Air fuel ratio sensors and ridiculou fuel trim readings

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  • Witsend
    Banned
    • Nov 2012
    • 2942

    Air fuel ratio sensors and ridiculou fuel trim readings

    2000 Lexus RX 300 3.0v-6 113K miles , no perceptible misfiring , but sluggish acceleration with exhaust leak and check engine light on . Codes P0171 bank one lean ,P1130 AF ratio sensor malfunction bank 1, and P1133 Bank one AF ratio sensor range performance. Customer wants T belt changed, but I advised needs root cause of CEL and poor performance checked beforehand. I drive and see that both AF ratio sensors do read close to the same voltage and appear to change but the Long term trims on bank 1 are ridiculously high. Both upstream AF sensors voltage do change , are close to the same voltage, but bank one hangs more at the same voltage for longer time than bank 2 , but the y pipe flex couple r does have a leak , and the car drives like the exhaust could be restricted. I clean the MAF see a little higher grams per second flow at idle but noticed little difference in acceleration . I pull front bank 2 upstream AF and using a small piece of cardboard lightly over the bung hole I sorta get a feel for the back pressure which did not expel forcefully enough like something does when corked up when I revved it up, so I reinstalled it the AF sensor . Then I unscrew the rear bank 1 upstream AF sensor and at idle could not tell there was any more back pressure from bank 1 bung hole than bank 2, but no assistant to rev car up I drove the car with the Bank 1 AF sensor removed from the hole , and there was no difference in acceleration, (maybe worse) so I assumed and ordered a new MAF and proceed to replace the timing belt and did the water pump because the weep hole had a lot of past seepage accumulation to risk not changing it.
    Having dream of a comeback haunting me last night ,Today I cleared codes and decide to swap the 2 upstream AF sensors between banks and sure enough the ridiculous fuel trims transferred to bank 2 voltage between the AF sensors, but voltage are still close to each other above 3 volts. What makes the trim go so high on one bank with voltages between them so close ?I'm not seeing volts of one flat line at 0 or 5 volts . Drove the car to the Mexican muffler shop and got a new flex coupler welded in for $80to fix the exhaust leak and ordered a new Denso AF sensor for $114 w no tax from rockauto,(local stores wanted $180plus tax, The new Denso MAF I ordered day before for $75 w no core fee is $133 plus tax from local parts store.9In all fairness the reman MAF was about $79 with a $50 core , but I'll see how much better she runs with just the a new AF sensor.
    I think next week I will hook up my Counselor 2 and try to freeze a couple screen shots of AF sensor activity of a known bad sensor to a new one to get a grasp on why. Maybe some screen freeze snap throttle shots of MAF sensor old and new, if it still popping a lean code and decide to use the new MAF sensor.
    Attached Files
    Last edited by Witsend; 08-28-2015, 04:25 PM.
  • Crusty
    Senior Member
    • Dec 2007
    • 2450

    #2
    The Air/Fuel sensors are oxygen sensors but they try to maintain a voltage and the amperage CURRENT is what the ECM is reading. Very small milliamps.

    Try getting a relatively cold start. Maybe an hour or more of engine off.
    Start engine, let run for about 1 minute. Bring Rpms up to about 2500 rpm and HOLD IT STEADY for about 2 minutes. Then, without backing off the throttle, SNAP it to WOT and release (over about 1,000 rpm difference....3500 or maybe a little more)
    WATCH the O2 A/F sensor voltages at this point.
    They should be very steady while you're at the 2500 rpm. Then as you snap the throttle, they should rise very quickly, then drop very quickly and steady out back to the same voltage.
    It's the speed the voltage rises and the amplitude it rises to is what to look for.
    Some Toyo's run around 3.1 volts to 3.3 volts. Nissan's tend to run around 2.8 to 2.9 volts if I remember correctly.

    Comment

    • greasybob
      Senior Member
      • May 2008
      • 1590

      #3
      check this out from the component test in the Vantage Ultra
      Attached Files

      Comment

      • Witsend
        Banned
        • Nov 2012
        • 2942

        #4
        The Air/Fuel sensors are oxygen sensors but they try to maintain a voltage and the amperage CURRENT is what the ECM is reading. Very small milliamps.

        Try getting a relatively cold start. Maybe an hour or more of engine off.
        Start engine, let run for about 1 minute. Bring Rpms up to about 2500 rpm and HOLD IT STEADY for about 2 minutes. Then, without backing off the throttle, SNAP it to WOT and release (over about 1,000 rpm difference....3500 or maybe a little more)
        WATCH the O2 A/F sensor voltages at this point.
        They should be very steady while you're at the 2500 rpm. Then as you snap the throttle, they should rise very quickly, then drop very quickly and steady out back to the same voltage.
        It's the speed the voltage rises and the amplitude it rises to is what to look for.
        Some Toyo's run around 3.1 volts to 3.3 volts. Nissan's tend to run around 2.8 to 2.9 volts if I remember correctly.
        Reply With Quote
        Thanks Crusty. I should have thought of monitoring both AF sensor pid voltages in graph mode besides watching the text changing. Maybe graph merge to overlay both.Even in text mode at idle I could see the good side AF sensor decimals in the hundredths of volts was more active than the suspect side. The overall voltage difference comparison both AF sensors was close and never saw the suspect one spiking up or down in relation to the other , but saw the LTFT was at times 50% and occasionally see Bank 1closed loop fuel control change to Bank1 open loop fuel control while driving.
        Bob , thanks for posting the Vantage Ultra tip that the AF voltage goes higher in lean condition and lower in rich condition.(opposite of regular o2s)

        Comment

        • Crusty
          Senior Member
          • Dec 2007
          • 2450

          #5
          I had to pop over to the shop for a few minutes and while waiting for a customer I grabbed the August 2015 issue of Motor Age magazine.
          Page 52 has an article on AF sensors-!! LMAO
          I read as much as I can but I don't necessarily believe everything. One has to keep some healthy scepticism especially with the fact that 90% of what's on the internet is BS
          You can get to the Motor Age magazine online
          sorry I don't have a link
          Just google the mag name.
          HTH

          Comment

          • Crusty
            Senior Member
            • Dec 2007
            • 2450

            #6
            Bob, yes, thanks for reminding me the AF sensors work opposite to conventional O2 sensors.
            They don't cycle the way conventional sensors do and my method shown above forces a full sweep (most times)

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