The DTC's in question were P0171 and P0174, which indicate that both banks 1 and 2 (left and right sides, in no particular order) were running too lean (too little fuel). This is primarily dictated by the Long Term Fuel Trim (LTFT) levels, which are largely controlled by the oxygen sensors. The fuel trim levels indicate whether the engine is running lean (positive values) or rich (negative values). Short Term Fuel Trims (STFT) bounce all over, providing momentary corrections as requested by the O2 sensors. LTFT's indicate more coarse corrections, and are the values considered when throwing these DTC's.
On the second consecutive ignition cycle during which these DTC's are triggered, the dash board's "service engine soon" light will illuminate. It will clear again after three consecutive cycles during which the codes are not triggered.
I followed the procedure on page 6-2346 of my Chevy factory service manual (which sucks in comparison to my Jeep manual -- just sayin'), which had me test the fuel pressure at the port on the fuel rail above the right cylinder head. I had to buy a new fuel pressure tester for that ($45 for an Actron kit unit that I'm sure I'll use again someday). It indicated that my truck was only seeing about 48-50 psi -- well below the 55-62 psi that the service manual expected. The pressure did hold steady after the pump was turned off, which ruled out a leak in the pressure regulator between the fuel supply & return lines.
My 12-year-old son helped me with the repair. He's been eager to learn how to fix things lately, and I love spending time with him. The fuel filter is bolted to the inside of the left frame rail below the front of the front door (right in front of the fuel tank). The filter is connected to the two steel fuel lines with two 5/8" flare nuts. The hex fittings cast into the filter itself are 13/16".
A little PB Blaster on the flare nuts allowed us to break them loose easily enough. Be sure to have a metal or glass container handy to catch all the fuel that will leak out of the open connections. The entire line between the filter and the engine will drain down in your face if you're not careful. Be sure you're wearing safety glasses! After the nuts were loose, I had a heck of a time getting the flared line out of the filter until I popped the front line out of the plastic clip that holds it to the frame. With that line out of the way, the filter just slides forward out of the bracket that holds it to the frame. There's no need to unbolt that bracket. After sliding the new filter into place, I brushed a little bit of antiseize onto the flare nuts to deter them from freezing up the next time I do this.
With everything reconnected, we turned on the fuel pump a few times (it runs for two seconds every time the ignition is turned on) and purged the air from the fuel line using the pressure tester's purge tube. With the air gone & the line full of fuel, we checked the gauge... and found that the pressure had actually dropped 2 psi from where it was before. What dismay! It now sat at around 46-48 psi, which was only 80% of what the service manual said it should register. Starting the engine and idling for a minute still yielded the same values.
In short, it looks like replacing the filter helped the situation, even if it didn't solve the problem completely. I'll keep the low fuel pressure in mind in case the problem reappears, and I'll keep my code reader in the Tahoe's center console for a few days as well. If the problem pops up again, I'll let you all know how we proceed.
Update: Four months later, my fears of a dying fuel pump were confirmed. The pump died suddenly, leaving my wife & kids stranded on the highway (fortunately only 5 miles out of town). Read the whole story in this article. I'm glad it didn't happen at a more inconvenient time, but I'm still kicking myself for not just biting the bullet and having the pump replaced back when the red flags first went up.
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