do you have a scan tool that can read Mode $06? If you can, and the car supports it, you can use to to check for misfire counts. Or, if you can monitor live misfire counts during the stutter, that'd be even better.

You could also eliminate a volumetric efficiency or air measurement issue by monitoring both MAFs during the lean/stutter. Two MAFs that more or less agree would suggest measurement and engine breathing are fine. A large difference would warrant further testing/inspection.

I did check mode $06 misfire counts with my scan tool, and didn't see anything more than one misfire. I think the stuttering is a result of the ECU pulling timing (maybe?). I'll try to get another Log today, with all of the missing PIDS. Like I said there were quite a bit missing, so that could help get to the bottom of it. 

If you can get it to stutter again during your drive, check your Mode $06 miss counts again before cycling the key off. That way you can see the miss counts in the 'current drive cycle' TID 0C before they get averaged into the EWMA counts in TID 0B.