Sounds like the MAF g/s is spot on or real close. I would suspect a vacuum leak or unmetered air of some sort.

Have you checked all of the pcv hoses carefully?

Have you sprayed it down with brake cleaner?

Try plugging any vacuum ports and rechecking trims. Pull the brake booster hose too, boosters do leak, or at least pull the hose off the booster and plug it with your finger and watch the trims.

One of the best tools i've come up with for listening for leaks is using a mechanics stethoscope with a piece of brake line stuck in the hose, you can use rubber vacuum line too but it's much easier to get to some places with the long piece of brake line. It's excellent for pinpointing leaks. A couple of weeks ago I had one come in that I was pretty sure had a leak but was hard to find, ended up being an injector o ring and I found it by using the stethoscope around each injector until I heard the hissing sound.

 

 

Ugh sounds like you're having as much fun as I am. I have a fuel trim pain in the *** in my bay right now. Working on a 98 Ford F150 and it's the only vehicle i've seen go to 35% compensation. Found a couple of large vacuum leaks and fixed them. No more leaks at all and fuel trim still at +35%. Fuel pressure is in spec, increased it from 28 to 40 and still no change.

On to the MAF. I actually started here but I went through and ended up testing everything else because I had rigged up some towels in the intake track to route more airflow over the MAF and the reading went to 6.8-7.3 ish g/s but my fuel trims were still at +5% on one bank +25% on the other bank, so it didn't seem a new/different MAF would be all of the answer, and I need to have all answers before I turn in the ticket.

With the MAF sensor mentioned above and no rigged up airflow it's only reading 3.48 - 3.52 g/s at hot idle on a 4.6 v8 (700 rpm), that's too low. I also checked the barometer and it's 26.8 which is an indicator of a faulty MAF. So I swapped the MAF and the new one is also reading 26.8 baro which is wrong but it went up to 5.3 ish g/s and the trims dropped all the way down to 5% on BOTH banks. 1.6 grams dropped the compensation by 30%. Usually it's not that dramatic.

But the crazy thing is with the original MAF rigged up to see 7 g/s it was still very lean, with the new MAF at 5.3 it's perfect. This is a first, but I usually end up with the craziest fuel trim issues on Fords so it's not surprising.

An autozone MAF ended up being the one that worked correctly, the other MAF the vehicle came in with was actually brand new also (no name on it so not sure what brand it is). That being said, run to autozone and grab a MAF and just try it and see what happens. I used to think I would never recommend autozone but lately (about a year) we have been using them more and more because we haven't had an issue with any of their parts yet and finding quality parts these days is getting hard if it isn't a OEM part or made by the original manufacturer that made it for the OEM (delco delphi motorcraft etc).

 

Very interesting and you have me thinking. I have always thought that the MAP is what is reading the barometric pressure ( after key on and before cranking) and not the MAF. Do you know this not to be true for the LS motors? Do you think that may be why the baro reading was still constant after you changed the MAF? I think your MAF was bad since it corrected the problem but seems like your MAP also needs to be replaced so it will read the 29inHg.

For my problem , I did swap out the MAF for a known working one on another 5.3 that was running fine. My fuel trims and g/s readings were exactly the same with both MAFs. But... I have not tried swapping the MAP. I'll try that right away just to see...

 

Okay so you substituted the sensor and got the same thing. Your g/s is real close to where it should be so I would move on from the MAF for now and make absolutely sure all other base items are correct.

Try the MAP just in case, but fords use inferred MAP/Baro through the MAF, GM uses an actual MAP..

On Fords checking the baro is one way to determine if the MAF is working correctly, if the implied baro is off then the MAF is off. The new MAF I put on it today showed 26.x baro initially, but if it functions correctly it will correct over the next day or two. It takes some driving to get a Ford's baro correct and some WOT driving. This particular truck was a **** box with a knocking motor and had a giant crack in the radiator that was puking coolant so I didn't drive it to set the baro.

 

Has it been like this since you did the swap, or did it just go lean one day?

 

Hi All,
I beleive the problem is solved and just wanted to post the resolution for the archives.
After running a few more checks with a good quality stethoscope to look for leaks and trying another MAP and MAF swap with know good ones ( without any improvement), I finally decided to buy a couple more credits on the HPTuner and reflashed the
MAF table adding 15% as Joecar suggested. This brought the long term fuel trims down from 24% to about 8%. The ECM seems happy with these and there are no more codes.
Just wanted to say thanks to everyone who offered ideas.
Jeff