Damned dreaded P0171 AND P0174

[chevman_88]

I should start off by saying I've been a lurker for a while and this is my first post. Lots of good information so far and hoping you all can help me out.

Some background:
I picked up this '97 k2500 burb with a 4l80e and 454 about two weeks ago and replaced the injectors with the bosch design as well as the FPR due to it leaking and having a hard time starting. I also changed the fuel filter, cap, rotor, plugs, wires, and oil filter adapter o ring.

After finishing this up I packed up the family and headed out of town with a travel trailer in tow. No issues for about 200 miles when the first ses light popped up for an o2 sensor issue. Thinking it was likely a transient issue i cleared it out and finished the drive with 60 miles remaining.

Didn't get the lean bank codes until the next day and they are persistent little sh1ts.

While driving around I plugged in my scanner and noticed I maxed out the LTFT on each bank at 25% with the STFT adding anywhere from -3 to 15%. Checked fuel pressure, got it to 58psi key on engine off then it dropped to about 52psi and held. At idle it held a solid 48psi. I went about spraying carb cleaner to try and find a vacuum leak and had no success. Took it to a shop (on vacation, only have basic tools) they noticed the lowish fuel pressure but that was it. We decided to just replace the fuel pump, to no success.

After getting the burb back to the campsite I pulled the negative battery cable thinking maybe i needed to reset the fuel tables... no dice. Finally I started going through live data on my scanner and noticed the MAF was reading 2.4 g/s key on engine off, so I unplugged the MAF sensor, got 0 g/s and low and behold fuel trims at +/-10%!! Cool! I rush to the autoparts store thinking i had a bad MAF sensor (had tried cleaning it several times already with MAF cleaner). Plug the new one in...Fk me... same damn thing as the old MAF.

Running out of ideas... the fuel trims are off at idle, but not maxed. Unplugging the MAF seems to clear up the issues, but isn't the issue suggesting I have a vacuum leak.

In park at idle I see about 10% LTFT and it climbs with load or RPM. At negative load STFT and LTFT swap and STFT maxes out at 25% while LTFT drops to about 10-15%. This last part to me look like fueling issues not vacuum, but everything in the fuel system has been replaced...

To add to this whole debacle, I have no real noticeable drivability issues. It starts right up hot or cold, sitting for any amount of time and seems to have decent power. There is a very slight miss at idle, but I didn't take note if the miss happened with the MAF unplugged or not.

 

[skylark]

What is the cam retard set at? I've seen cam/crank correlation issues cause fuel trims to be off because it is injecting fuel at basically top dead center instead of before top dead center.

 

[RawbDidIt]

I'd clear up the low fuel pressure issue, sounds like your fuel pressure regulator either needs adjusting or replacing. Doesn't sound like it's the problem considering the fact that it corrects itself with the MAF unplugged, but a problem that can pop up later. Throw the vacuum guage on it and check for a vacuum leak, if you don't find one, try unplugging the MAP sensor and see what that gets you.

Sent from my SM-N976V using Tapatalk

 

[stutaeng]

Ok, so definitely a fuel delivery problem if the LTFT increases with load....and you have lean codes on BOTH banks, meaning neither bank is getting enough fuel (possibly.) What do you mean "negative load?" That would be a good call of fuel pump being the culprit because it was low on pressure, and you changed it. Did you confirm fuel pressure increased?

On these lean/rich codes it's best NOT to clear codes, because all you have done is you have to drive more and the SES comes back on. It's better to make adjustments/repairs and keep monitoring the fuel trims to see if the ECU is correcting them...

Do you have any other codes?

 

[chevman_88]

Only those codes, new delphi pump has the same fuel pressure as before. By negative load I mean downhill coasting at the same rpm, sorry.

Burb is back in the shop, looks like the sending unit lock ring didn't seal properly as I started dumping fuel all over the ground after filling the tank completely (could explain the lack of fuel pressure change).

Gonna dig more into it when I get it back.

 

[Supercharged111]

48psi at idle seems low, try pinching the return hose and see if pressure goes up. If it does, I'd be looking at the regulator.

 

[Schurkey]

I picked up this '97 k2500 burb with a 4l80e and 454 about two weeks ago and replaced the injectors with the bosch design as well as the FPR due to it leaking and having a hard time starting. I also changed the fuel filter, cap, rotor, plugs, wires, and oil filter adapter o ring... ...No issues for about 200 miles when the first ses light popped up for an o2 sensor issue.quote

How old are the O2 sensors?

I had a TBI truck that I dicked with on-and-off for two years trying to solve a "lean" condition that set like clockwork at highway speed. As time went by, it set at lower and lower speed. Everything I tested seemed to be good.

One day, I got angry. Tore out the O2 sensor, even though it gave reasonable output at idle and low speed. Put in a new one. Problem solved, and has stayed solved for years..

 

[Supercharged111]

How old are the O2 sensors?

I had a TBI truck that I dicked with on-and-off for two years trying to solve a "lean" condition that set like clockwork at highway speed. As time went by, it set at lower and lower speed. Everything I tested seemed to be good.

One day, I got angry. Tore out the O2 sensor, even though it gave reasonable output at idle and low speed. Put in a new one. Problem solved, and has stayed solved for years..

I've had one fail causing a rich condition and one fail causing a lean condition. Only reason I didn't throw out O2 sensors just yet is he indicated that both banks were messing up, not just one. Unless that statement is incorrect.

 

[chevman_88]

O2 sensor aren't off the table, but it is both banks causing an issue. I think I'm going to have to limp it along until I get back home and can start digging into everything. While at the shop for the leaking sending unit seal I noticed the ground had been spliced by a PO and needs to be repaired the right way and verify fuel pressure again.

I've also found a slightly wet surface along the lower intake and front of the block, so looks like it's also time for a new set of intake gaskets.

I'm just glad this isn't my daily right now and can take time when I get home to go through it with a fine toothed comb. I'll be sure to update this thread as I hate finding threads that are similar to issues I'm having with no resolution

 

[chevman_88]

Got it home and ordered a full Mahle intake gasket set and both upstream o2 sensors AC Delco. I also noticed the ground wire on the fuel tank needs some loving, so gonna drop the tank again and get that cleaned up.