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Thread: Dropping out of closed loop @ idle question

  1. #1
    Electronic Ignition!
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    Dropping out of closed loop @ idle question

    Hi,
    I have bone stock 93 C1500 5.7L TBI with a 16168625 ECM/PCM and BDJZ std code.
    I failed emissions and was looking thru Tunerpro datalog to see if I could see why.
    I noticed that I'm dropping out of closed loop when sitting at hot idle due to the O2 sensor changing to "not ready".
    I checked coolant sensor, cleaned up grounds and confirmed in datalog I'm @ 90C when this happens.
    It appears it happens when O2 sensor doesn't fall below 400mV when in closed loop. Possible the other criteria is over 700 mV. Can't tell if it's an either or both from trace.
    I've possibly never replaced the O2 sensor and assume I have a "lazy" sensor from reading/googling - so I'm going to replace it.

    I was curious if anyone knows the specific criteria for the "ready" status of the O2 sensor?
    I've read that the other two criteria to be in closed loop is a coolant temp and a engine on time criteria.
    If anyone knows those also, I'd appreciate it. I've seen many different coolant temp criteria - I'm assuming it may be different for different engines.

    also
    I noticed that if I cold start and just let it idle it never gets into closed loop. The temp comes up to about 90C and @ 10 min it's still in open loop.
    I can see the O2 sensor slowly rise with temp from around 450mV to 550mV with little noise as INT is not changing.
    If I blip the throttle hard so force a change in O2 below 400mV then it gets into Closed loop and INT changes and O2 changes as I'd expect.
    It seems this is normal behavior assuming the O2 sensor ready criteria of changes under 400mV / over 700mV is correct. Is this also right?

    Thanks for help. perhaps not needed to verify O2 sensor is bad but curious about how the system works.
    Ed

  2. #2
    Fuel Injected! PlayingWithTBI's Avatar
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    Sounds like your O2 sensor isn't staying hot enough while idling. Try a heated one?

  3. #3
    Electronic Ignition!
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    Quote Originally Posted by PlayingWithTBI View Post
    Sounds like your O2 sensor isn't staying hot enough while idling. Try a heated one?
    Thanks for suggestion. I was thinking about that also but, in the end, I decided to stay with single wire as the stock location is in the big cast iron manifold close to the block. That plus, like I said, I don't think I ever replaced it and I've owned it since new. I'm also being a bit lazy - I won't have to mess with adding wires for power, ground if a new single wire O2 works. If it doesn't- then I'll go 3 wire and wish I had taken your advice first.
    I'll let you know how it turns out

  4. #4
    Electronic Ignition!
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    I replaced the O2 sensor with a new ACDelco - single wire. It fixed the problem.
    Big difference in range the sensor swings. On new one it ranges between 800mV and 100mV. On old, roughly 700mV to 400mV. so it was indeed "lazy".
    Also - it did drop into closed loop @ idle on cold start. The O2sensor became ready when sensor reached about 735mV. This occurred on same sample as when the "closed loop timer flag" passed. This was @ 210 sec after cold start. - I don't know what criteria (O2 mV, time, ?) was satisfied to enable the O2 sensor ready.
    The system went into closed loop @ idle after temp reached about 79C/174F. It stayed in closed loop.

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