2015 QX70 RPM surging issue

A forum for lovers of Infiniti's hot-rod crossover, the FX! In 2014, all FX models will be named Infiniti QX70, in line with Infiniti's new naming structure.
fox911eg
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Joined: Wed Sep 08, 2021 2:33 pm
Car: 2015 Infiniti QX70 S

2015 QX70 RPM surging issue

Postby fox911eg » Thu Sep 09, 2021 2:39 pm

Hi,
I have a 2015 QX70S with 140k. The vehicle RPM sometimes fluctuate up and down when going uphill, which makes it act as if its jerking. It goes away if I let of the gas pedal or if I slammed the gas some.

I have no loss of power, maintenance up to date with original or higher quality items. Throttle bodies been thoroughly cleaned, I did perform relearn for both of them. cleaned both MAF sensors, I also did idle relearn.


Did some research and some suggested MAF sensors. Took it to Infiniti dealer in the area where they were able to replicate the issue. However, the technician said he will consult with Infiniti tech line for advise on how to proceed.

He came back with this:

-We most commonly see similar concerns caused by improperly adjusted brake switches, erratic ABS wheel speed sensor input, or rich or lean condition

Data monitor the stop lamp switches and confirm they are adjusted per the ESM.

Disconnect the ABS actuator electrically to see if this eliminate the issue, if resolved, check speed sensors signals with CONSULT

-If there is no change after disconnecting the ABS, please inspect Alphas, and A/F sensor voltage when concern occurs. we should see the alphas stay between 90 and 110%

So, first of all,, what are the ESM parameters for adjusting the stop lamp switches since I can do it myself?

I did check the Alphas on bank 1 and 2 and it stays between 90-105%, but I wasn't able to monitor it when the issue occurs, anyone knows why checking that might be related to the issue??

A/f sensor, I would assume this is the O2 sensor voltage. what are the parameters for those??

Any other information you guys want to share will be greatly appreciated.
Thanks


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VStar650CL
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2004 Nissan Altima 2.5 S

Re: 2015 QX70 RPM surging issue

Postby VStar650CL » Thu Sep 09, 2021 4:08 pm

The brake switch adjustment procedure in the ESM may not help, N/I brake switches are notorious for sticky contacts and for the two throws in the lamp switch getting "out of sync" with one another from contact abrasion. When that happens you need a new switch and not an adjustment. However, I really doubt that's the problem. N/I vehicles are very good about throwing DTC's if there's a brake switch conflict, although they often blame the pressure sensor for what's actually a bum switch. Since you don't have DTC's at all, that's probably the wrong tree to bark up. The only thing that might be happening is the switch "falsing" and turning on the lamps when they shouldn't be on, causing the tranny to refuse to accelerate. That's easy to diagnose if you can see the reflection from your third brake, just look in the rearview next time it acts up and see if the lamp is on. If so, replace the switch.

I also doubt it's the ABS, unless you're hearing actuator noises from one or both rear wheels when the surging happens. When there are no codes it's generally the steering angle sensor acting up more often than wheel sensors, because N/I ABS's throw wheel sensor codes at the first missed tooth, they don't wait for a repeat. So same deal, without DTC's the ABS isn't the most likely suspect.

Most scanners read short and long term fuel trim (STFT/LTFT) and not A/F-alpha, which is peculiarly Nissan. So I'll stick with STFT/LTFT. Generally STFT will sit slightly rich (typically 1~3%) at warm idle and go slightly lean when you raise and steady at a higher RPM. What you really need to check first is LTFT. If it's more than a few percent rich or lean on either bank then there's an underlying issue of some sort, possibly fuel quality, vacuum leaks, PCV valves, etc. If the LTFT's are normal then it means your surging is an intermittent problem that the ECM doesn't recognize and isn't attempting to compensate. If one or both LTFT's are very rich and you don't find a vacuum leak on that bank, there's a good chance the A/F sensor is the culprit (dying A/F's almost always cause rich compensation). A/F's are what used to be front O2's, by the way. They work differently, directly metering rich/lean above and below a set voltage threshold (usually 1.5V or 2V). They respond much quicker than HO2S's, so they're much better for direct mixture control. Nowadays HO2S's are pretty much relegated to working for the government, monitoring the cats. A/F's are used in front, where it matters to the engine.

