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O2 Sensor Thread (possible sticky?)

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  • O2 Sensor Thread (possible sticky?)

    Based on a few other people's threads where they are fighting start and idle issues, and the question of whether or not the O2 sensor may be contributing or even serves any purpose on our cars keeps coming up, maybe we can make a short sticky that has just the relevant info.

    More experienced guys, feel free to correct this if I have anything wrong, or add anything I left out. I have an otherwise perfectly running NA with no other issues and very few variables in play, so I did some testing. It's been well-attested that turbos just don't need the O2 sensor but that wasn't as clear for NA cars.

    The short version:
    • If your car, NA or turbo, won't stay running without the O2 sensor plugged in, the O2 sensor is not the problem.
      • Turbo cars run just fine without the O2 sensor, as the turbo fuel mappings are rich by default.
        • NA cars should have a working O2 sensor, as the NA mappings are considerably leaner.

        If you are having starting and idle problems on a turbo car, unplugging the O2 should not make it worse.
        If that happens you may have a harness problem. Best of luck with that. If the problem improves, simply leave it unplugged, and continue troubleshooting if necessary.

      If you are having starting and idle problems on an NA car, the O2 sensor may be the problem.
      If this is the case you may not notice a difference between a bad O2 sensor and an unplugged O2 sensor. You may want to just replace the O2 sensor to eliminate it as a potential problem.

    I can confirm that an otherwise perfectly running NA will miss and stumble a bit on a cold idle with the O2 sensor unplugged (confirmed on both 84 and 87 ECUs.) This car has no other idle controls installed (IACV, air regulator, and FPR solenoid all deleted) and idles perfectly ~750 (set at throttle body) with the O2 plugged in. Nothing changed between tests (conducted on sequential mornings), running 91 gas, outside temperature approximately 70 degrees.

    Note that the car without the O2 sensor plugged in was still driveable, had no problems over about 1100RPM, and ran fine once warmed up. Repeat: if your car won't stay running without the O2 sensor plugged in, the O2 sensor is not the problem.

    O2 sensor and ECU interchange information can be found here: http://www.redz31.net/pages/fuel.html. Pay special attention to this page if you or a previous owner have been swapping ECUs, or the year "1986" ever comes up in relation to your car

  • #2
    Good info! I've had my 02 unplugged for months. Happy to read here that it's not just me; it's a turbo thing.

    1985 300ZX Turbo GLL ^Click for log^

    Originally posted by nater86zx
    No such thing as hoarding Z parts. When it comes to z parts, its not hoarding its preparedness!

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    • #3
      Good info.

      I have an extra brand new in box oem o2 sensor for an 86T (should work in most other models I'm not entirely sure which), if anyone is having problems and needs one, pm me
      A work in progress....

      >>>My Build Thread<<

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      • #4
        NAs run perfectly fine without an O2 sensor too....in my experience.
        - VG30DET (HE341) 86 300ZX - 1982 280ZX Turbo - Headered NA 1986 300ZX 2+2 - 2000 Xterra -

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        • #5
          michaelp;338904 wrote: NAs run perfectly fine without an O2 sensor too....in my experience.
          Mine was 100 per cent driveable without it - you wouldn't think your car was broken. It basically goes from "showroom idle" to "29-year-old-car idle." If you got the car that way and you weren't anal you'd probably just hope a PRW-2 would fix it and shrug when it didn't.

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          • #6
            Yeah my 85 na definitely runs better with the o2 plugged in.
            If you have the o2 plugged in and it runs worse than when it was unplugged, then you should just replace the o2 sensor.
            http://z31performance.com/showthread...2-2-(-now-NA2T
            My build thread (:

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            • #7
              Minor thread necro for an interesting observation. I now have a freshly rebuilt VG33ET, stock turbo injectors with very slightly raised fuel pressure, running the stock 87T tune on a Nistuned 87T ECU. The car now cares a whole lot if the o2 is hooked up. I was running the o2 because I always have, and then swapped it out for a wideband when I wanted to make sure that the stock tune was playing nice on the new motor. It ran so rich that it was barely driveable and was giving me idle AFRs <10. Put the stock O2 back, everything was fine. I think my adjustable FPR and Walbro must be making me run slightly rich normally, and then without the O2 it gets really bad. Will find out when I get another bung in for the wideband but in the meantime she's running great otherwise. So, yeah.

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              • shromy
                shromy commented
                Editing a comment
                old news, good job correcting your earlier info though.

            • #8
              Curious though, because when I put this thread up everyone in the community claimed that the turbo tune doesn't need the O2 sensor... my experience with the NA at the time was that it didn't matter much but it did matter. Like, I wonder what these guys' threshold for driveability is lol. What I'm gonna do anyway is try bumping up the injector size in Nistune a tiny bit to match the increased fuel. I bet when I do that,the o2 sensor makes less of a difference.
              Last edited by FrozenZ; 05-03-2017, 08:04 AM.

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