I checked fuel pressure at the injector rail on my 5.4 2001.
Doesn't register at key on engine off, but goes up to 13 psi at idle.
This truck turns into a pig when kicking into passing gear, or going uphill.
So, fuel pressure is low, BUT... My fuel trims hover right around +/-1-2 for both banks.

I was thinking that if my fuel pressure is really that low, it should show in my fuel trims. What gives?

 

Fuel pressure is what the pump is delivering to the system. OTOH fuel trims are what the computer determines the engine needs per the reading of various sensors.

Put a new fuel filter in it then retest, if still too low you will have to trouble shoot what is causing the problem of the low pressure.

 

Thanks for the reply.
I understand pressure, and thought I understood ltft and stft, but I guess I don't.
It doesn't make sense to me that my pressure could really be that weak, and not reflect in my fuel trims.
Does the pcm adjust to poor fuel pressure somehow, and keep the fuel trims relatively normal?

 

It shouldn't even be running with only 13 psi - spec is 28 to 40. Try a different gauge. Pull the vacuum hose off the regulator and see if fuel comes out of it.

Also, check your cats for blockage.

 

Well, I don't have cats, so that's ruled out.
I don't have another pressure gauge, but this one seated properly and is new. The gas would just barely squirt out of the purge tube when I pressed the purge release.

I still haven't figured out it could have such weak pressure and not be reflected in my fuel trims.

The truck has a slight shake at idle, but no misfires.
But when you start up a hill, it loses about 10 miles per hour. And if you kick it down into passing gear, it's still slowing down.
Also when I hit overdrive during highway cruising, lots of spark knock.

No codes and no light. I've checked both.

 

Also forgot to mention, I can't hear and humming noise from the pump when I turn the key on.

 

You do not know the whole story about how the system works.
At idle the motor does not need very much fuel pressure at the injectors to maintain idle.
This is the purpose of the vacuum control to the fuel regulator but not the that extent (15 psi).
As you call for power by opening the throttle, the mass air meter and TPS signal the PCM to lengthen the pulse width of injection adding more fuel to support the power the engine is supposed to be making.
In general the fuel rails are supposed to be fuel regulated at about 35 psi.
If the pressure falls to low, every time just one injector opens, the next one has less pressure to work with.
Bottom line is the motor runs out of 'food'.
It's very obvious your seeing the motor falling down on power from lack of fuel.
Fix the fuel issue you know you have and don't worry about the fuel trims.
The short term trims are instantaneous, the long term trims reflect an average of those supplied by the short term signals 'over time' and affect the fuel tables, where the short terms do not on an 'instantaneous' basis..
When the long terms shift more than 25% they will set a code for each bank
by Ox sensor detection of to much Ox IN THE EXHAUST read lack of fuel.
Good luck.

 

Alright, some more info from testing today.
Had a mechanic friend put his snap on gauge on the port.
It came up to 29 psi idling, but I took it a few seconds to build up to that, probably 25 or so seconds.
When I turned the engine off, it climbed to 32 psi. When I then unplugged the regulator with key off, it climbed very slowly up to 41 psi.
Don't know if it's supposed to build slowly or very fast in idle, he said fast and I needed a new filter or pump.

Something else strange I need help with- when I got home I depressed the schrader valve with a valve stem tool, and LOTS of air came out along with some gas.
There were also a bunch of bubble in his tester purge tube, and it didn't seem like a lot of volume.
Where would the air come from?

 

rusty,
the fuel pump is normally capable of near 100 psi pressures.
The fuel system is a 2 pipe system, meaning what does not get used past the regulator gets returned back to the tank.
The pump should rise in pressure quite fast at cranking and remain there as long as the motor is running.
At shut down the pressure should remain at the stopping level at least 5 minutes.
If not the pump check valve is leaking or there is an outside leak in a line.
There should be no air in the fuel at any time.
If you have some different action, it's a fault.
On a good system, the regulator drops the main line pressure to the 35 range.
The vacuum applied to the regulator further reduces the pressure at idle when the motor does not need the pressure.
It is also possible one or more injectors are leaking.
That's the whole story, find the issue from this info.
Use the pressure gage to access the performance of the fuel system.
If the 'volume' is insufficient the motor will run out of fuel or go lean at speed.
Good luck.

 

I'm going with leaking injector too. I wouldn't run the engine until you have determined for sure they are not leaking. A leaking injector is what destroyed my 1st V8.

 

Hmmm. Not sure about a leaking injector.
The system holds 42 psi with key off for at least five minutes.

 

I guess I missed that part of your post. You had said earlier "The gas would just barely squirt out of the purge tube when I pressed the purge release." and coupled with the air in the line.

It's possible the the rubber hose in the fuel pump\gauge sender assembly has degraded. Maybe an injector driver in the PCM is shorted to ground - though I can't see how that would not trigger a MIL.

 

The slow build of pressure makes me think of a fuel filter. Not sure if you have an external that is not in the tank, fuel filter. Back in the carb days the first thing you would check would be the fuel filter with driving symptoms like yours. The basics don't change, just some of the details.

 

The 01 has an external fuel filter, it's on the driver's side at the frame rail. Have you changed it yet?

 

Yep sounds like an over complexion of simple. Change the fuel filter