When I datalog my AFR's are not the same every pull, not even close sometimes. I would say 80% of the time they are where I set them, but the other times they are 0.5+lower than normal. I'm doing all the pulls in 3rd gear and basically everything else is constant, boost etc. even when the afr's drop, so it should be in the same cells. It also seemed to do this mostly in the mid range, once it got up to around 7000rpms it would lean back out to what it should be.

I talked to the dyno operator a few weeks ago when I was there and he said he didn't know that he has seen people come by that can't figure it out either. All my logs are street pulls btw. Any ideas on what could cause this?? All I can really think is fuel pressure.

 

Are you doing your pulls at the same ambient air temperature? Different temps can change the AFR's.

 

Yes unless the temps drop 30-40 degrees within 5min and me not knowing about it.

 

Your O2 WB floating ground is fluctuating causing an instable output voltage.

Now if you want a real answer, why have you not posted two real logs? DOH!

 

It's not just the votage from the WB, it was proven on the dyno by having two different power curves with it losing mid range power when it went rich.

I'll try and post some logs when this weekend.