Sudden addition of 20+% O2 Corr??

[S351 R]

Data logged drive in to work and all cruise light throttle O2 Corr % were single digits, usually right at 1-2%. All was well. Later in the evening (&next morning) did more data logging and suddenly noticed that light throttle input had the O2 Corr dumping 20-24% under the same basic conditions.

Heavier throttle could bring those back down, but the light stuff is now is lean.

Program is clean from some phanotom value as far as I can tell (didnt change anything).

Noticed it first right after a WOT run but have already changed O2 sensor from one header side to other and same readings, suggesting that its not mechanical ie. piston/valve/injector etc.

The other variable was filling up with BP Premium. how possible is it that oxegenated fuel, or ethanol blend would effect O2 readings in the very light load cells. 20% of such little fuel in the effected area isn't alot, and once heavy load is introduced the slight impact of the alcohol goes away. ( Maybe? gonna take a bit to burn thru the tank to try new...)

O2 UEGOS value is .45, so sensor should be good.

Upper intake and TB has changed recently, but that would not account for the normal readings earlier in the day with same parts...

Car behaves as usual, just a bit edgier driveability, consistent with leaner mixture.

Pls help me brainstorm stuff to check. I have a new 427W due to go in and want to make sure the basics are healthy.
Thanks

 

[Craig Smith]

Check your fuel pressure and filter! Classic sign of a problem with one or both.

 

[S351 R]

can do. fuel pressure stayed consistent between both days. Maybe I got a bunch of trash from that fill up and finished off the fuel filter, but would'nt that impact only have gotten worse the heavier the load and fuel demand? In any case I will direct my efforts toward fuel system since it also has new fuel lines/regulator/rails (basically braided Aeromotive underhood items plugged into factory chassis hardlines - will finish full system soon.)

My biggest fear was a bad/clogged injector that Id be putting on the new motor, but given that both banks behave the same, I'll focus on things that would effect all cylinders, like what you said.
Thanks