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Question about a 355 OBD2 car's pre-cat O2 sensors

Discussion in '348/355' started by hjp, Jun 3, 2017.

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  1. hjp

    hjp Formula Junior

    Feb 23, 2013
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    #1 hjp, Jun 3, 2017
    Perhaps you guys with more experience than I can answer this.

    Is it possible for a pre-cat O2 sensor to begin to malfunction without generating a code, sending the wrong information to the ECU telling it the mixture is too rich or lean when in fact it is not. In other words, if an O2 sensor begins to fail due to age or whatever, does it totally fail or can it just subtly begin to act up? If so, how would you know if anything is wrong? Something like a mixture too rich for example, creating pops/backfiring in the exhaust? All other things being normal, would this be a sign an O2 sensor is at fault? Or, as far as O2 sensors go, do they either work or they don't?
     
  2. INTMD8

    INTMD8 F1 Veteran
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    #2 INTMD8, Jun 3, 2017
    They degrade over time but even slow switching, generally stay close to maintaining stoich.

    Really slow switching will result in some surging. Stuck at high or low voltage will result in pegged fuel trims in one direction. (And resulting in poor running)

    You can scan it to check o2 function and fuel trims. Of course skewed fuel trims are usually a result of another problem rather than the sensors themselves.

    I consider them a maintenance item and will replace at every engine out.

    Are you having some type of issue?
     
  3. kenneyd

    kenneyd Formula 3

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    #3 kenneyd, Jun 3, 2017
    Short answer, yes they can degrade in performance without failing to the point of a trouble code.

    On many cars they are a periodic maintenance item. If you are trying to trouble shoot and issue, I would start with the simple stuff like an 02 sensor, especially if they are near 20 years old
     
  4. hjp

    hjp Formula Junior

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    #4 hjp, Jun 3, 2017


    I think I might be. When I wind it up at full throttle to near red line and shift to the next gear (first to second) there is some backfiring (one or two pops). It didn't used to do this. It implies the mixture may be too rich leaving some unburned fuel in the exhaust that ignites at shifts. I don't know how old the O2 sensors are. Maybe I should just prophylactically replace them.

    Where is the best place and price to get plug ready replacements? How much should I expect to spend?
     
  5. Dave rocks

    Dave rocks F1 World Champ
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    #5 Dave rocks, Jun 3, 2017
  6. hjp

    hjp Formula Junior

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    #6 hjp, Jun 3, 2017
    Just checked. These were replaced with new Bosch OEM by the previous owner. There are only about 1,500 miles on them. This leads me to believe they are still OK (don't think they would go bad that fast). What else could cause what I am experiencing?
     
  7. Dave rocks

    Dave rocks F1 World Champ
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    #7 Dave rocks, Jun 3, 2017
    I would get an OBDII monitor such as the Kiwi app and check out the fuel trims as Jim suggested. You could have an intake vacuum leak or other stuff causing a rich condition but you first need to see if you truly have a rich condition.
     
  8. johnk...

    johnk... F1 World Champ
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    #8 johnk..., Jun 3, 2017
    Last edited: Jun 3, 2017
    When are you experiencing the problem, all the time or every so often? If the the O2 sensor was pegged at high or low voltage you should have a code. If fuel trims are pegged, you should have a code. Are you running cats? Did you remove the cats and then this started?

    Oh, also, if this happens only at full throttle I doubt it has anything to do with O2 sensors because at WOT the ECU goes full rich.

    Best place for plug and play replacement is probably Rock Auto.
     
  9. kenneyd

    kenneyd Formula 3

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    #9 kenneyd, Jun 3, 2017
    I'm just thinking as I'm typing, the injectors should close as soon as you let off the throttle so if it's backfiring after you let off, it's not cuz it's Rich it's because your injectors are still putting in fuel. So maybe leaking injector?

    Sent from my SM-G930P using Tapatalk
     
  10. INTMD8

    INTMD8 F1 Veteran
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    #10 INTMD8, Jun 3, 2017
    As I read it, it back fires after shifting and in the next gear.

    Agree with John that it doesn't sound like 02 sensors for the reasons he stated.


    A video of this happening may help. I would be looking at plugs/wires first myself, just to cover the simplest possibility first.
     
  11. hjp

    hjp Formula Junior

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    #11 hjp, Jun 4, 2017
    I don't usually wind it up to near red line at full throttle very often but, yes, I think it does this most if not every time, just not positive. It does not do this when driving normally.

    John, I didn't know it goes to full rich at WOT. Interesting and good to know. Oh, and it has not thrown a code (at least so far). You know, I did put in Fabspeed test pipes a while ago (now back out but haven't had the car on the road yet to see if it still does this....been up on jack stands all winter fixing/improving a bunch of little things). That may very well have been when this started but not sure. However, when I bought the car, it had test pipes and did not backfire. However, these were not normal test pipes. They looked somewhat like a cat and had a perf'd pipe down the center with fiberglass surrounding the perf'd pipe, a sort of resonator. Since then I put cats back in for a while and then out again when one developed a loose core. I think that's when the popping started

    To be clear how this happens, only after WOT near red line in first gear and then immediately as my foot comes off the gas it pops/backfires once or twice while momentarily shifting into second gear. It immediately stops when the shift is complete and the WOT begins again. It only happens during that brief pause between gears. I've never tried to see if it would do it again between 2nd and 3rd (no good place to get it up that fast very often around here). Otherwise the car seems to run fine (except it sometimes [not always] lopes a bit while idling at a stop light when its very hot out [high 90's]). Maybe all this is more or less normal and nothing to worry about?? Thoughts?
     
  12. Rifledriver

    Rifledriver Three Time F1 World Champ

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    #12 Rifledriver, Jun 4, 2017
    Last edited: Jun 4, 2017
    Trims do not need to be pegged. Motronic allows a large adjustment but will set a code long before then. There is an expected normal maximum. Beyond that it set an error.
     
  13. johnk...

    johnk... F1 World Champ
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    #13 johnk..., Jun 4, 2017
    Yes, I realize that. I was just trying to kept it simple. FChat you know. Point being, no codes, most likely, no sensor problem.
     
  14. Ferrari Fanatic

    Ferrari Fanatic Formula 3

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    #14 Ferrari Fanatic, Jun 4, 2017
    My 1998 355 did the exact same popping noise between shifts, but only with the test pipes on. BTW, on a spirited club drive the guys behind me said it looked awesome, LOL.

    No cats = high hydrocarbons. Hydrocarbons are unburned fuel emitting from the tail pipes. CO is unburned fuel in the cylinders. So high HCs with no cats could probably cause this. Mine did and went back to normal when I put my cats back on.

    Give her another go with the cats back on when you get a moment.

    Just my .02.
     
  15. INTMD8

    INTMD8 F1 Veteran
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    #15 INTMD8, Jun 4, 2017
    Good to know. I've seen some cars at 25%+ forever and not throw b1/b2 lean. Others at plus 10 will set codes. (Talking gm stuff, no idea on thresholds for Ferrari)
     
  16. Rifledriver

    Rifledriver Three Time F1 World Champ

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    #16 Rifledriver, Jun 6, 2017
    Well on Ferrari it is a moving target. 355 had several software versions even just for 355 5.2 and it seems they are different in that respect. Some are as low as 8% +- but some are much higher.
     

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