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RPM bounces up and down after new o2 sensor

Discussion in '1st Gen. Tacomas (1995-2004)' started by teppla, Oct 12, 2018.

  1. Oct 12, 2018 at 1:08 PM
    #1
    teppla

    teppla [OP] New Member

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    Howdy,

    2001 prerunner v6 3.4 auto

    CEL was on for bank 1 sensor 1 so I went ahead and replaced it and unplugged battery/EFI fuse to reset the ECU. Now with the new one in (Denso 234-9001) the truck bounces up about 100 rpm constantly at idle and even during acceleration. Ive only driven it about 40 miles so far so I'm wondering do I need to give it more time for the computer to relearn or is the part faulty/wrong?

    Thanks
     
  2. Oct 12, 2018 at 1:16 PM
    #2
    Noelie84

    Noelie84 What Could Possibly Go Wrong?

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    Was it doing this before you installed the new sensor? Usually rpm surging like that is caused by the o2 sensor 'hunting' for the proper air/fuel mix and is an indicator of a bad sensor. I'd give it about another 50 miles or so to sort itself out, but it may be possible that the other sensor is bad as well.

    Another possible cause is a vacuum leak, but you should be able to hear it when you're under the hood. Plus, there aren't any vacuum lines that I remember near the O2 sensors so it's not likely you knocked one loose.
     
  3. Oct 12, 2018 at 4:28 PM
    #3
    cruiserguy

    cruiserguy Well-Known Member

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    California emissions Tacoma? If so, then that part number is the incorrect O2 sensor.
    Does the rpm actually change? Or does the needle just bounce?
     
  4. Oct 12, 2018 at 4:32 PM
    #4
    Leggo

    Leggo slow is smooth, and smooth is fast.

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    Last I checked, 02 sensors report, they don't hunt.
     
  5. Oct 15, 2018 at 5:12 AM
    #5
    Noelie84

    Noelie84 What Could Possibly Go Wrong?

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    It's a simplification, hence the quotation marks around it. Your Oxygen sensor constantly reports to the engine computer about fuel-air ratios and the computer makes adjustments based on them. However, mechanics refer to the surging caused from a malfunctioning or poorly functioning O2 sensor as 'hunting' because the engine bounces back and forth from too lean to too rich while it tries to find the right setting, which causes an rpm bounce.
     
  6. Oct 15, 2018 at 6:18 AM
    #6
    vern650

    vern650 Well-Known Member

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    A properly working O2 sensor ALWAYS bounces from Rich to lean, that is how it works. It is impossible for an engine to run perfectly at the proper fuel air mix at all times. This does not cause a fluctuating rpm or every efi engine out there would be fluctuating. Neve heard any mechanic use the phrase "hunting for the right mixture" and you can not always hear a vacuum leak.

    Op, was it an actual code for the sensor itself or a generic lean code? If it just said lean it could be any number of things. If this problem just showed up with the new sensor though then it's possible the sensor has issues or it could just be a coincidence.
     
    Leggo likes this.
  7. Oct 15, 2018 at 6:50 AM
    #7
    Noelie84

    Noelie84 What Could Possibly Go Wrong?

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    Yes, but a properly functioning O2 sensor lets the computer make adjustments in milliseconds, not in tenths, so you don't get bouncing in the RPM's.

    And if you'll bother reading my initial response, it said "SHOULD" be able to hear a vacuum leak; not "WILL" be able to hear one.


    But, it might be irrelevant anyway, since OP appears to have ghosted on all the followup questions directed his way.
     
    Last edited: Oct 15, 2018

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