This smells pretty much like a MAF issue, so if you find short or long term alphas out of kilter on one bank, try swapping the MAF's and doing an IAVL. Then drive for awhile to see if the problem changes banks. If it does, then that MAF is lying to the ECM and causing your problem.

fox911eg
Posts: 6
Joined: Wed Sep 08, 2021 2:33 pm
Car: 2015 Infiniti QX70 S

Re: 2015 QX70 RPM surging issue

Postby fox911eg » Fri Sep 10, 2021 7:22 pm

VStar650CL wrote:
Thu Sep 09, 2021 4:08 pm
The brake switch adjustment procedure in the ESM may not help, N/I brake switches are notorious for sticky contacts and for the two throws in the lamp switch getting "out of sync" with one another from contact abrasion. When that happens you need a new switch and not an adjustment. However, I really doubt that's the problem. N/I vehicles are very good about throwing DTC's if there's a brake switch conflict, although they often blame the pressure sensor for what's actually a bum switch. Since you don't have DTC's at all, that's probably the wrong tree to bark up. The only thing that might be happening is the switch "falsing" and turning on the lamps when they shouldn't be on, causing the tranny to refuse to accelerate. That's easy to diagnose if you can see the reflection from your third brake, just look in the rearview next time it acts up and see if the lamp is on. If so, replace the switch.

I also doubt it's the ABS, unless you're hearing actuator noises from one or both rear wheels when the surging happens. When there are no codes it's generally the steering angle sensor acting up more often than wheel sensors, because N/I ABS's throw wheel sensor codes at the first missed tooth, they don't wait for a repeat. So same deal, without DTC's the ABS isn't the most likely suspect.

Most scanners read short and long term fuel trim (STFT/LTFT) and not A/F-alpha, which is peculiarly Nissan. So I'll stick with STFT/LTFT. Generally STFT will sit slightly rich (typically 1~3%) at warm idle and go slightly lean when you raise and steady at a higher RPM. What you really need to check first is LTFT. If it's more than a few percent rich or lean on either bank then there's an underlying issue of some sort, possibly fuel quality, vacuum leaks, PCV valves, etc. If the LTFT's are normal then it means your surging is an intermittent problem that the ECM doesn't recognize and isn't attempting to compensate. If one or both LTFT's are very rich and you don't find a vacuum leak on that bank, there's a good chance the A/F sensor is the culprit (dying A/F's almost always cause rich compensation). A/F's are what used to be front O2's, by the way. They work differently, directly metering rich/lean above and below a set voltage threshold (usually 1.5V or 2V). They respond much quicker than HO2S's, so they're much better for direct mixture control. Nowadays HO2S's are pretty much relegated to working for the government, monitoring the cats. A/F's are used in front, where it matters to the engine.

This smells pretty much like a MAF issue, so if you find short or long term alphas out of kilter on one bank, try swapping the MAF's and doing an IAVL. Then drive for awhile to see if the problem changes banks. If it does, then that MAF is lying to the ECM and causing your problem.
Thanks for your response, I will start addressing your recommendation and will update as needed. I have an Autel maxisys Elite and cant find the ST OR LT anywhere but I will keep digging. It does however show the A/F percentage and voltage as well as Alphas but since I dont have a reference to all these values, I will start with the simple stuff first.
Dont hesitate to share your knowledge Much appreciated.

fox911eg
Posts: 6
Joined: Wed Sep 08, 2021 2:33 pm
Car: 2015 Infiniti QX70 S

Re: 2015 QX70 RPM surging issue

Postby fox911eg » Sun Sep 12, 2021 7:42 am

I got the following information from the scanner(at idle) Hope it can shed some light...sorry its little long

Throttle position sensor 2 bank 2 580 mv
Throttle position sensor 1 bank 2 565 mv
Throttle position sensor 2 bank 1 560 mv
Throttle position sensor 2 bank 1 560 mv

A/F adjustment bank 2 -0.05
A/F adjustment bank 1 -0.07

A/F sensor 1 heater bank1 26-31 % AT 2.17 V

A/F sensor 1 bank 2 2.2v

HO2S2 BANK 1 0.3 V
HO2S2 BANK 2 0.3 V

A/F Alpha bank 2 103-104 %
A/F Alpha bank 1 100-106%

VVEL timing bank 2 7.03- 7.14 degree...sometimes jump to 25-27 degree then back down
VVEL timing bank 1 7.04-7.38 degree jump the same

VVEL position sensor bank 2 0.79 v--------------constant at idle
VVEL position sensor bank 1 0.85-0.86 v

