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I towed a 15 Ford f250 in on hook today.  Will start, but won't stay running. Smoothly stalls, as if running out of fuel, but it will pop a few times out of intake if you really try to keep it running.  I didn't get far into it at the end of the day today to diagnose the problem, but did manage to waste 20 minutes trying to figure out the fuel system.  From both Shopkey and Alldata wiring, and from the scanner pids, there is no FRP sensor.  It does have the standard fuel pump driver module.  I'm trying to figure out on what basis the ECU determines when to rev up or slow down the fuel pump via the driver module when it doesn't see pressure?  Or am I missing something(I'm sure)?  BTW, I tapped into the fuel rail and it holds fairly steady pressure above 50 psi all the while its trying to stay running.  It wont surprise me if one of the cats are fubarred, as it has a bank 2 cat efficiency code, and misfire codes for cyl 1,3,7.  Will know more in the morning.  Myron

Do you have a wiring diagram?

      Thanks for the replys guys.  This truck is gone now. To remind everyone, this is a 2015 6.2 Ford with Flex fuel.  It was a logger truck thoroughly abused, with 120,000 on the clock.  When I grabbed it with the wrecker, they had already removed the air filter, so I'm assuming that was fairly plugged.  I didn't waste a lot of time figuring this one out, but more than I should have. It would start every time, and if i snapped the throttle quickly, I could keep it running barely.  It didn't act as if it was skipping, it would just smoothly want to stall, exactly as if it was running out of fuel.  Thats what ended up costing me a good hour of wasted diagnostic.  KOEO would show around 50 psi pressure.  That seemed like enough to me.  The spec was weird of 50-70 psi.  I did what  was not recommended( I wasn't feeling kind at this point), I went to the fpdm and gave the fuel pump battery voltage and ground, just to see what pressure would go to, and see if it affected the performance.  Weirdly, it still only showed 56 psi while trying to idle, and did not effect performance. I then popped a 3/16 hole in front of each cat and took a look at pressure, and it was barely over 1.5 psi while trying to keep it run. So, backpressure was ruled out.  I was then trying to figure out my next move, which was either possibly pull a plug and take a look, or grab the pico and get a capture with crank, spark and injector.  I pulled first plug and it was black.  Pulled a plug from another cylinder and it was also very black.  At that point, I had one of the guys go at changing all the plugs(16).  This has two plugs per cylinder with 8 coils. With plugs changed, I cleaned the MAf, and reset KAM.  The reason for Kam reset was that I was suspicious of Flex fuel % I was getting a 40% reading.  What I can tell you of the flex fuel vehicles are that they use a dual speed fuel pump strategy via the fuel pump module.  I have no idea yet after a lot of digging, when they decide to pick high or low speed. I would think they would use low speed at idle, but when I gave the pump direct battery, it was the same pressure as when the ecu was in control.  So, that was weird.  Also it uses a calculated flex percentage, and re evalutes after there is a major fuel fill. (over 40% fill up). The only way to reset flex fuel is with the fill up followed by several miles of drive time, or a Kam reset and the same drive time.  I still dont have a good idea of what the pump strategy is.  I would love to hear from a Ford engineer to shed some light on this.  It would seem to me to add complexity to the fuel maps when fuel pressure is purely inferred, as there is no FRP pid or input to ECU.   Anyway, after getting this thing running, trims looked very good at pretty well all throttle loads other than at idle, where there is a minus 12 % on LT on both banks.  It wasn't enough for me to warrant chasing at this point, but after the KAM reset, inferred Flex stayed right at 0%.  I ran some fuel into a clear container and it had no ethanol in it. So, I think a combo of plugged air filter, and a Flex fuel % way off were the main culprits. Another bit of confusion for me was on the FRTD, I graphed what I usually do, including the load pid. I'm not sure what the criteria is for load on this vehicle, but I would get it to 100% at about 3/4 throttle.  At idle, it also seemed high at around 20 percent if I remember corrrectly. So, that was a bit different to what I'm used to seeing on the 50 or more FRTD that I have captured in my library.   I'm going to include a few quotes from Ford Technical on the workings of this system for your consumption.
     As far as Flex fuel:  "In order to maintain proper fuel control, the PCM strategy needs to know the Stoich Air/Fuel ratio for use in the fuel pulse width equation.  On pre-2000 MY flex fuel vehicles, the percetn alcohol in the fuel was determined by reading the output of the Flex fuel sensor.  The percent alcohol was stored in a register called Percent Methanal(PM).  Although current alcohol-blended fuels only include ethanol, the percent methanol nomenclature has persisted.  On 2000 MY and later vehicles, the Flex fuel Sensor has been deleted and the PM is inderred.  The strategy to infer the correct A/F Ratio relies on the oxygen sensor input to maintain stoich after vehicle refueling occurs."
    As to the fuel pump, it uses a Mechanical Returnless fuel system(MRFS)-dual speed.  "The FP signal is a duty cycle command sent from the PCM to the fuel pump control module. The FPCM uses the FP command to operate the fuel pump at the speed requested by the PCM or to turn the fuel pump off." 
   So, in closing, this MRFS system of dual speed pump and flex fuel capable uses neither a FRP sensor nor a Flex fuel sensor. There is no schrader, so for fuel pressure, one has to physically disconnect the plastic line at the rail and T into it.  Hope this saves someone some wasted diagnostic time.   Myron