04 silverado

I have a 2004 silverado 1500 with the 5.3l. It throws p0171 and p0174 sometimes. Mostly when it's thirty degrees or lower. I've done all the checks for vacuum leaks and didn't find any. I've cleaned maf. I did a fuel pressure test and pressure does drop on a snap test. Just wanted to put up the data of when the code tripped and get some opinions. This fuel trim stuff is new to me. I would of just change o2 sensors until I learned about fuel trims
Engine Load = 9.412 %
Engine Coolant Temperature = 201.2 °F
Fuel Trim Bank 1 Short Term = 6.25 %
Fuel Trim Bank 1 Long Term = 25 %
Fuel Trim Bank 2 Short Term = -2.344 %
Fuel Trim Bank 2 Long Term = 23.438 %
Intake Manifold Pressure = 6.382 psi
Engine RPM = 1,362.5 rpm
Speed (OBD) = 46.603 mph
Mass Air Flow Rate = 165.401 cfm
Throttle Position(Manifold) = 21.961 %

check the intake gaskets for leaks when cold. with a smoke machine. or spray brake clean along the intake when first started when cold and look for trims to go way negative when it sucks in the brake clean in...

I will do that later tonight and see if they change. Thank you

You can also try the water test if you don't have any brake cleaner handy. Works perfectly for finding intake leaks.

I've done the water test before but maybe I didn't spray enough. I'll try it again after it sits over night. I'm just wondering if it's more a fuel problem. The code tripped while I was driving and the engine was hot. Also the trims don't get better if I raise the rpms. They go higher.

What where your fuel pressure readings @ idle and during the snap?

Oh, and forgive my ignorance, but could someone please explain the 6lbs of pressure in the intake during cruise?
I still have a hard time with the relationship between manifold pressure and barometric pressure and manifold absolute pressure.

Noah wrote: Oh, and forgive my ignorance, but could someone please explain the 6lbs of pressure in the intake during cruise?

I was thinking the same thing. Is this based on 14.7 psi atmospheric pressure at sea level?

Noah wrote: I still have a hard time with the relationship between manifold pressure and barometric pressure and manifold absolute pressure.

To my understanding, barometric pressure is atmospheric pressure, 29.92 inches of mercury (14.7 psi) at sea level . Manifold pressure is the pressure inside the manifold. Absolute pressure is the difference between the two.

Normally, we think of atmospheric pressure as zero pressure. Anything above 14.7 psi (29.92 inHg) @ sea level is pressure. Any thing below 14.7 psi ( 29.92 inHg) @ sea level is a vacuum. Looking at it this way, with 6psi inside the manifold and 14.7 (29.92 inHg) on the outside. The 6 psi (?? inHg) would be a vacuum.

This is, merely, my own reasoning and may, very well, be wrong.