Ok so my dad has been using my 98 while I was fixing his 92 4.0. I had the 92 perfect and then someone pulled in front of me and totaled it out. Well now I got my 98 back and it's all messed up. Check engine is on p0171. Fuel pressure test done and it's steady 40-42 psi. I remember last year I checked it was like 68-72 psi. It hasn't been a year or two since fuel pump and strainer were replaced. Filter probably 15,000 miles ago. It's currently running good sometimes it cuts off when you start it then next time it stays running. Is this another fuel pump failure? How much longer will it last on 40 psi? The fuel lines and tank have 336,000 miles, engine maybe 150,000 and injectors have about 20,000 miles on them.

 

PO171 = System too lean bank-1. SO, with fuel pressure reported to be so low (40-42 psi) (for 98+ vehicles it needs to be 64 +/- 8psi), its no wonder the fuel injectors are giving a lean squirt.
SO, you need to do some basic trouble shooting to find out why its so low. Maybe measure the fuel pump voltage at the easy to get at in cabin inertia switch, located passenger side foot well, on the firewall just above the carpet I believe. Check operating voltage on both sides of the inertia switch, to make sure you don't have a voltage drop there. Measure voltage when you turn the ignition switch from Off, to Run & the computer operates the fuel pump for a couple of seconds to prime the system for a start. If B+ voltage is good that far, you've checked most but not All of the pump wiring. If B+ looks ok, then move on to check the fuel lines for kinks or bends that might restrict flow. If that checks out ok, try thumping the pump with a broom handle or the like & see if you get a change in fuel pressure.
How long will the 40-42 psi pressure hold, before bleeding off???? If its quick to bleed off, then suspect the pump located fuel pressure regulator, or the pump itself. With the mileage on this puppy, maybe tank sediment/gunk has clogged the fuel pump intake sock/pre-filter???? You did clean the tank when replacing the pump last time....right????
Some trouble shooting thoughts for consideration, let us know what you find.

 

I'm sure this time it's going to need a whole fuel pump module. Last year I had a shop replace just the pump and strainer but I had to do the same thing a year before too. Today I changed the fuel filter. I noticed the gas coming out the filter was gray not clear. Then I rechecked the pressure and it was fluctuating rapidly from 40-56 psi, the needle was moving rapidly on the gauge. I doubt the tank was cleaned but I don't know for sure. The fluctuating needle leads me to believe the regulator is bad. Unfortunately I have to return the truck to my dad until we can get the 93 we have roadworthy. It needs brakes, valve cover gasket, timing belt, tune up, fuel pump, tires, and a/c hoses so it will all take a while.

 

With the pump having been replaced recently & something going on with it again & with the mileage on this ride, I'd sure check the tank for trash. When the last pump was replaced, how did its sock/prefilter look for debris????

 

Last time the strainer wasn't clogged excessively it looked normal to me but I had it a shop that was replacing it for me while I was at work. I looked at the pump and the strainer and it didn't look clogged or excessively dirty. Last time this happened, the pump just totally quit pumping the truck wouldn't even barely move. The time before that the only reason I replaced the pump was because the plastic fuel pump retaining ring had become brittle and it was moving around causing gas to overflow from the tank when you fill it. Now I'm thinking it's just going to need a whole new module and pump assembly. Unfortunately I had to return the truck back over to my Dad until we get this settlement on this 92 Ranger that got totaled so I can get another Ranger or something else to drive. I told him to not be surprised if it quits running or barely moves or maybe not start at all. When I did the fuel pump on that 92 4.0 that got totaled we removed the bed to swap it out. It was so much easier. But then like a week later the fuel pressure regulator on the fuel rail started leaking gas, it wouldn't even run. So since this 98 2.5 pump regulator is part of the fuel pump module assembly what is it going to do when totally quits? Is it going to start leaking out gas from the vacuum hoses like my 92 when it went out? I was told by some mechanics that running the take near empty and then filling it up shortens the life of the pump too. We always do that in our family. The tech at Airtex said that's still debateable why the pumps keeping going out. I've asked some shops in the area and they say they have so many failed Airtex fuel pumps after they've been replaced, they keep having to replace them under warranty. That's the brand I used.