Long term Fuel Trim whacked out, with GRAPH

Discussion in '348/355' started by ThomasK, Apr 15, 2018 at 11:28 PM.

  1. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    High all,
    I have high (running lean) LTFT on bank 1 however, the STFT (short term fuel trim) seems to be manageable. The STFT for bank 2 is +/-5% and LTFT is about 3%.
    As you can see my LTFT for bank 1 is around 20%. how can LTFT be so whack but STFT be more normal?
    the only thing i can really suspect is possible throttle linkages out of sync or a vacuum leak but it idles perfect at 1050. i didnt do this test at constant RPMs "stages" (idle, 1.5k, 2.5k), i should have, but for now here is just a short test drive with graphing RPM (blue), bank 1 LTFT (gray) and bank 1 STFT (orange).
    any ideas? as far as i know the injectors have never been cleaned or replaced. i would like to smoke test it, but with 20% its gotta be something obvious but i dont see anything. maybe ill pull the plenum in the next few days.
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  2. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    i got code P1128 after a few hundred miles, which prompted me to do some diagnostics
     
  3. taz355

    taz355 F1 Rookie
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    Feb 18, 2008
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    Grant
    Could be that your not grtting enough fuel, like you said maybe injector not working properly.

    Also could be a exhaust leak drawing in air.

    What year and model is my it. What obd system
     
  4. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    yes it could be any of those things. i think ill just have to pull the plenum to take a look around, all exhaust bolts are tight and i cant hear any leaks.
    its a 99, so obd2.

    i did just do a major service along with some internal engine work and didnt have this code before. upon first start up and drive, i had the knock sensor and throttle body potentiometer plugs swapped by mistake. i wonder if from there something went wrong. ive read of that happening with no ill effects and ive done a full system reset since then, and it fixed the idle issues i was having due to the plugs being swapped, but now i have this LTFT issue on that bank 1.
     
  5. taz355

    taz355 F1 Rookie
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    Feb 18, 2008
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    Grant
    The throttle pot is just a backup position sensor. Ni it shouldnt cause any grief.
    Did you put in new o2 sensors?.
    What mods ie exhaust, etc are done. This will just tell us id it could be other things.
    Intake and exhaust are stock?
    Did you take off headers?
    If you know everything is s put back as it once was then smoke test intake and exhaust
     
  6. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    Thanks. I didn’t put in new o2 sensors but I do have some I purchased last week. Was going to put them in anyway. No intake mods, only mod is capristo muffler and forza valve controller which all this data was gathered with it in normal stock mode. Stock headers, stock secondary cat pipes, stock cats although they were replaced a few years ago and are the spiral core type. I had everything apart to replace crank and rod bearings along with head work. There is a chance something isn’t put back how it was but I am quite methodical.
    I do agree smoke test is my next step or regather the data on solid idle, then 1.5k then 2.5k to see if the trim levels decrease with higher rpm.
    I guess my question is I thought ltft was determined from stft, how could it be so far off from the short term? What sensor is determining the long term?
     
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  8. INTMD8

    INTMD8 F1 Rookie
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    Jun 10, 2007
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    If you reset learned trims and started it, long and short terms on both banks would be at zero. O2 voltage on bank 1 would be low as it enters closed loop so short terms would increase until o2 starts switching.

    If short terms stay positive it learns those values and applies it to the long term trims, so once long terms are at 20%, short terms are back to around zero.

    Not likely a vacuum leak IMO as it would drive trims up on both banks as fueling of both banks is based on a single mass airflow sensor.

    Most likely issue for one bank being high on a 5.2 car is (as Grand mentioned), exhaust leak ahead of o2 sensor and second fueling. (I think you will probably find an exhaust leak).

    Or, anything else that will introduce air into the system. Possibly a leaking check valve for air injection system?
     
    taz355 likes this.
  9. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    I had exhaust leaks on initial start up because I didn't have all the exhaust post headers tightened up because I wanted to align bumper with muffler, but all those have been tightened since then and I've shut off battery cutoff switch a few times since then.

    I unplugged battery last night then plugged it back in. Then started engine and logged data. The bank 1 long term trim was still through the roof and short terms were around zero at idle.

    Maybe I just don't understand how long term trim works. I see short term as a real time check for the upstream O2 sensors and the long term as a average of previous short term values. But if I don't have anything crazy short term what is going on long term? I'm thinking I need to get long term reset somehow but don't you need an SD 1 or 2 for that?
     
  10. INTMD8

    INTMD8 F1 Rookie
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    Short term is the first mode of correction, if short terms stay positive then it is learned to long term.

    Either way, it is still adding 20% more fuel than the base map.

    If the problem was fixed you would see short terms at negative 20% until it learned long terms back to zero, in which case short terms would go back to zero.
     
