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O2 sensor on Rich at idle when the car is at orerating temperature

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  • [Corsa B 1993-2000] O2 sensor on Rich at idle when the car is at orerating temperature

    Hello,

    I have a 1998 1.0 Corsa B (X10XE). I have an OPCOM that I bought as I had pollution issues.

    Basically, the motor seems to work well, except when the car is at idle and hot, where the O2 sensor stays around 800 mV. What is stange is that all other operating conditions seems OK
    - when I start the cas, and the motor temperature increases, the O2 sensor oscillates, and the mix alternates between lean/rich,
    - when I drive the car with the OPCOM connected, the O2 sensor oscillates,
    - but when I park the car back home and check, then at Idle, then the O2 sensor is stable around 800 mV / Rich.
    - If I press slightly the gas pedal, then the the O2 sensor starts to oscillate again,
    - If I relase the pedal, the O2 raises "high" and then slowly decrease (over severql seconds)... to stabilize around 800 mV ... Injection times around 4.4 ms, stable.

    All parameters seem to behave correctly: air massflow, coolant temp, throttle angle....

    I block on that since several weeks, and don't understand. Any help welcomed !

    Best regards,

    JM

  • #2
    Check your basic service items, filters, plugs etc. Check there are no leaks around the exhaust, especially near the front end. For the price of one I'd try a new coolant temperature sensor as the default ECU mix is rich and it leans out as the CTS tells the ECU that the engine is warming up. That's the CTS on top of the water pump, not the gauge temperature sender on the EGR valve.
    1972 Viva restoration thread - http://www.thecorsa.co.uk/projects-b....html#post1534

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    • #3
      Hi,

      Thanks for the proposal. All is new: plugs, air filter, fuel filter.

      For the coolant Temp sensor, it looks good to me. The mix is OK during all the warmup phase, with an O2 sensor readout that oscillate and a mix that appears alternatively lean/rich on the OPCOM. The value given by the OPCOM for the coolant Temp is also OK.

      What would be the effect of a leak at the exhaust ? It is after my O2 sensor. Could such a leak after the O2 sensor affect the reading in the way that I have ?

      My issue is:
      - when I park the car back home and check, then at Idle, then the O2 sensor is stable around 800 mV / Rich.
      - If I press very slightly the gas pedal, then the the O2 sensor starts to oscillate again,
      - If I relase the pedal, the O2 raises "high" and then slowly decrease (over severql seconds)... to stabilize around 800 mV ... Injection times around 4.4 ms, stable.

      Best regards,

      JMF

      Comment


      • #4
        Any exhaust leak, even after the O2 sensor, can throw the sensor out of parameter. Since it's a feedback system the ECU then tries to compensate - so you end up trying to fix the symptom rather than the cause. (Hope that makes some kind of sense.) So if your exhaust is leaking sort that first - then check the readings again.
        1972 Viva restoration thread - http://www.thecorsa.co.uk/projects-b....html#post1534

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        • #5
          Thanks for the explanation.

          Are there methods to follow to find those exhaust leaks ?

          Comment


          • #6
            Block the tailpipe with a damp rag - you'll find any leaks quickly enough that way.
            1972 Viva restoration thread - http://www.thecorsa.co.uk/projects-b....html#post1534

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            • #7
              Thank you Taurus,

              I will try this week-end

              JMF

              Comment


              • #8
                Hello,

                I tried to find a leak in the exhaust system yesterday. I blocked the tailpipe with a damp rag. I have a small hiss, just before the catalyser:
                Click image for larger version

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                I also captured the screen of the OPCOM to show my issue: At idle, the O2 sensor is blocked around 800mV. If I very slightly accelerates to 1200 RPM, the sensors starts to oscillate. Then, if I come back to Idle, the O2 sensor raises again:

                First graph: Red = O2 sensor, Yellow= injection pulse, blue = actual value idle air, green = calculated airflow
                Second graph: Red = O2 sensor, Yellow= injection pulse, blue = aThrottle valve angle, green = RPM

                Click image for larger version

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                Could this help?

                Best regards,

                JMF
                Last edited by jmf; 13-03-2016, 08:44 AM. Reason: legend of graphs

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                • #9
                  I would patch that hole with exhaust repair paste, run the test again ,and see how the car behaves. With the hole that close to the clamp you could remove the clamp, plug the hole with paste,put a small patch of thin metal over the top and clamp it back up. I'm not an mot tester but I think it would pass its mot like that. But whatever the expert opinion on this the hole needs fixing even if you have to replace the pipe. If it cures the problem thats a bonus.

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                  • #10
                    Hi Bugman,

                    I think the the hiss comes from the gap between the tube and the fitting (not sure about my english here: the small external tbe that helps to connect to two tubes). What looks like a hole is the hole at the end of the fitting "cut".

                    JMF

                    Comment


                    • #11
                      Hi
                      You might be able to seal the joint just by tightening up the clamp a bit more.Or changing its position slightly. But if the hole is actually the end of the 'cut' it may be the tube from the catalyser has not been pushed deep enough into the split 'female' tube to cover the hole internally . If you suspect this is the case its best fix this before you try tightening the clamp. You can probably find enough free play in the system to push it in a few more millimeters but it may not be easy with old joints. Be careful not to damage the catalyser or yourself.
                      Edit .I have just realised from your comment that english may not be your first language . Your english is excellent. Better than many english people can manage. But if you are not in the UK your vehicle testing may not allow using exhaust repair paste.
                      If you do decide to repair the hole with paste bear in mind there may be a similar weak area on the 'cut' on the top of the pipe. Better to re-make the joint.
                      The clamp is usually called a U bolt clamp.
                      Last edited by Bugman; 15-03-2016, 10:29 AM.

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