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  1. #1
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    Jake D Janssen
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    Default Engine running rough

    Ok, here's the situation. After about 500 miles of driving with the new heads and cam the engine ran real rough today on my way to and way home from work. It has run great up until now. The engine ran great up until about 5 miles into the drive, probably about when it reached operating temp and it went into closed loop. I cannot verify this as I was not scanning the PCM at the time and now it's storming and I'm not sure I should drive it at this point anyhow. It was missing real bad and barely had enough power to pull itself to 55 mph. Every once in a while the check engine light would come on and it would run better then until it went out and then it would misfire again. I was thinking it seemed like it was running on 4 cylinders. When I scanned the car after work while idling it seemed to idle okay like it is supposed to and the only error code I saw was bank 2 O2 rich. After about 3 minues into idle the bank 2 O2 stuck at over 1.000 mV and barely changed. BLM for that bank was pegged at 108. Should I change the O2 sensors with new delco units, could a bad O2 sensor cause this issue? Could an injector have stuck open and killed the O2?

    My wideband was pegged out at 20:1 also, it's installed in the Y pipe before the merge after the standard O2.

    Thanks,
    Jake
    Last edited by Jakegsxr11; 07-07-2020 at 04:26 PM.

  2. #2
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    Default

    It looks like there are 3 versions of the AC Delco sensor, the heated sensor $50, the OE sensor $55, and the professional sensor $75. Which one is preferred?

  3. #3
    LTx Guru


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    Default

    Shoebox shows:

    1994-1995 heated oxygen sensor
    AC-Delco # AFS75
    GM 19178930

    Check the wiring harness first to make sure there isn't a short to 12V.

    A bit of work, but you can swap the O2 sensors side-to-side. If the problem/code follows the sensor, it's a sensor problem. If the problem/code remains on Bank 2, it's not the sensor.

    The fact the stock narrow band sensor was reading pig rich, and the PCM responded by pulling out the max possible 15% fuel with the 108 BLM, but the wide band was reading extremely lean, could be indicating the stock sensor is reading false rich.

    94 Formula A3+1: 381ci forged stroker - Callies Stealth, Oliver 5.85 billet rods, BME nitrous pistons / CNC LT4 heads / Comp Cams solid roller / TH400+GearVendors OD / 4.11's/Strange 12-bolt / 300-shot (dry) N2O / Spohn Suspension / 6-pt roll bar /AutoMeter instrumentation / MoTeC M48 Pro engine management system /a few other odds 'n ends.

  4. #4
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    Default

    I don't think it's a short, the sensor responds and appears to be "working" until 150 seconds after running then it pegs rich. I'll swap the sensors and see if the issue follows to the other side. My sensors are pretty easily accessible.

  5. #5
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    Default

    I swapped the O2 sensors from side to side last night. When I put them back in and ran the engine while scanning it ran fine and the O2 sensors worked as expected, even once I got into closed loop. When I unplugged the sensor in question it seemed a little moisture was in the plug. So I expect the plug was wet and the sensor was actually shorting out. It was really hot and humid the day I drove the car and it ran rough, the A/C was on high. I think the A/C condensation was dripping on the plug and soaked it, the new sensor plug location is right below where the condensation drips. I'll try to seal it up using some dielectric grease, some self fusing silicone tape and some electrical tape and hopefully that helps.

  6. #6
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    Default

    If the wires get wet, or coated with any fluids at the entrance to the sensor body, it can screw up the sensor readings. Ambient air has to reach the inside of the sensing element thimble. The difference between the O2 content in the ambient air inside the thimble, and the O2 content in the exhaust on the outside of the thimble is what creates the voltage that adds to or subtracts from the 450 millivolt bias voltage supplied by the PCM. My wife's Grand Cherokee was running like dog crap, and it turned out the PS reservoir was leaking and the fluid was running down the exhaust pipe and coating the sensor. Sensor replacement and reservoir replacement solved the problem.

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