Lean Condition Codes P0171 P0174

I've spent the last couple days reading all the thread I can on these codes.

My symptoms:
When the truck is towing and under load (going uphill) it seems to trigger a limp mode. It acts as if the fuel is being cut and won't accelerate beyond limited throttle input. Once in this mode, it doesn't seem to want to come out of this mode unless the battery is disconnected for a while. The truck seems to drive normally without a trailer (even after towing and going into this mode) and has never triggered this while not towing. It doesn't always throw lean condition codes, but it has a couple times now. It even threw a misfire code and went to blinking check engine light a couple times. I'm assuming this is caused by it being lean.

What I've done:
Research...read all forum posts I could find. It seems like it has to be MAF sensor, vacuum leak, or fuel pump. Once of the first things I did was clean the MAF...no change. I've got another MAF on order to test it out. I looked for vacuum leaks with propane and then pushed and wiggled on anything that looked like it went to the intake or intake manifold. I look at fuel trims and noticed that the STFT was negative, which supported the theory of fuel being cut. I've since left the battery unplugged for an hour and checked them again. LTFT is at zero and STFT are hanging at around 3ish percent while idling and aren't going further negative under more throttle.

I've don't currently have a way to check the fuel pressure.

I'm really hoping this isn't a fuel pump, but I don't really think it is the MAF. I saw someone ended up replacing their ECM to fix a similar issue. Any suggestions welcome.

 

Well besides fuel pressure as you said still needing testing - there is the potential for a poorly operating fuel injector (more tests) - or an air leak in the bellows from the MAF to the plenum (sometimes under heavy throttle there is a lot of engine movement causing this). I'd rule all these out before replacing anything. Luck.

 

I checked everything over, including the bellows. I replaced the MAF sensor. I towed again and monitored the STFT and LTFT. Interestingly, the STFT never went crazy (+5-7%), but the LTFT started building over time. I started have the same symptoms and LTFTs were hitting as high as 40%. At this point I think it must be the fuel pump.

 

I ended up replacing the fuel pump. I haven't towed with it yet, so I don't know if my problem is solved.