Goood morning and welcome to JustAnswer!. This is Ed.
Change out your upstream oxygen sensor. Your problem is likely being caused by a slow-to-heat 02 sensor, causing an erroneous negative shift in adaptive fuel values as the system goes to closed-loop mode.
02 sensors require a great deal of heat to work properly and even though they live in the exhaust, they need assistance from an electrical heater built into them. Over time, the heater's efficiency degrades and the sensor takes longer to come up to temperature, but that doesn't stop the fuel system from acting on the 02 sensor's signal when it wants it (closed-loop mode). Taken too early, the information sent from the sensor will suggest that the fuel system is running hog rich, inspiring the fuel system to start trimming fuel back to bring things into line. If the info is wrong... as it sometimes is... the result is that the fuel system takes away fuel that's actually needed, by a margin sometimes of up to 66%. While self-diagnostics should be able to pick up on this mistake, the sad fact of life is that it usually doesn't. Unless the heater completely fails, you probably won't set a code to identify the bad part.
The 02 sensor in question lives in the exhaust right at the point where the two exhaust banks come together, right in front of the catalytic converter. Another sensor is located farther back from this one, but it's not the one responsible for direct fuel control (it's the downstream sensor). If at all possible, replace your upstream sensor with a factory unit or at least one made by the same vendor we use, which would be NGK, NKK or Denso.