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2001 Infiniti QX4 Air/Fuel Alpha values 110-140%, low power at high rpm.

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30 Apr 2019 14:41 #29145 by SHIMHEAD
Hello,
First a bit of background. I am a nearly 40 year veteran tech working in a multi-franchise dealer (Infiniti, Maserati, Alfa Romeo, Rolls-Royce), ASE Master, L1, Infiniti Master, Rolls Master, Porsche Gold Tech. I own a PicoScope 4423. Now on to my questions. I just installed a JDM engine pull in a 2001 Infiniti QX4. As far as I was able to research, the core engine (Pistons, crank, cams) are the same as the original. I did have to swap over the intake cam sprockets, front timing cover, upper oil pan, crank pulley, and flex plate to accommodate the different cam/crank sensing setup as the original engine. Using E3 spark plugs. Shortly after I had it running, I noted it would not rev freely into the upper RPM's (4-5K). The original engine had a head gasket issue on bank 1. There was a very steamy exhaust stream for the first several minutes after new engine start-up. Have driven it several days so far, with no codes set yet. Looking at O2 waveforms (Using my own Consult II in line graph mode), it is apparent that the bank 1 cat is not functional (rear O2 mirroring front). Rear O2 on bank 2 staying low/lean. Air/Fuel Alpha readings 110-115% both banks at idle, climbing to the 140% range on moderate to hard accel., then falling to 120-130% at cruise. Have cleaned air flow meter and new air filter. I have a home made in cylinder transducer available. New fuel filter and fuel pressure test are next steps planned. Looking for suggestions as to least invasive/simplest/best bang for the buck diagnostic steps. OE cats are $400 each my cost, not too keen on aftermarket ones unless you have a known QUALITY suggestion. I do have an exhaust back pressure gauge available as well. Through the years have seen cats "survive" a coolant bath and keep on trucking, hoping to hear your thoughts.....

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01 May 2019 05:12 #29156 by Andy.MacFadyen
My instinct is first go for exhaust back pressure I will at least give a definitive yes/no

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03 May 2019 12:03 #29248 by Tyler
Backpressure would be relatively easy to check with your in-cylinder pressure transducer. Easier than pulling the upstream O2's, anyway. :silly:

Because of your high Alpha readings on both banks, I'd suggest staying on the fuel path. A fuel pressure reading would be nice, but it's work on this vehicle. I've also never seen a bad fuel pump in one of these.

What's the highest Engine Load reading you can get at WOT in 1st gear? If it's lower than 85%, then you may still have a skewed MAF sensor, despite cleaning it. If both upstream O2's are stuck lean doing the same WOT run, then that's just more evidence for a fuel problem.

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03 May 2019 13:36 #29255 by SHIMHEAD
Thank you Tyler. I agree with all you have said. I have never replaced a fuel pump in a QX4 except for a no start. This particular vehicle had 242K+ on it when I got it. Hopefully later today I will get it in here at work and do a bank 1 in cylinder waveform since that was the bank that had the head gasket issue prior to the replacement engine. Also a new fuel filter and a fuel pressure test. Have you by any chance had any experience with aftermarket catalysts for QX4's? Hoping a previous employer can get hold of some MagnaFlow replacements for me to look over before I commit to them if that turns out to be my issue. Have been avoiding any full throttle just in case a cat is thinking about falling apart on me. It set P0171 and P0174 codes yesterday morning finally, and would reset them pretty quickly if erased. Have not yet played with the Active Test for Fuel Injection where you can add or subtract 20% from the base fuel schedule (usually use to check O2 function-response). Just trying to invest as little time in it as possible for a change, and testing my ideas against others....

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03 May 2019 14:09 #29258 by Tyler

SHIMHEAD wrote: It set P0171 and P0174 codes yesterday morning finally, and would reset them pretty quickly if erased


I was wondering why it wasn't coding. :lol:

Hoping a previous employer can get hold of some MagnaFlow replacements for me to look over before I commit to them if that turns out to be my issue.


I'd suggest holding off on the cats for now? If you truly had a B1 cat that was plugged or otherwise not flowing, you'd have negative trims on that bank. Equal trims on both banks suggests equal flow across both banks. If you get a P0420 after solving the P0171/P0174 issue, then that's a different story.

