Engine - 1998 SE O2 sensor pegs lean (P1131) after running a while | Ford Escort Owners Association (FEOA)

Engine 1998 SE O2 sensor pegs lean (P1131) after running a while

Discussion in '3rd Gen 1997-2002 2.0L SOHC' started by Greenie, Nov 4, 2019.

  1. Greenie

    Greenie FEOA Member

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    Got a check engine light during otherwise normal drive. OBDII shows P1131. No other codes incl. pending.

    The graph for upstream O2 and short term trim shows oscillation, then if I let it warm up and rev it a few times, upstream O2 pegs lean, near 0V, trim maxes out. Stays that way for several minutes to forever. Idling is irregular, engine is a bit jiggly at those lower RPMs. No unusual sounds. No stalls. Exhaust a little stinky. 86K miles.

    OBD reading:
    https://drive.google.com/file/d/1O3bLQAH6JuAHyNQCukuriWNDk5laGVor/view?usp=sharing

    Eventually the computer gives up and resets trim to 0%, engine runs better.

    Saw no obvious vacuum leaks though I wasn't thorough, wiggled the connectors on engine, O2 sensor, wiggled wires near what appears to be a grounding block near passenger-side fender. No dice.

    Sometimes the oscillation manages to kick back in but usually goes out with a rev. and may never come back.

    Ideas?
  2. marclar

    marclar Moderator Staff Member

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    what is the threshold for the pegging of the graph vs coolant temp? does it peg lean as soon as it hits 160*
  3. Greenie

    Greenie FEOA Member

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    Ran another test, brought a CO detector with me. Outdoors, 15mph wind, windows closed, heater on floor, idle only, no revs this time.

    Temp climbed to 186F then O2 voltage dropped to .01V flat. Once that happened the smell came back and CO levels climbed rapidly. Had to abort the test at that point.
  4. marclar

    marclar Moderator Staff Member

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    interesting. any variations in the MAF signal?
  5. Greenie

    Greenie FEOA Member

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  6. marclar

    marclar Moderator Staff Member

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    is it always at the same 180ish degree area?
  7. Greenie

    Greenie FEOA Member

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    3rd test, O2 oscillations stopped and pegged to 0.01V at 170F this time. Colder outside.

    MAF seems to be oscillating healthy, no breaks even when O2 sensor cuts out. Its average value correlates tightly with RPM.

    Gave it a few revs, O2 sensor spiked to 0.9V, stayed there for a while, then returned to a sickly, weak, uneven oscillation around 0.4V - 0.6V before dropping back to 0.01V ~30s later.

    Car does seem a slightly weaker than normal: It doesn't move forward on its own without a little gas after releasing the brake but that might be the snow.
  8. marclar

    marclar Moderator Staff Member

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    sounds like a heat soaked oxygen sensor. the weird oscillation in the maf signal could be beausse of a change in suction durring the transition.. i would start with the hego, then maf.
  9. Greenie

    Greenie FEOA Member

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    Thanks a lot

    UPDATE: It was the O2 sensor. Replaced O2 sensor, power-cycled ECU, idled up to 190F, O2 sensor was fine, oscillating nicely. Seemed to be oscillating faster and the engine idle wasn't roller-coastering as much. Went for drive, O2 readings were good, accelerator seemed more responsive.

    Interesting that the old sensor failed to 0V. Literature I've found said it would fail to 0.5V bias. This made me under-confident to simply change the sensor before consulting the forums.

    Apparently either this sensor is biased to 0V or it failed to a short.

    Also found it odd that it would fail when the engine warmed up. My understanding was that these sensors are heated, so perhaps this means the pre-heated state is actually cooler than the naturally-heated state?

    Also, MAF reading is much smoother now that the O2 sensor was replaced.
    Last edited: Nov 5, 2019
  10. marclar

    marclar Moderator Staff Member

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    correct on the heater part.. the hego coil only gives it a bit of a jolt initially, i can not remember the exact temp but its around 200*.. its purpose is to try and get the zirconia core warm, and keep it warm during some conditions like over fueling where the exhaust is cooler.
    inside the sensor is a 'can' that contains a zirconia or porcelain insulator that 'reference' outside air in order to 'compare' ambient oxygen to exhaust oxygen. sometimes the can corrodes and allows air in, or exhaust out into the 'sensor' which can throw off the reading. the ect correlation is odd though. the pcm ignores the o2 signal until about 160*ish...
  11. Greenie

    Greenie FEOA Member

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    Could the sensor pegging have been mechanically triggered and not thermally? As the Escort warms up its RPMs go down and I can hear it going through its many (heh) resonance modes. I call it "The dubstep rattle."

    Then perhaps the shaking engine started exacerbated the failing sensor's malfunction, and that's why when it was colder out the failure happened at a slightly colder temp...? A rev would then let it settle or shift something internally, and sort of work until RPMs came down again.

    I imagine that if left in the engine the sensor would eventually just peg low constantly.
  12. marclar

    marclar Moderator Staff Member

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    that is very possible. these vehicles are known for many NVH issues. it is possible the zircon internals were rattled about as the resonance changed porportional to rpm.,

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