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Thread: Idle Mixture Adjustment

  1. #21
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    Thanks all. Well I’ve done my testing and here is what I found. I clean all the plugs and when in open loop it idles perfectly. I then tested the lambda system by applying 1.5v across it and then grounding it and that all works (it goes to each end of the dwell range). Now here what is odd. I tested the O2 signal on a mv scale and it’s basically sitting at a fixed voltage. When I tested it (engine all warmed up) it was 450mv static. When I increase rpm it would slowly increase mv up to 800mv but when I let it idle again it would stay static at that mv level then very slowly come down. Isn’t the O2 sensor supposed to swing voltage quite fast? If so that may be the issue as the lambda system is trying compensate. Am I on right track?

  2. #22
    Senior Member Bitsyncmaster's Avatar
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    Yes the O2 sensor responds to mixture changes very fast. The narrow band sensor just looks for any oxygen or not. So the lambda ECU is making the mixture richer if that O2 sensor voltage is above 0.45 volts or leaner if below.

    Using voltage to adjust your mixture really does not zero in the mixture adjustment because the lambda system will keep that O2 sensor voltage holding by changing the mixture via dwell changing. The same goes for checking with a wide band sensor on the exhaust. You want to zero in on the dwell number so your starting and wide open throttle mixtures are set to intended values. Remember the O2 sensor takes time to warm up and produce voltage even with a hot engine.
    Dave M vin 03572
    http://dm-eng.weebly.com/

  3. #23
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    Thanks Dave - sorry just to be clear do I have a faulty o2 sensor based on my results? I wasnít trying to set the mixture taking a direct voltage reading off the sensor I was just trying to see if the sensor was working. I recall testing the sensor last year and it would show rapid voltage changes on my digital gauge but now itís very sluggish and mostly shows a static voltage with slow Increases and decreases when I change rpm.

  4. #24
    Administrator Ron's Avatar
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    Quote Originally Posted by Jonty View Post
    Ron just picking up on a comment you made about setting up with a gas analyser. Would if not be better to disconnect the FV and set the mixture with a modern gas analyser then simply plug the FV back in again? Would that not set the perfect baseline for the 02 sensor and FV to do its thing? Would that not rule out a dodgy narrow band 02 sensor?
    I've never found the need to on a D, but if using a gas analyser, you would want to disconnect the O2 sensor (not the FV). And then you would potentially be left with the brass screw(s) open...

    Quote Originally Posted by Jonty View Post
    Thanks all. Well Iíve done my testing and here is what I found. I clean all the plugs and when in open loop it idles perfectly. I then tested the lambda system by applying 1.5v across it and then grounding it and that all works (it goes to each end of the dwell range). Now here what is odd. I tested the O2 signal on a mv scale and itís basically sitting at a fixed voltage. When I tested it (engine all warmed up) it was 450mv static. When I increase rpm it would slowly increase mv up to 800mv but when I let it idle again it would stay static at that mv level then very slowly come down. Isnít the O2 sensor supposed to swing voltage quite fast? If so that may be the issue as the lambda system is trying compensate. Am I on right track?
    300 milliseconds is typical.
    The O2 sensor maybe sooted too.

    Does "all that" mean these:
    DPulseRatios.jpg

    I'm guessing not "A".

    And "full range" = 20į (1.5v) to 87į (grounded)?
    What was the dwell with everything in place??



  5. #25
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    Hi Ron

    All the tests in the manual for the lambda system check out. Applying 1.5v I get a constant 87. Grounding the system I get a constant 12. Applying full WOT I get 54. What I can’t get right is with the 02 plugged in a closed loop dwell swinging between 45 and 35. I disconnected the O2 sensor and measure the mv coming off the sensor on a fully warm engine and it was pretty much static at 450mv. If I revved the engine it would slowly climb up to about 750mv but when I brought the engine back to idle it would hold 700mv the very slowly start coming down. On that basis is my O2 sensor faulty or not?

    When I plugged the O2 sensor back in the dwell just sat at 45. This is incredibly frustrating so I’m hoping it’s just a faulty 02 because I’ve nothing else to check!
    Last edited by Jonty; 05-18-2020 at 06:32 AM.

  6. #26
    Administrator Ron's Avatar
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    Sounds like it to me -- Sooty O2 sensors have slow reaction times.

    Hang in there!

  7. #27
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    Great thanks Ron. This process can certainly add to the grey hairs but luckily kind people like yourself can help narrow down! Cheers

  8. #28
    Administrator Ron's Avatar
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    You're welcome! Hopefully that will do it. (I'd probably get plugs too tho...)

    They can't screw with my hair's color anymore. However I do have an occasional problem with temptations to pull it.

  9. #29
    Delorean Guru
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    To use a gas analyzer you need the hose adapter hook-up per the procedure on D:04:01. The Workshop Manual actually doesn't use the dwell meter to set the mixture, it is only used to confirm the Lambda system is operating. With that said you can set the mixture very well using just a dwell meter if everything is working correctly. You are not supposed to measure the exhaust at the tailpipe, the adapter taps off ahead of the catalytic converter.
    David Teitelbaum

  10. #30
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    The Lambda controller constantly adjusts the mixture through the frequency valve. The mixture will swing back and forth between too rich and too lean constantly too. As a result the O2 sensor voltage will swing back and forth between 0.1 and 0.9 volt accordingly as well. Most, if not all, digital volt meters will read and display peak voltage so it is not usable for monitoring how the Lambda system works. What's needed is a volt meter that gives the average voltage it sees. That's why I used an analog meter to measure the sensor voltage. Analog meter displays average value the meter sees. If it shows 0.45 volt it says the mixture is controlled right on target swinging between rich and lean with an average right in the middle.

    I remember I disconnected the O2 sensor when I did the measurement while tweaking the mixture adjustment screw. I guess that was not right because the Lambda controller might try to correct the mixture in an extreme manner. I will soon do another round of adjustment for my B28F. I will do it with the O2 sensor connected.

    My Bertone was able to pass California smog test in 2004 with excellent numbers. I remember the test technician told me that he was surprised that such an old brick passed so well.

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