Ignition timing 10-11 BTDC
Intake valve solenoid bank 1 0%
Injection pulse 2.3-2.37 ms
Base fuel schedule 3.24-3.28 ms

MAF 3.98 - 4.00 g/s

MAP 1.02-1.08 V

MAS A/F SE BANK 1 0.95 v
MAS A/F SE BANK 2 0.96 V

purge volume c/v 24%

MALPRVU 112 %
MALPRVL 88%

Still trying to see the STFT AND LTFT

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VStar650CL
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Re: 2015 QX70 RPM surging issue

Postby VStar650CL » Sun Sep 12, 2021 4:29 pm

I don't see the B2 A/F sensor, but that B1 A/F-alpha looks suspiciously jumpy even though the A/F sensor is nominal (2.2V is the centerline on your sensors, above is lean and below is rich). Now that I'm thinking about it, has your engine ever been de-carboned?

fox911eg
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Joined: Wed Sep 08, 2021 2:33 pm
Car: 2015 Infiniti QX70 S

Re: 2015 QX70 RPM surging issue

Postby fox911eg » Sun Sep 12, 2021 10:09 pm

Chemically, yes. I have used high quality fuel additives (red line, royal purple and even BG 44k) to the fuel at least twice each since I got the vehicle. Not sure if previous owner used premium fuel all the time though, but I do. But I am not sure if there is another way / another product tht might be more effective???

Will try to post the B2 A/F

fox911eg
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Joined: Wed Sep 08, 2021 2:33 pm
Car: 2015 Infiniti QX70 S

Re: 2015 QX70 RPM surging issue

Postby fox911eg » Mon Sep 13, 2021 6:45 am

STFT bank 1 at idle 8.6 - 10.2 %
LTFT bank 1 at idle negative 6.2 %

STFT bank 2 6.8 - 8.6 %
LTFT Bank 2 negative 3.1 %

STFT Bank 1 sensor 2 99.2%
STFT Bank 2 sensor 2 99.2%
Lambda bank 1 sensor 1 1
Lambda bank 2 sensor 1 1

IAT 145 F

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VStar650CL
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Re: 2015 QX70 RPM surging issue

Postby VStar650CL » Mon Sep 13, 2021 7:50 am

That's a big difference lean between the banks on LTFT, but the STFT's are pretty much nominal. That means something is going on above idle speed on bank1. LTFT's that don't move together rule out things like fuel pressure and fuel quality that affect the system globally. If it isn't carbon, given the rest of what we know, I'd suspect the bank1 MAF. However, I'd de-carbon it first and clear the LTFT's, then drive it 100 miles or so and look at the new LTFT's. Overheated carbon can easily cause the sort of intermittent surge problems you're seeing.

The best de-carboning method I know of is a teaspoon of ATF (any kind) down any handy vacuum port with the engine fully warm and throttled to 3500~4000 RPM. Whatever carbon is present will literally explode itself off the metal from the sudden temperature differential, and since the agent is non-reactive mineral oil with a high flash point (335F), it won't hurt the cats or anything else. We do it routinely at our shop with tune ups or whenever we think heavy carbon might be the issue. Since your ride has dual intakes, use one squirt on each bank.

fox911eg
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Joined: Wed Sep 08, 2021 2:33 pm
Car: 2015 Infiniti QX70 S

Re: 2015 QX70 RPM surging issue

Postby fox911eg » Tue Sep 14, 2021 6:41 am

I went a head and did what you recommended and cleared the fuel trims. I know it is little early to judge the results but I hooked my tool after driving around 30 miles with the following results

STFT bank 1 0 - 4
LTFT bank 1 -1.6

STFT bank 2 0 - 4
LTFT bank 2 -2.3

Car drives better but will need some more driving before saying for sure if problem resolved or if there are any improvement

MAF sensor wise, there is one sensor that constantly responds faster and marginally lower by 0.03 than the other. I mean it keep adjusting frequently while the other changes but noticeably slower though. Not sure if this indicate a dirty sensor on the slow one or its negligible.

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VStar650CL
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Re: 2015 QX70 RPM surging issue

Postby VStar650CL » Tue Sep 14, 2021 7:01 am

MAF response should be pretty much instantaneous with the throttle plate. "Laggy" ones can happen if the heat transfer substrate for the element becomes microscopically detached or cracked, causing the element to retain heat when it shouldn't. It's very likely the laggy one has a problem.


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