2001 F150 5.4l 2v (L VIN) 134,000 miles running lean on both banks (no DTC yet)

Hello ladies and gents. I've got kind of a long one for you today, so sorry in advance. I'm trying to provide as much detail on what I've done and noticed with this truck since I've purchased it in early January.

Initial TL;DR: Started with rich codes on both banks under throttle, normal trims at idle. changed MAF and now it's a lean condition at idle with trims evening out quickly at 1200rpm.

More details below.

Prior to resolving(?) this rich issue, I had changed the fuel filter, PCV Valve + hose (passenger valve cover to intake) fuel pressure regulator, and ran injector cleaner through the tank. I have also cleaned the IAC and Throttle body and have replaced the throttle body gaskets on the truck.

please keep in mind that during the course of this, for whatever reason, I did NOT clean the EGR passageway. I do smell gas from time to time on startup. regardless of cold or warm startup. Not every time, though. and I do not smell it while driving/idling at lights. I'm not sure if all of this would be caused by a clogged EGR pipe/elbow, however. (specifically the lean condition that is now occurring)

One day after all of those above changes, I realized that the air readings were higher than people had suggested at idle (7 to 7.5g/s at 600rpm vs the 5.5-ish I should be seeing) I took this as a sign that my MAF wasn't reading correctly, and thought I'd try a 'new' one from the junkyard, as mine was looking rough and had no change after cleaning.

Immediately with this 'new' MAF, the trims began to go toward 0... And then right past it. this issue is now a lean condition on both banks.

Now to the fuel trims/ other random numbers with this newer lean condition I'm currently experiencing:

At idle I'm at 10 to 20% LTFT on both banks. this appears to get worse as the truck warms up. I've heard the opposite tendencies for intake manifold gasket leaks but I haven't fully ruled that out yet. I've replaced most of my old vacuum hoses, Including the PCV valve and hose mentioned earlier, driver side PCV elbow, and all rubber connections to hard vacuum lines on the engine side of the tree.

I've got numbers with the trims at each RPM for more info:(LTFTs only in this section, bank 2 is generally 2% leaner than b1 but close enough so I'll count both)

550-600rpm (warm idle) : 10% to 20% - seems to be worse when fully warmed up

1000-1400rpm: 4% to 8%

1500-2000rpm: 6% to 9%

the 'sweet spot' seems to be in the 1000-1400 rpm range. not sure what that suggests. I've heard trims with vacuum leaks improve with RPM.

(If anybody has any input on this specific tendency, the hint might be enough to give me some inspiration on this.)

I have used a vacuum pump to test the vacuum reservoir side of the vacuum tree, on the passenger side, and when testing from the manifold tree to the vacuum reservoir, could note a small leak in vacuum pressure. maybe 5mm/hg per second, or a little slower.

I assume this is too slow of a leak to cause the numbers I'm seeing, is that correct?

If I've left out any important information, or if you need any other information, please let me know and I'll do my best to get some numbers/info for you. I really appreciate it.

Did you clear all codes and the keep alive memory after installing the other MAF? If not, you're seeing the correction of the fuel trim table. Drive for a while then look at the trims.

I hadn't actually reset KAM or unplugged the battery after replacing the MAF. I have driven 250 miles since then and let the vehicle idle while pinching /plugging hoses and these numbers persist, though.

it'll also vary heavily during a single drive. on a warm idle I may see 12% at the highest, but after driving and stopping it may go to 16%, and during that 2 minute idle it can creep up to the highest I've seen so far, to 21%.

I keep a close eye on the RPM but warmed up at 500rpm vs hot at 550rpm I can tell a difference for sure.

I never see these higher numbers (above 13 to 14%) until I'm fully warmed up, and going from a cruise to a stop light. this makes me think that a KAM reset wouldn't resolve this. but please let me know if you think differently. I'm down to give that a shot.

Disconnect the MAF... this will put the truck in FMEM mode (failure mode and effects management) the system will estimate air flow via other sensors and reset the Baro value. If the lean condition goes away. The MAF is dirty or contaminated. Clean or replace the MAF. You got the MAF from the junkyard, it maybe dirty or no good. After replacing the MAF disconnect the battery for 5min.

I'll give this a try tomorrow after work. Thanks for your time man.

I gave it a shot; unplugged the MAF and ran the trick at a warned up idle.

LTFTs stayed the same, but the short trims went from hovering back and forth between 4 to - 4 to jumping up to near 15% on both banks. truck idled a lot more rough.

feels like a decent bet that the MAF is not the cause of my issue here. which does help to confirm that it WAS the issue previously, so I'm not just masking the new issue with my assumptions.


so far my biggest guesses are: Evap purge valve stuck open, possibly a clogged EGR pipe/elbow, and then down to the intake manifold gasket leak. I've tried the water trick before on the intake manifold and didn't have any luck. but I'll give it another try this weekend I think.

also going to the junkyard to try and salvage a purge valve hose because I know the second I take mine off it's going to break. so to test the purge valve I'll need a backup hose/connector first.

any other ideas I may be missing?