Going to put some backstory to this, so bear with me. It's been a bit of an ongoing issue. 1998 Expy, 5.4L Auto Eddie ("Project Workhorse" in the sig...):

Started seeing codes P0171/0174, and having idle/shut off issues. Changed PCV, IAC, EGR tube (done when headers were installed) and EGR valve. Helped, but didn't fully solve the problem. Changed the mass airflow sensor, which also appeared to help, but only for about 24 hours.

Last Tuesday night, the only way I could keep it running at idle was to keep my foot on the throttle. It was fine on the way to work in the morning though. Got home, and had to leave again - but the Expy wouldn't run. Mash my foot to the floor, and I'd get about 1000 rpm out of it. Threw a shedload of codes, mostly related to O2 sensors *but no vac leak related codes*. Flatbedded off to the shop.

In the shop they found the intake box cracked, and a split in the PCV tube that goes to the back side of the intake. Changed both of those, then the fuel filter was changed as basic maintenance, but it was still reading between 30-40psi fuel pressure which for what I know is normal range, so no fear of a fuel pump issue. Now here is where the real fun and problem begins.

Without the MAF hooked up the motor runs strong - in fact they considered letting me drive it for the weekend without the MAF hooked up to test it out for vacuum leaks but there's fear that other stuff possibly goes bad when you do that so we nixed that idea. With the MAF hooked up, it runs pretty bad. And there have been several MAF's tried on it - so the odds of three or four (I forget how many we tried this morning as it happens the shop had a couple of Expys sitting around for routine service stuff like tires and oil and such) being bad are pretty low, I would think.

The other thing is that we're getting odd and inconsistent readings for voltage to the MAF - at times it will read 3.8, 4.2, etc. and the motor runs great and at other times it will read 1.8, 1.5, 1.6 and the motor responds accordingly by running poorly.

We're (me and the mechanic) stumped. Could this be signs of the PCM going bad? Simply a short/pinch in the wiring? And where does all this wiring go? I have access to a set of service manuals for the F150's of that vintage (as I also own one of those) and I'm assuming there's some crossover between the two vehicles since they're on similar platforms, but it's not clear.

Clues?

 

First off try a new MAF because the used ones can be contaminated to some degree. Second, after replacing a MAF you are required to disconnect the battery for at least 5 minutes to reset the KAM. Failure to reset the KAM will force the ECM to use fuel trims and such based on the previous MAF sensor's output readings.

 

Specifically what codes are showing after these changes?

Its difficult to make much sense of the MAF voltage with a DVOM. The pulses within the intake manifold make the voltage jump each time an intake valve opens. You really need a scope to see how the MAF is behaving.

A another way to diagnose a MAF fault is to watch the short and long term fuel trims. If the Fuel Trims are high in all operating condictions the MAF is contaminated. If the fuel trims are high only at idle/light load there is an intake leak after the MAF.

Any P1100/P1101 codes present?

 

check the whole MAF wiring and the plug. i've seen at least on 6.0's they could melt/damage the plugs over time and cause very odd readings thus running bad.

running without the MAF is fine it puts the truck into a rich running limp mode so to speak. i did this with a '94 explorer for a month or so before the person i was helping could afford a new MAF.

 

The MAF on the truck when it went in to the shop was brand new (remanned Cardone actually). I did the reset procedure at home after replacing it as well. I will make note of the reset thing and make sure we did that.

 

After the vac leaks were fixed, it wasn't throwing any codes. I drove about a mile or so before it started flaring up, and again, no codes.

He plans to scope it Monday - It was late yesterday when this all started going on, and we gave up for today after some basic diagnostics. No P1100/1101 codes.

 

The one mechanic in the shop went on a big rant about how if the MAF is unplugged it won't send info to the computer to control things like the tranny cooler and it might screw up the transmission...which prompted the "let it sit" discussion. When I test drove it, the MAF was hooked up.

We did trace some of the wiring, but not all. I will make sure to pass that along since he will get back on it Monday and I will be at my desk-jockey job unable to help him.

Thanks for the replies, guys.

 

makes for a good story but not true, but i've done it on more then one ford for weeks even months at a time over the years with zero issues. the whole idea behind it is the computer goes into a "fail safe" mode and runs the truck off pre-set fuel/air ratios that is pretty rich to keep it very safe.

 

Your mechanic needs to use a smoke machine and check for additional vacuum leaks before you go buying a MAF sensor. Eliminate the obvious first.

 

Remanufactured is not new, it is a cleaned-up used part.

 

That's one of the things we did this morning. Vac leaks are all sealed up.

 

No offense taken, nor none intended here, but my understanding is that a reman is something that is refurbished to OE "new" spec. Is that not correct?

 

Sometimes you're lucky and it is, then sometimes you're unlucky and it's not. Personally, I don't trust refurbished anything when it comes to electrical.

 

Just for the sake of closure.

Turns out after 2 months' time, a dealership and a breakout box, there was a short in the main wiring harness for the Expy. Fixed said short, aside from making sure all vacuum leaks were sealed up and it runs like new. Took it on a 350+ mile jaunt on Saturday and it never flinched - even at 168k miles.

On to the next, and more fun, projects.

 

Many Thanks for taking the time to report the solution.

BTW, was the short in the MAF circuit?