09 f-150 5.4 lean and rich then swaps banks

need4speed · Dec 21, 2016

Hello Folks, We have an 09' f-150, 5.4 that runs good part of the time then the fuel trims will go to -30 on 1 bank and +30 on the opposite bank and the truck will run poorly. Then after several (4-5) time of this the fuel trims will swap banks and go to +30 and -30 and run poor again. As we drive the truck and it is running good we can watch the fuel trims as they climb/drop and see that the truck is going to start running poor.
We have tried new converters, resinator, MAF, timing chains, PCM, both ft. 02 sensors. We have reset the alcohol percentage, we have unplugged the cam actuators/solenoids and unplugged the purge valve all without success. I know many things can cause this but to have the fuel trims switch, go opposite, has us baffled. We tried the dealer and after $800 dollars (they rechanged our 02 sensors etc.) they want to now do our timing chains again. We are lost. All repairs were done because of this situation, we did not create it. If it helps/matters this truck was imported from Canada. Anything?

FTZ HAIC · Dec 21, 2016

Did anyone work on the truck before this happened? Because it sounds like classic symptoms of someone swapped the front O2 sensors from one bank to the other (left to right).

What happens in this case is bank 1 may see slight lean (normal), so it richens.
But with the sensors swapped it's actually reading the wrong bank... and richens the wrong bank.
So the other bank sees too rich, and leans.... but leans the wrong bank.
That bank tries to richen the wrong bank, and so on until the trims exceed -25% and throw a check engine light.

09 f-150 5.4 lean and rich then swaps banks

need4speed

FTZ HAIC Staff Member Oregon Chapter Founding Member

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