  11. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    any idea why long term would go from 20 to zero and back? short term does fluctuate +/-, its not always +.
    im just going to tear the plenum off and see if i did anything stupid, but if the leak was that bad i would think it would raise my idle and be kind of obvious. same thing with exhaust leak, i would think i would hear it. Ive heard it before and its quite obvious. car idles and runs great, but i dont want to drive it like this until i figure it out.
     
  12. INTMD8

    INTMD8 F1 Rookie
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    #11 INTMD8, Apr 16, 2018 at 11:46 AM
    Last edited: Apr 16, 2018 at 11:51 AM
    What do you mean why long term would go from 20 to zero and back? I thought it was basically stuck on 20% positive long fuel trim on bank 1?
     
  13. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    see the gray line in the graph i posted. It is 20 most of the time, but drops to zero or close it, randomly. i dont get it.
     
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  15. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    well, maybe not randomly. it kind of seems like if i get on throttle harder thats when it slams to zero or so, but then when i let off throttle it spikes...that would tell me a vacuum leak. i am going to pull this plenum first. or maybe ill hook up my boost leak tester to the plenum and see if i hear any air leaks.
     
  16. INTMD8

    INTMD8 F1 Rookie
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    Oh, because you're getting into an rpm range that have either not learned long term values yet or are not referencing them because you went from closed loop to open loop operation.

    (also possible that under heavier throttle/rpm the learned values are just lower)
     
  17. INTMD8

    INTMD8 F1 Rookie
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    A vacuum leak on one bank would reduce maf output and fueling to both banks. I don't think it's likely you would have a vacuum leak on one side so severe it causes a 20% positive fuel trim and bank 2 stays perfect but obviously I would check everything you can.
     
  18. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    oh really? thats interesting. my spreadsheet was corrupt this morning otherwise i would provide more data on bank 2, which looks fine on long term and short term.
    what would be so specific to bank 1 causing this kind of a fuel trim? i wish there were an easy way to test fuel pressure in one bank.
     
  19. INTMD8

    INTMD8 F1 Rookie
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    Well it has one mass airflow sensor that reports total airflow for both banks so generally a vacuum leak on one side will reduce fueling for the entire engine due to reduced maf output.

    Do 5.2 cars have 2 fuel pressure regulators? If only one regulator highly unlikely if not impossible for one bank to have low pressure.

    So still, in my opinion the most likely issue is an exhaust leak or air being introduced into the exhaust system ahead of the primary o2 sensor. Doesn't take much at all to drive up fuel trims.
     
  20. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    correct, 2 regulators.
    i would think if there was a leak maybe at one of the throttle bodies on bank 1 it would only make that bank/cylinder lean and not affect bank 2 at all, but maybe i have that wrong.

    someone needs to make an inline gauge adapter for the flared fittings for the fuel inlet so i can test the pressure on each bank.
     
  21. INTMD8

    INTMD8 F1 Rookie
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    Sure if it was carbureted or a dual maf 2.7 car. Both banks of your engine are being reported by the same mass airflow sensor so not possible for a vacuum leak on one bank to effect fueling for only that bank.
     
  22. jjtjr

    jjtjr Karting

    Aug 29, 2016
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    Vermont
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    john truskowski
    If in doubt as to what you are seeing from bank 1 vs bank 2 you could try swapping the precat o2 sensors, and see if the high LTFT follows. But remember, disconnecting the battery doesn't clear anything on an OBD2 vehicle. Only with a scan tool. The ECM looks to the O2 sensors for adjusting fuel trim.
     
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  23. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    ill drop the new O2 sensors in tonight and see what happens. after looking at even my STFT in the graph, they probably should be switching up and down more often.
     
  24. drbob101

    drbob101 F1 Rookie
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    leaking injector/s?
     
  25. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    i would think they would be clogged if anything on bank 1 since its running lean and overcompensating via increased pulse width. the O2 sensors may be lying so im going to swap those first and see.
     
  26. yelcab

    yelcab F1 Veteran
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    some 355 have this, some don't. There seems to be no consistency.
     
  27. ThomasK

    ThomasK Karting

    Dec 8, 2009
    102
    slc, UT
    i swapped the secondary o2 from one side to the other, i still see lean measurement on the 2ndary o2 sensor on bank 1. i also replaced the upstream o2 on bank 1 with no change.

    although...i was playing with the throttle linkage and noticed that if i barely move it, there is some play on the driver side and linkage doesnt move, however the passenger side opens a little. and with linkage fully open the mechanical stop hits first on the passenger side. so from what im seeing, the linkage may be out of alignment? it also must affect idle because my LTFT is messed up at idle too.
     

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