If you want to troubleshoot this yourself, get a cheap OBD scanner ($15) and a good OBD app like OBD Fusion.($15)

Then you can find out your fuel trims which will tell you if your engine is running lean or rich. You can also see your air/fuel ratio in real time. Also, OBD fusion has a very good diagnostic report that will show the mis-fire counters for each cylinder.

Low fuel pressure would result in a high fuel trim as extra fuel is being added trying to get the engine back to the proper air/fuel ratio.

Also as mentioned, the factory spec for fuel pressure is 51 PSI and a special tool (fuel tap) is needed to check it. The old style "T - tap" gauge won't work as all our fuel lines are steel or hard plastic - no rubber.

The codes P0171/P0174 indicates that both banks are too lean.
This tells us that issue is common to both banks. So look for intake leaks, torn intake boots, loose hoses.

If your fuel pressure is 30 PSI, that is too low and I would start there.

So today I the third day without my car. The dealer has diagnosed a bad fuel pump. We're waiting on parts. Fortunately I sprung for the extended warranty when I bought the car so the $1100 bill will be covered plus they're covering the cost of a rental. Hopefully they diagnosed it right. I'll keep you guys informed.

The dealer replaced the bad fuel pump and so far, even on a 90 degree day after driving for 40 minutes, parking, then starting up again, no problems. I've tried a couple of times to re-create the problem and so far no problem. I think they got it right. If anything changes i'll get back to y'all.

Over a month since the dealer replaced the bad fuel pump and the car is working fine. There haven't been any 100F degree days but so far no problems with sputtering and misfiring on restart. In fact, the car seems to have gained 20 or more horsepower. Low speed response is much better. The best part is I found out that the extended warrantee I bought was still in effect and I ended up paying $0.00 for the whole thing. The dealer got it right. Hope this helps.

:driving: :tup: Glad to hear you got the problem fixed. $0.00 is nice.

That is amazing to hear man! Im getting ready to pull mine out for the summer and hopefully get it fixed as I still have the issue as far as I know... at this point ive done so many little things ive lost track... I have some extra cash to spend that I have been saving so this might help, but I hope I dont have to keep dumping money into it, I really wish I had a very knowledgeable Z person local to help me fix this haha, I don't trust the dealer here, they werent ever able to give me a solid answer.

My P0171 and P0174 problem was caused by fake Denso Iridium FXE24HR11 spark plugs. The electrodes on all of these fake plugs had melted to a gap of .075 to .080. I did buy new genuine plugs from NGK, the Laser Iridium DF8H-11B plugs and this fixed my issue. I also replaced the front O2 sensors in the midst of troubleshooting my troubles. I had rough and misfiring issues with the fake plugs. I checked and tightened the entire intake tract and secured all vacuum hosing with fresh clamps to eliminate and vacuum leaks. The fake plugs were confirmed by the fact that the writing could easily be scraped off. The real Denso plugs do not let the lettering scrape off, the letters are under the glaze on the insulator. The fake plug issue is widespread on the web.