***FIXED*** '14 Infiniti QX80. Occasional P0171 or P0174

For the last several weeks, I've been trying to diagnose these DTCs: P0171 and P0174 on my 2014 Infiniti QX80. Whats throwing me off is when either DTC turns on, only then the idle jumps up to over 800-900 rpms as if there was a vacuum leak. Once I clear the code, the truck idles normal as if nothing happened. I've driven it close to 500 miles and all of a sudden, either of the lean DTCs pops back up; but both have never come on at the same time.

I'm wondering if the idle jumps up because at that point the ECU decides to ignore O2 sensors and forget fuel trims and go to a preset injection pulse? As per my signature, getting involved to learn more

The other thing I'm wondering - after a search I see mention of the maf sensor being the cause most of the time. If it was marginally dirty then perhaps fuel trims would both slowly creep up but due to the many variables it would be too much of a coincidence for both banks to throw the code at once - and random chance as to which does it first. I'm sure someone knows the numbers but perhaps you can check maf sensor signal voltage to compare with what it should be.

Desmond6004 wrote: I'm wondering if the idle jumps up because at that point the ECU decides to ignore O2 sensors and forget fuel trims and go to a preset injection pulse? As per my signature, getting involved to learn more

Desmond6004 wrote: The other thing I'm wondering - after a search I see mention of the maf sensor being the cause most of the time. If it was marginally dirty then perhaps fuel trims would both slowly creep up but due to the many variables it would be too much of a coincidence for both banks to throw the code at once - and random chance as to which does it first. I'm sure someone knows the numbers but perhaps you can check maf sensor signal voltage to compare with what it should be.

Thanks for the feedback. I'm in the same boat as to why I joined this forum. I'm no mechanic, actually a PT in ortho/sports medicine. But just like my profession requires hands on work, I've been the same with all my cars for the last 27 years as a long time DIY'er. I've never really ran into mechanical/diagnostic issues I couldn't figure out but this one has me stumped.

Nissan & Infinitis are notorious for the failing MAF sensor and one of the first things I did go over. Everything from voltage and g/s were within range. Even both low and high psi were in range and no vacuum leaks. I recently bought an Autel MP808TPMS diagnostic tool and noticed the LTFT1 and LTFT2 were both very high, between 27-30%+ at idle. I'm still learning the ins/outs of this tool so if the DTC returns, I'll save the freeze frame.

Have you checked fuel trims at higher rpm's?

johnathon.balderas wrote: Have you checked fuel trims at higher rpm's?

I have not. But the P0174 returned this afternoon, while I was at a stop light. I manually cleared the DTC after parking my truck in order to lower the idle back down. Once I got home, I plugged in the MP808 and saved the freeze frame. I'm attaching the pdf and just looking at the fuel trim values... something is definitely not right.

Before I forget, Nissan/Infiniti starts their stoichiometric at 100. So, my QX is definitely leaning out since my LTFTs are showing 30%+ on both banks.

I was looking over the Live Data pdf and one thing I noticed was the A/F Alpha B1 reading was blank/0%. Meanwhile, the A/F Alpha B2 was 102% which is within the accepted range.

I'm wondering if this could be a reason as to why my truck is throwing these DTCs
According to the QX80 Engine Control pdf I have, the meaning of A/F Alpha B1/B2:

The mean value of the air-fuel ratio feedback correction factor per cycle is indicated

.

This data also includes the data for the air-fuel ratio learning control

.

Also, I just read from another forum about how Nissan programmed their Consult diagnostic tools to read LTFTs at 0 because they wanted their mechanics to diagnose using A/F Alpha B1/B2 readings instead.

Monde wrote: According to the freeze frame data, the lean condition occurs at idle. What happens when you raise the idle speed?

With the rpms at 3050 for about 10 seconds, the A/F Alpha B1 reads 128%, B2 reads 130%.

With the rpms at 1600 for about 10 seconds, the A/F Alpha B1 reads 131%, B2 reads 135%

Since I'm driving my wife to work tomorrow morning, I plan to record the live data feed. If this issue isn't because of vacuum, I'm thinking it may have to be fuel related since the LTFTs aren't dropping under load.

At idle, the low pressure and high pressure fuel pumps are within range. I'm going to see if the fuel pressure drops on either pumps when load is applied and/or cruising.

The morning drive didn't reveal anything except the same high LTFTs staying around 30+ and up. Otherwise, everything else related to fuel were all within their specific values.

The only thing I can't check right now are the actual fuel injectors. Since the QX80 has a GDI engine, the fuel rails and injectors are located under the intake manifold. Even though the fuel injection timing is on point at 16 degrees, could a clogged injector(s) be the cause of these erratic DTCs?

primov8 wrote: could a clogged injector(s) be the cause of these erratic DTCs?

I imagine it would be difficult for that to cause the code on both banks at random.
Desmond6004

I'm not sure that it's your problem that's erratic, but the way the PCM is setting the codes. All the data you've posted shows a lean condition at all times. I think it's more likely that you're meeting the code setting criteria for P0171/174 at specific times, hence the intermittent codes. I think it's also likely that you're only getting one code or the other because one bank may be a few percentage points leaner than the other when you meet the code set criteria.

Any chance that this vehicle got filled up with E85 at some point?

Tyler wrote: I'm not sure that it's your problem that's erratic, but the way the PCM is setting the codes. All the data you've posted shows a lean condition at all times. I think it's more likely that you're meeting the code setting criteria for P0171/174 at specific times, hence the intermittent codes. I think it's also likely that you're only getting one code or the other because one bank may be a few percentage points leaner than the other when you meet the code set criteria.

