I desperately need help with this. I am starting to get frustrated with all the money going in to this problem with no solution in sight. About March, my wife was getting ready to take a road trip with the kids to see her mom in Florida. A check engine light had come on a couple weeks prior. So I look into the code and it is throwing 171 and 174. Too Lean, Bank 1 and Too Lean, Bank 2. I google the issue and see several people who have had it before, which (I guess) is a good sign as it generally means easy repair. First fix people were suggesting was Mass Airflow Sensor. I go out to my local Autozone and buy a sensor and throw it in ($110 after core charge). Reset CE light, everything is fine for a couple days and boom, back on, same codes.

Next thing I see is vacuum leak. My brother in law and I look and find a couple elbows and possibly a leak at the PCV valve grommet. I call up my local dealer (other parts stores don't carry the pcv valve and grommet). Dealer says can't buy just the elbows, need to buy the whole assembly. $80 later and I have a new vacuum assembly on. Couple days later, I'll be damned if that CE light isn't back on.

At this point, my brother in law calls a mechanic he knows that was formerly a Ford master mechanic that now runs his own small shop (convenient right?). Mechanic says all the stuff we had already replaced. Also, mentions that it could be fuel filter and possibly the MAF sensor from Autozone could be defective because they are known to not play nice with these vehicles. Navi is due for her spark plugs anyways so I schedule an appointment with this guy to get those done as well as the fuel filter. When it is in the shop, he calls and says while doing the fuel filter, he noticed the fuel pump was going (low fuel pressure, immediately drops to 0 pressure when shut off). This sounds legit because I have noticed it was getting harder and harder to turn the beast over after it sat for any length of time. $900 later, vehicle has a new fuel pump, fuel filter, spark plugs, and wires.

I was like, "Woohoo, this crap is finally done, the fuel pump explains why it was saying to lean, it wasn't putting out enough gas!" I eagerly watch my mpg on the way home on the dash and finally it is hovering around 10ish mpg when previously I was only getting 7-8 trying very hard to be fuel efficient. Vehicle ran like a charm until today (picked it up from mechanic on Thursday). Pulling out of my subdivision, boom, check engine light.

I am now beyond pissed. This vehicle shows absolutely no symptoms (and never has the entire time of the problem) other than the gas mileage has been ****tier than a dump truck. Even the 10mpg seemed a little less than what I was used to getting (11-12) around town. I chalked it up to maybe bad tire pressure and figured the real test would be a highway drive as mpg can fluctuate tremendously with city driving. Still seems to start a little slower than it should, but only just barely and after it has sat overnight or while I am at work. I guess my next step is to see if I can get autozone to give me another MAF under warranty and see if that fixes it and then get a Motorcraft one and try it if that doesn't fix it. I am absolutely out of ideas and am leaving on vacation in two weeks. I need to get this thing figured out so I don't spend more on gas than I already have to for this monstrosity. Does anyone have any tips, insights, ideas? I will take anything at this point, I just want to figure it out as my bank account is starting to suffer because of it.

Just as an aside, I have gotten mixed answers on replacing the oxygen sensors. I have seen some answers that said those codes mean the O2 sensors are working fine (since they are reporting the too lean condition), some say it could be one or both are bad, others still even say when it is those two codes together it is never the O2 sensors as they likely won't fail at the exact same time.

 

When its reporting lean the LTFT plus the STFT is over ~+30%. That will pull your MPGs down. At least its now moving in the right direction.

Wires? I can't remeber - does the 2004 Navi have the DOHC engine, the SOHC with 2-valves, or 3-valve engine?

Shouldn't have a fuel delivery problem with all those new parts. But, a vacuum leak would be more pronounced at very low engine speed, i.e, while cranking as opposed to under heavy load, as in pulling up a steep hill.

I'd take a look at the freeze frame data when the code sets. If it sets under heavy load I'd think fuel supply (but not with all the new parts) or MAF, maybe, or dirty injectors, depending on vehicle miles. But if it sets under low load (when manifold vacuum is high), I'd be thinking vacuum leak. How is the vacuum line going over to the EVAP Purge Valve? Sometimes the vapors make that line go pourus. Or, depending on engine type, some intake manifolds can cause vacuum leaks. Might be worth it to have the intake smoke tested to remove all doubt.

Don't forget, there are two sides to the PCV system. The "dirty"side and the "fresh air" side. (Took me a while to figure out why my Navi was throwing a 171/174 code about every three months. The whole fresh air side was missing!).

FWIW, the PCM keeps a close eye on the voltage, frequency, and amplitude of the O2 sensors. If it sees a fault you should find a DTC in the P013x range. Only then would I change the O2 sensor.

 

thanks for the detailed insight into this. My brother in law has a pretty decent OBDII scanner, so maybe we can take a look into these items this week sometime. I will report back with updates when I can.

 

following your advice (I haven't yet had a chance to grab the obd scanner from my brother in law), I checked the line over to the evap purge and when I moved it a little, the outside coating crumbled off. I feel like this could be part of my problem as well as that is most likely causing the line to be porous as you described. Also, at the green connector, it is a very loose fit, and I am not sure if it is supposed to fit better than that or not. I will be giving my local parts store a call today to see if they have those parts available. Hopefully this will fix it once and for all although I feel like there may be other parts of the vacuum assembly failing considering I have had a bad grommet, two bad elbows and a porous line already.