308 QV running without Lambda sensor, any effects, duty cycle?

Discussion in 'Technical Q&A' started by 308GTS, Jun 10, 2004.

  1. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    I have been wondering about this and would like to discuss this. I have seen many late model QVs that have their lambda sensors disconnected and the wires just sitting there. The car still runs fine. What are some of the effects? Is it safe for the car?
    Here is what I have been thinking. With the lambda sensor disconnected there is no V going to the Jetronic box. Therefore the duty cycle increases to a max set limit with the duty cycle at 95% or so, right? Now this makes the frequency valve open 95% of the time leading to a much richer mixture. Now is this a safe rich mixture or a too rich mixture? Is this a correct hypothesis of how a disconnected lambda sensor would effect the mixture?
    Now with the sensor disconnected what does the Jetronic box do besides complete an electrical path for the frequency valve? Any info would be appreciated.
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  3. Steve Magnusson

    Steve Magnusson F1 World Champ
    Lifetime Rossa

    Jan 11, 2001
    Full Name:
    Steve Magnusson
    Are you sure that it doesn't default to a 50% duty cycle (like after cold start-up) when no useful O2 sensor signal is available? I believe a disconnected O2 sensor line (no current and no voltage) can be detected by the ECU as diffferent than a true working 0V (lean) output. I confess I don't know the K-Jet with FV as well, but that's how a KE-Jet with lambda effectively behaves (it almost needs to behave that way otherwise you could never set the static warm-running idle mixture).
  4. slewman

    slewman Karting

    May 4, 2004
    With O2 sensor disconnected you can set fuel mixture anywhere you please and computer will not compensate. I have also seen O2 sensors disconnected and the CO value at idle set to 1.0% , this gives a smooth idle (no cycling up and down under lambda control. No danger to catalyst at this value, no power increase either.
  5. 308GTS

    308GTS Formula 3

    Dec 27, 2001
    I got the 95% duty cycle value from Probst's Bosch Injection Book. Thanks. I am not sure from what I gathered when disconnected it was the same as a bad 02 putting out no V. I could be wrong but one of the tests required removing the harness to the 02 sensor and measuring the duty cycle in a full rich condition. If it defaulted to 50% that would be great. It would only make sense that it should rather than 95%.
    So, at 50% it be safe. Keep me posted if you find out extra info.

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