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HUNTING IDLE, O2 Sensor related 700

posted by  Brick Skinner on  Thu Jul 23 14:55 CST 2009
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last visit: Fri Jun 13 13:31 CST 2014
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I've had this small nagging idle hunt for some time now. Between, say, 600-900 rpm. Over the months I have eliminated all possible causes except one; the O2 sensor. I replaced the head gasket a few months back because it was in real bad shape and leaking into the cylinders. I figured I was immune to the possibility of silicone contamination to the sensor so I didn't think it was a concern.

Well I stuck an o-scope on it today and a few things came to light. The first thing is that I do have about a 0.1-1.2 volt pulse except that it is very slow, about once every 1 or 2 seconds. Also, it seems like the sensor heating circuit stays on at around 13.2vdc.
NOW, here's what I don't understand: when I disconnect the red and white (heater) wires, the idle surge goes away and the pulse flatlines at 0.0 volts and the engine runs smooth as silk. When the red/white pair is re-connected the pulse gradually widens, over a period of about 15 sec's, along with the idle surge.

Questions:
1. Should the pulse be much shorter?
2. When should the heater circuit turn off?








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    HUNTING IDLE, O2 Sensor related 700

    uid 18083's pic
    posted by  lucid on  Fri Jul 24 04:31 CST 2009
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    last visit: Fri Oct 24 12:31 CST 2014
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    "NOW, here's what I don't understand: when I disconnect the red and white (heater) wires, the idle surge goes away and the pulse flatlines at 0.0 volts and the engine runs smooth as silk."

    Disconnecting the red and white wire connector also opens sensoer the ground side, resulting in no signal voltage to the ECU. The ECU should set the CEL and a 2-2-1 code if no O2 signal is detected...
    "...for more than 3 minutes under constant load, for example during idling...".
    See if you get that ECU response after 3 minutes of that "smooth as silk" idle.

    The smooth running is perhaps from to a rich mixture due to a no-signal default ECU reaction.

    The sensor voltage swing is within limits, but I can't explain the slowness.
    I don't recall any bad Titania reports here in 6+ years, but who knows?

    The heater circuit doesn't turn off. It's powered from the Fuel relay.

    --
    Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.








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    HUNTING IDLE, O2 Sensor related 700

    posted by  david hunter on  Thu Jul 23 16:15 CST 2009
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    last visit: Mon Jun 30 22:10 CST 2014
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    The sensor heater never turns off, 1.2 V is not normal, slow response usually indicates a bad sensor. It is most likely bad. Cannot explain why idle smooths out but I would go ahead and change it the O2.
    --
    David Hunter








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    HUNTING IDLE, O2 Sensor related 700

    posted by  Herb Goltz on  Thu Jul 23 15:09 CST 2009
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    I've encountered this before-- my bet is that you have voltage bleeding over from the heater circuit to the sensor side. You should only see swings from about 0.1-0.9VDC from the sensor itself. I had a similar problem with a nearly new universal Bosch sensor-- I was seeing 1.3 or 1.4V on the sensor line. Replacing the sensor fixed it.

    With the bad sensor I was getting 1-1-3 on the OBD. My car was a '92 245. This car was intermittently undrivable-- it hesitated very badly and would suddenly surge back to life.

    Good luck!








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      HUNTING IDLE, O2 Sensor related 700

      posted by  Brick Skinner on  Thu Jul 23 15:27 CST 2009
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      I'm not getting any codes except for a 1-4-3 once in a while (unrelated?). And I do have the titania sensor P/N 25002 from NTK.








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        HUNTING IDLE, O2 Sensor related 700

        posted by  Herb Goltz on  Fri Jul 24 03:30 CST 2009
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        last visit: Fri Oct 24 13:23 CST 2014
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        Ah, that's a Regina car-- then all bets are off about the voltage issue-- check the 700/900FAQ for details on checking the titania sensor-- I've only ever worked on one Regina car, but I didn't do anything with the 02 sensor.

        Good luck!







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