I have the 1998 E350 5.4 engine that is throwing lean code P0174. However, both banks 1 & 2 are running close to 25% as shown in this freeze frame. Under normal driving conditions, I do not get any codes. It throws a code while driving up a steep grade. It's thrown pending P0171 and P174.

I was getting DPFE sensor codes for 2000 + miles until I replaced the pressure tubes and the EGR pressure feedback sensor (DPFE).

I checked for vacuum leaks with propane even though it's doubtful that it's a vacuum leak since both banks have the same problem. I used a propane bottle with a vinyl hose attached to the end of the torch bit. I sprayed near all vacuum lines and connections. I used propane to prevent a fire inside the engine compartment even though it's flammable, it's not as volatile as carb or brake cleaner. Propane is easier to control and will not cause deterioration of the hoses.

I have not checked fuel pressure. I downloaded OBD Fusion to my iPhone and the fuel rail pressure is available although it's reading 0. Obviously, I am doing something wrong.

The MAS was replaced by the previous owner in January. However, that does not mean it's working correctly. I cleaned it several times during the course of the 5000 miles I put on it. I did this because I suspected bad fuel. I put a bottle of Chevron Techron in the tank too and refilled only at Costco and Chevron after the suspected bad fuel.

 

If you've not already posted this in the 5.4 Forums do so---there's a ton of accumulated experience to be found there.

I myself would suspect the MAF either not working as those LTFT's are off the charts too rich. Tough to say about the STFT's as those are constantly changing which is normal; the values you show assuming that image is just a snap shot of the fuel injector behavior aren't troubling.

I would definitely check the fuel pressure via the test port on the fuel rail---If I understand correctly the OBD-II reading isn't all that useful determining the usable fuel pressure. (If I'm wrong there someone will or should correct me--don't wanna give bad info here.)

Just FWIW and in case you'd like a bit of education about fuel trims I've found this video one of the best explaining it all. In fact this guy is a fantastic teacher. It's a bit lengthy but times goes quickly when something is so well presented. Hope others find this useful too: