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Hey all,

Been trying to track down a cause for P0171 and P0174 codes. Due to the fact that the STFT's are low and the LTFT's are high I am beginning to wonder if its fuel pump related. A vaccum leak would have the fuel trims do the opposite I believe. Below is the freeze frame data taken this morning.

Freeze frame information:
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Engine Load = 35.686 %
Engine Coolant Temperature = 197.6 °F
Fuel Trim Bank 1 Short Term = -2.344 %
Fuel Trim Bank 1 Long Term = 20.312 %
Fuel Trim Bank 2 Short Term = -3.125 %
Fuel Trim Bank 2 Long Term = 24.219 %
Intake Manifold Pressure = 7.397 psi
Engine RPM = 2,510.5 rpm
Speed (OBD) = 77.671 mph
Timing Advance = 49 °
Intake Air Temperature = 71.6 °F
Mass Air Flow Rate = 239.759 cfm
Throttle Position(Manifold) = 11.373 %

Haven't checked fuel pressure yet but just wondering if anyone has any input. I feel like I'm close but any other ideas would be appreciated.

Right away, something is driving your LTFT's which is the genesis for this code. If they're greater than 19% for a minute, along with some other operating parameters (which you've met just by my quick scan), this sets. Had you been giong 85mph or faster it wouldn't have. Anyway, those LTFT's are what are driving it.

One thing I do question, did you get this code after you replaced the injector(s) in the other thread? a leaking or not properly/fully seated injector "O" -ring would cause a post-MAF vacuum leak that could generate this. Something to look at...

What I don't see is your O2 sensor mV readings. They should fluctuate between say 200mv and 800mV, with pre-cat O2's being fast moving and post-cat moving slower. If you monitor these and you notice anything out of kilter, start looking at the O2 sensor, electrical harness to the sensor, a short in any of those wires. If their switch-rate looks normal, leave them alone.

Ok, I went and looked it up as I got to thinking, and I thought my mental rolodex was getting confused with another DTC. It wasn't, I was recalling correct info;

Step 1: Did you perform a Diagnostic System Check? Yea, yea, yea (this is like asking if you plugged it in)

Step 2: Monitor LTFT's are they greater than the specified value (the 19%)? Review Freeze Frame data. Yup, we did this, I can see that it's greater than 19%. Move to step 3

Step 3: Monitor the HO2S's. Are they within the specified range (200mV - 800mV) and fluctuating? Need you to answer that. Lets assume they are to move to Step 4. If not, go to step 5.

Step 4: Insepct vacuum hoses for splits, restrictions, and improper connections. Ensure that there is sufficient fuel in tank. I'm sure you have, and I'm sure you have enough fuel in the tank. So from here we go to Step 6. If you did find the problem, go to Step 7.

Step 5: Inspect the HO2S for proper installation. Make sure the harness/wires are not contacting the exhaust. Insepct for a short between the signal circuit and the low reference circuit (I'm sure I can find that if need be). If this fixed it, I think you be done. If not, start looking into the fuel system (we can cover that if need be).

Step 6: Any missing, loose, or leaking exhaust? Any vacuum leaks at the intake manifold and injector O-rings. Check the air induction system and intake ducts for vacuum leaks/restrictions. Check the Crankcase ventilation system for leaks.

Step 7 is post repair and clearing the code, then running the vehicle for the conditions to run this DTC (or your freeze frame data), and see if it re-triggers. If it failed, start over. If it didn't, go to fridge, obtain beer, open beer, enjoy.

Thank you, sir. That cleared the confusion for sure.

- The O2 sensor reading are within specified parameters.

- I verified there are no leaking injectors (I do have replacements on the way though).

- I have replaced every vacuum hose so I'm sure its not those. (Yes, even the clean and dirty air vent lines)

- I replaced the intake manifold gaskets.

- I am waiting on the intake coupling (boot between TB and intake manifold) as this appears to be a common problem with these N*s.

Will update you this weekend and hopefully reach step 7.

Cool.

Don't know if this is related to the codes I am getting or not but today I checked the fuel pressure at idle and it was 30psi. Engine off it was at 32psi. What's the normal supposed to be? 45psi or greater?

30 - 32 is what most GM vehicles is considered normal. I could look up 4.6 specific info tonight and see what comes up.

Usually when you do an injector balance test with T2, it requires static pressure to be in the 30psi range, when measured at the end of the rail. Then you cycle each injector and monitor the psi drop. You figure each injector should be about a ~5psi drop.

45psi just seems like a lot, but maybe it is that much on the LD8's.

To save cmntmxr some time, I looked it up. It's 41-47 psi key on, engine off or when commanded with a scan tool. Accepted drop with engine running at idle is 3 - 10 psi.

With the engine warm, key on engine off is at 32psi. Engine on 30psi. Remove vacuum hose from regulator jumps to a very shaky 41psi.

Have you changed the fuel filter recently?

That was one of the first things I did.

Archon wrote:To save cmntmxr some time, I looked it up. It's 41-47 psi key on, engine off or when commanded with a scan tool. Accepted drop with engine running at idle is 3 - 10 psi.

Thank you.

Oh, FYI, you may have just found your problem too! Low fuel pressure can cause a lean code. Obviously 30psi isn't enough.

I'll be replacing the pump tonight so we'll see. Thanks guys!

Well, fuel pressure is now around 37psi with key on and engine off. Next question; what else could cause the fuel pressure to not be within spec?

FPR, clogged/pinched line...

Thanks. I have a new FPR on the way.

Looks like the issue has been resolved. The codes have not resurfaced in a week. Fuel pressure is at 42psi EOKO.

Excellento.

Thanks for all your help bud.