So, new guy here. I've owned some Ford pickups over the years, and recently bought this FX4 Supercab for my son, who is headed to Alaska on military orders early next year. He needs 4X4, so I jumped on this truck. It has almost 151K miles on the original engine and tranny. Pretty decent truck, considering the current truck market.
I bought it with a lit CEL for Bank 1/ Bank 2 too lean, P0171 and P0174 respectively. I thought I had it licked last week when I found the air filter drawer missing any kind of seal, but it came back today once the fuel system went closed loop.
The previous owner changed 3 of the 4 O2 sensors with Bosch brand sensors. At idle, the sensors all change wildly from one extreme to the other; 0.845 to 0.075 in one update. SHRTFT stays around 48-50, so it's trying to add a bunch of fuel to correct the lean condition. I also noticed the fuel rail pressure staying around 5.7 PSI. That seems really low.
I feel like I'm circling the drain here... any ideas of where I should start looking for problems?

 

I would change the fuel filter if it hasn't been changed and go from there.

 

Check the FPDM and its connector for corrosion. Very common issue with that generation truck. You have to unbolt it and check the backside.

 

It was the FPDM! Replaced it today, and now have correct fuel pressure and much better SHRTFT numbers. Runs much better now.

 

Way to go glc on the advice!! Nice work Diesel Power on getting that swapped out!

 

Mostly...

The FPDM was bad. It was so corroded the bolts broke off in the crossmember when I tried to remove them. We had 5.2 PSI before changing it, and it had no power; it was drivable, but couldn't get out of its own way. After changing the FPDM, we're getting 36-40 PSI and it has power again.
That being said, it has thrown the codes again for bank 1/2 lean. When they're set, the SHRTFT is high again, like 35-50. I clear them out, and they stay gone until my son hammers on it in traffic, then they come back. Not sure what's going on...
I'm going to change the fuel filter next, and run some fuel system cleaner through it. I tried the old carb cleaner spray trick on it, hoping to find a vacuum leak, but she's tight as a drum.
I actually tried to change the fuel filter first, before replacing the FPDM, but the filter I got from Advance Auto did not fit. It was spec'd for the truck, but fittings were too big for the lines to connect. They were physically larger diameter than the filter that is on it. I had already ordered the FPDM from Rock Auto and had it on-hand, and after seeing the corrosion back there I went ahead and swapped it out.
Something else I don't like is the MAF reading on a live data pull is only .1 lbs, and goes up to .3 with throttle input. Is that normal for these trucks? No MAF codes are set, but it seems odd to me.

 

Ok, so I’m still having problems with this thing. I replaced the fuel filter and the FPDM, and have not found any vacuum leaks.
it’s still throwing the lean codes. This morning it made it all the way to work without any check engine lights coming on, then the light came on when I drove it home. I did a data pull on it again, and the fuel trim is sky high again. At idle, I’m getting 0.0 from the MAF and the FRP is back down to 5.5 PSI again!
Please let me know if you have any similar experiences or ideas. I don’t want to take this thing to the dealer, but I’m running out of ideas.

 

Ok, IMHO, 1) scrap the Bosch O2 sensors and go with oem...... years ago Bosch was the "cadilliac" of components, for the past few years they are "junk" and Bosch itself has been sued (and lost) in international courts (multiple) for a variety of frauds and international fraud scemes..... went through this with the O2 sensors before with my own vehicle.... just like a script IMHO of what I went through.

2) Get/build/borrow a smoke machine! Despite the "old school ways" of looking for leaks (which does work on the newer gen vehicles to a certain degree, they are much, much more sensitive and we simply don't have the abilities of "humans" to find the tiniest of leaks. Went through this with my wife's car years ago...... finally surrendered and bought a halloween smoke machine and contacted the mfg....guess what, same company builds the commercial smoke machines for mechanics! They also advised to use a water based product as compared to the solvents so to avoid internal contamination. I built an adapter to connect a vacuume cleaner hose from the smoke machine to the intake and it works really well!!!!!!! FYI, the hose cannot fit snug up against the smoke machine because 1) where the discharge is it does get a bit too warm and 2) it needs to draw fresh air in to create the smoke expansion... otherwise all you get in water droplets.

 

I did finally get it to stumble when I sprayed the passenger side of the intake with carb cleaner, so I ended up replacing the intake manifold and gaskets. When I removed the manifold, I found the unused vacuum port on the backside broken off, and the hard line supplying vacuum to the brake booster was leaking at the metal 90 degree joint. I fixed the line and got the new intake installed, but the damn codes are back again!
I downloaded Torque Pro and bought an adapter so I could watch the O2 sensors with more detail, and have found B2S2 switching wildly like the upstream sensors, while B1S2 is steady between .6 and .7 volts. Today I swapped the downstream sensors between the two cats, hoping a bad sensor would follow, but it did not move to the other side with the sensor. It looks like maybe a bad cat. I’m headed back out to inspect and clean the PCM connectors, then recheck my readings. Not looking good…

 

Cleaned the PCM connectors. Afterwards it stalled twice in the garage, and now both bank 1 sensors are lean (0.0V). I’m going to officially wave the white flag and look for a computer diagnostic…

 

So for anyone still following this disaster, I can finally report a good fix. Vacuum, vacuum, vacuum! I didn't throw good money at chasing ghosts at least...the intake manifold was broken, the brake booster vacuum line was leaking, and the PCV hose on the driver side was also bad. Ford tech told me about the PCV leak; I had heard something under the hood one day while I was chasing my tail, but never went to check on it. It was the PCV sucking air. With all the leaks fixed, the lean and O2 codes went away. I now have to take it to an exhaust shop, because some knucklehead welded the cats on at a position that is allowing the seniors to contact heat shields and front driveshaft, then we gotta put a block heater and a battery blanket on it so it can report for duty in Alaska in about a month.

 

A while back I bought a cheap smoke machine. Comes in handy for that kind of stuff. Glad to hear you got it fixed though!