I think the WOT test through 1st gear will get you the most maximum diagnostic bank-for-your-buck. If Load is low, you know there's either a volumetric efficiency issue or an airflow measurement issue. Lean suggests an airflow issue, not VE. Alternately, you can record your highest airflow rate and put it into a VE calculator . Again, if it's low, I'd suspect a MAF.

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03 May 2019 14:18 #29259 by SHIMHEAD
I do also have another MAF available in my '03 QX4 that I wrecked before Christmas, but I have not yet checked FAST (parts catalog) to see if it is the same part number, will do that later tonight. Not yet sure if that hood will open though, been putting that off...

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03 May 2019 19:14 #29273 by Tyler
Let us know what you find. B) I'd suggest just unplugging the MAF, but this generation of Nissan/Infiniti didn't have great backup strategies, and might be misleading.

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06 May 2019 10:35 #29334 by Tyler
Any luck with this one? I put all my money on the MAF, hoping it pays off. :silly:

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07 May 2019 07:11 - 07 May 2019 07:14 #29357 by SHIMHEAD
You were right on it! I finally got a chance to get the hood open on my other QX4 and robbed the MAF sensor. Alpha numbers right in spec now. Fuel pressure was OK, fuel from inlet side of filter a bit dirty, might cut it open later today because I'm curious. Have a bank 2 Bosch rear sensor on the way from Amazon. Front O2's were dead zero on WOT before, now OK. Still a bit puzzled as to why I don't have a Bank 2 rear O2 code, Or a Bank 1 Cat code yet based on the line graphed O2 values I'm seeing, but I do know the ECM has the latest software that was associated with a TSB for P0420/P0430 codes, so maybe that threshold for Cat diagnosis got raised significantly. Will give it a few days after I get the Bank 2 rear sensor in. For whatever reason I also could not get the Field Test Mode in my Consult II to record Calculated load values, so I don't have that info for you. Might try a manual record later. In any case runs strong now, and I am learning how to better utilize some of the data I have available. I can easily see how VE can be a valuable tool in the toolbox!

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Last edit: 07 May 2019 07:14 by SHIMHEAD. Reason: spelling

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07 May 2019 08:54 #29359 by Tyler
Great to hear. B) Hoping your cats pass the monitors with no problems.

You're probably aware that Nissan/Infiniti sometimes won't run the downstream heaters when just sitting there? Might explain some of the O2 sensor behavior you're seeing.

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07 May 2019 09:04 #29360 by SHIMHEAD
Nope, Bank 2 rear pretty much sitting at .28 volts ALL the time, barely a ripple in Line Graph mode. Using Fuel Injection active test to richen/lean mixture +/- 25% does not phase it. It may very well be the original 242,000 mile sensor. For as old as my Consult II is, it still is a very competent tool, compact and quite powerful. Might see if I can post some line graphed O2's after I install the new sensor. Any suggestions for verifying the secondary Cats functionality? Thermal imaging?

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07 May 2019 12:11 #29362 by Tyler

Any suggestions for verifying the secondary Cats functionality? Thermal imaging?


Without a five gas analyzer, that's about the best option you have. Or just a infrared thermometer. If the outlet is hotter than the inlet (after a test drive), I'd call it good. B)

Are you in an area that still does tailpipe emissions testing?

Nope, Bank 2 rear pretty much sitting at .28 volts ALL the time, barely a ripple in Line Graph mode. Using Fuel Injection active test to richen/lean mixture +/- 25% does not phase it. It may very well be the original 242,000 mile sensor.


Ah yeah, it's probably dead. :silly: I gotta say, I really wish other manufacturers had that Fuel Injection Active Test available. Toyota has a similar test, but that's about it. It's a very powerful test in the right hands.

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07 May 2019 12:51 #29367 by SHIMHEAD
Well, the last State Inspection I remember was in 1996, so no exhaust analyzers to be had readily. Around here, if it smokes it passes.....

Thanks for your input so far, very helpful! I will post up my final results when I am done.

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08 May 2019 07:54 #29398 by SHIMHEAD
Codes P0158/159 set last night and here are a couple of line graphed O2 prints for your perusal....

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