I've searched on this and turned up a BUTTLOAD of data. Here's the deal:

This weekend after getting my oil changed and replacing my headlamp assemblies (which involved removing the battery, thus resetting my PCM) I suddenly found myself with a CEL. The codes are the P0171 and P0174 (too lean on both banks).

After researching this, I've decided it would be really freaky to loose 2 O2 sensors at once, and I just cleaned my UIM about 8000 miles ago and haven't had a vacuum leak at all since then (new gaskets, the works, except I didnt' replace the fuel pressure regulator). So I'm figuring either it's a dirty MAF, OR it's a problem with my fuel delivery system.

I also wanted to mention that I'm getting GREAT gas mileage with the CEL, and the engine runs cooler. In fact, she runs just below the N in NORMAL. But I do seem to notice a slight loss of power at WOT, but it may just be a plecebo effect.

I'm going to try cleaning my MAF with electronics cleaner today, but I'm wondering if it's that Fuel Pressure Regulator I never changed or possibly the fuel pump? I mean the car DOES have 104k miles on it :/

How do I test for proper fuel pressure?

have you tried clearing the code to see if it was a fluke?to test the fuel pressure youll need a fuel pressure gauge and adapter.

I stopped at an autozone and had them clear the codes. It came back a couple hours later.

Can I get one of the guages and adapters at any autoparts store?

SVTCANUK has had this problem and got rid of it by tourquing the UIM to the upper end of the spec range. We just did mine yesterday and I can't recall now what it was (around 95 inch pounds IIRC) ... care to chime in here Rob?

10nm or 89 in/lb.that was from fords QC dealer online shop manuals.

Does the cel come on intermittently. Before changing my o2 sensors at 100,000 miles, I would get a cel usually on the highway. It would go on and off every minute or so. On my obd 1 system it was codes 172 and 176 if I remember correctly. After I changed the front sensor I did not get any cel's. But I changed the rear one anyway, major pita.

About the fuel regulator, as far as I know if it is bad the main problem you will get is surging.

And finally, the o2 sensors would not cause a loss of power at wot because the engine doesnt't use the reading from them at wot.

Once the CEL comes on, it stays on till I either clear the codes with an OBD-II scanner tool or by resetting the PCM.

It's probably not hte fuel pressure regulator then becuase there's no surging. Could it be a failing fuel pump?

I had the exact same problem last week. The same codes, the same great gas milage and the same cool running engine. You can search for my posts. I solved my problem by re-torquing my UIM/LIM bolts to the max 108 inlbs (previously 72 inlbs) and I also replaced all the small hoses on the PCV valve and put a clamp around the large one on the bottom for a better seal. I don't know which of these fixed the problem because they were done at the same time but it is a place for you to start looking. BTW when you fix the problem the engine runs warmer and the great milage goes away .

Well I went out and bought myself a torque wrench and torqued the bolts all down. I just guessed before, looks like I actually over-torqued them though. All the bolts where just over 100in-lbs already. Looks like I'll have to look elsewhere for the cause of this problem

If it was the fuel pump, I would get surging right? Or could the fuel pump be starting to fail and not push enough fuel to the injectors?

Did you check all the hoses for an air leak? I would suspect an air leak before a fuel problem. Spray some carb cleaner around the UIM/LIM as well as anything that runs into the UIM. If you have an air leak the car will hesitate when the carb cleaner is sucked in.

Update on this:

I performed the intake manifold overhaul How-To, which means I replaced all gaskets, seals, o-rings, fuel pressure regulator, and cleaned everything (uim, lim, IAC, and MAF) really good.

Unfortunately, problem is still here. Car is still running lean.

I've so far narrowed the problem down to somewhere in hte PCV system. Afterall, the only places I DIDNT clean, or replace is the PCV system (I cleaned every other part of the intake system doing the overhaul and tightened everything down to the higher-end of spec)

I'm seriously thinking its a crack or hole in the hose that connects to the top of the PCV valve, and goes around the backside of my engine bay along the firewall. Thats a plastic flex hose and is extremely brittle being 7 years old and over 100k miles. I've ordered a new PCV valve just in case, and the hose from Bill Jenkins and will keep you all posted on any changes/fixes.

The rubber elbows that come off the top of the valve covers for the PCV tubes develop cracks sometimes. If you're suspecting the PCV system, might want to check these...