Any chance that this vehicle got filled up with E85 at some point?

Definitely no E85, since I only frequent (4) specific gas stations in the NYC/LI /NJ area and none have E85 pumps.

But I also wanted to post... Problem "hopefully" solved.

I decided to remove the aFe CAI to check if the tube had any cracks and to re-align the t-bolt clamps. This aFe CAI for my truck has (3) clamped connections; throttle body <--> CAI tube <-->filter<-->housing. Once I disconnected the t-bolt tension clamp from the throttle body, I removed the entire CAI as 1 piece. I couldn't believe what I saw when I removed it today. The t-bolt clamp that connects the CAI tube to the filter wasn't tightened correctly. The last time I removed it was this past summer to clean the filter. I didn't realize my mistake and didn't seat the CAI tube correctly on the filter. There was a gap, a good 5-6mm gap. I couldn't believe I had missed this and have been driving the truck like this since then.


After re-installing the CAI and torquing the t-bolt clamps, I started up the truck. I had my doubts as I was thinking "too good to be true" if this was the reason why these DTCs kept popping up. I plugged in the MP808 and performed the self learn control values reset along with the idle air volume learning. I let it idle for about 15 mins and the values had already improved. Both B1 and B2 alphas were floating around 109-111.

We're driving out to NJ tomorrow so I'm looking forward to see if this truly was the fix.

Still a bit out of tolerance, but much better.

In your Freeze-Frame, i've noticed, that the fuel pressure is too low (402 PSI instead of 429 PSI). Maybe this is another Problem?

BR Matthias

After driving a bit over 300 miles the last two days, so far, so good. Once we got back home, no DTCs, the status of the A/F Alpha B1 and B2, floating around the 105-106% respectively. Considering the CAI, TBS and catback exhaust modsI have on my truck, I'll take those numbers any day over the previous 130%+up.
Thank you everyone for all the advice. I'll keep this thread updated if anything changes.

Great to hear.

Just curious, do you recall the MAF g/s reading at hot idle? I'm interested because in your case of pirate air, I believe the MAF should have shown a low reading.

Tyler wrote: Great to hear.

Just curious, do you recall the MAF g/s reading at hot idle? I'm interested because in your case of pirate air, I believe the MAF should have shown a low reading.

I'm sorry Tyler, but I don't recall the MAF g/s readings at that time. BUT, I do have an update and can finally say without any doubt, problem solved.

About two weeks after thinking I had solved the issue and my last post here (1/21), the fuel trims began to climb up once again. Even to the point where I had removed the CAI, reinstalled the OE air filter box and the numbers only dropped 2-3%. Again, it was sporadic and had only triggered the P0171 once. Although it was enough to drive me crazy because on some days, the numbers were somewhat decent and of course the sporadic higher than normal fuel trims.

While on a trip down to VA in Feb, I brought my Autel MP with us and had my wife look at some live data while comparing the #s to Infiniti's specific range. One thing she pointed out was that the L/Fuel Pres Sen wasn't reading within range. At 2000 rpms, the specific range for the the L/Press Sen is 0.45 map = 65.267 psi; the low pressure fuel pump was either showing 69-75 psi at 2000 rpms, or would actually even drop to a low of 60-62 psi at 2000 rpms. Of course, I was excited knowing it was the low pressure fuel pump but at the same time, I had my doubts. Nonetheless, I didn't want to overthink it and just wanted to enjoy the trip.

Within an hour of leaving VA to drive back home to NYC, it was obvious the low pressure fuel pump was the culprit because of the sporadic drops and increases in psi even at the recommend rpm range. BUT, I also found the other culprit to these issues and it happened to be the purge control valve. While idling at a rest stop, I noticed it was reading 15% when according to Infiniti's specific range, it was supposed to be at 0% at idle and only 5% at 2000 rpms. Again, this wasn't constantly happening because it wasn't always staying open and obviously not long enough to trigger a purge valve code.

About two weeks ago, I finally installed a new LP fuel pump and also replaced both the purge control and vent valves. With over 700 miles to date, everything has been great. The truck drives amazingly well once again and the most current fuel trim #s I have to report was yesterday:

A/F Alpha B1 and B2 (STFTs): 100, 101

Self Learn Control B1 and B2 (LTFTs):105, 104

I just wanted to say thanks to everyone contributing on this thread and really on this forum entirely.

Thanks for the update! :woohoo: Very valuable information, especially combined with all the data and testing you included.

I wouldn't have suspected a low side fuel pressure issue, but I also don't know what kind of high and low pressure these engines should be running. :silly:

Do you happen to have any of the scan data saved from when the engine was showing high trims? Mostly curious to see what the fuel tank pressure sensor was doing, and if it'd show an excessive purge condition.

Tyler wrote: Thanks for the update! :woohoo: Very valuable information, especially combined with all the data and testing you included.

I wouldn't have suspected a low side fuel pressure issue, but I also don't know what kind of high and low pressure these engines should be running. :silly:

Do you happen to have any of the scan data saved from when the engine was showing high trims? Mostly curious to see what the fuel tank pressure sensor was doing, and if it'd show an excessive purge condition.

Unfortunately, I don't. When I had my wife looking over the #s while I was driving, she had pressed the save data and created two files on the Autel but for some strange reason, all the parameters were blank. Of course, I didn't know until only after coming back home to NYC from our trip to VA.