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Do these O2 sensor graphs look ok?

Discussion in 'VT - VX Holden Commodore (1997 - 2002)' started by PappaYeet, May 23, 2020 at 1:54 PM.

  1. PappaYeet

    PappaYeet New Member

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    Hello. After getting a random DTC76 code, I've been looking into checking the O2 sensors of my '99 VT L67. Today I cleaned the ground connections for the sensors, reset the PCM and went for a drive.

    When I got back I checked for codes (all good) and then logged the voltage of each sensor while holding the engine at 2k rpm. I've read that the sensors should contantly alternate between 100mv and 900mv, which it seems they do. At idle however, they both stay steady at around 800mv.

    As I don't know much about these sensors, I was just wondering if someone else could have a look at the graph to see if all is good. I can provide more readings if needed. Thank you for any help.

    [​IMG]

    XDL log download: here.


    Side note: My long term fuel trim is at -11 for both banks. Is this within a normal range? Car feels like it runs fine and I get around 10L/100km.
     
  2. Utel67

    Utel67 Active Member

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    Graph looks good to me.
     
  3. PappaYeet

    PappaYeet New Member

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    Does that include the voltages at idle (both constant at 800mv)?
     
  4. Utel67

    Utel67 Active Member

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    Yeah mine does the same.
     
  5. mechanic

    mechanic Active Member

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    The single graph in your post looks ok, but constant 800mV at idle is not right. It’s running rich at 800mV.

    If your LTFT is -11 your engine is running stupidly rich.

    Putting the info together I’d imagine you have an over-fuel situation at idle.

    Dripping injector
    Fuel pressure regulator diaphragm leak
    Fuel pressure regulator stuck
     
    Last edited: May 23, 2020 at 6:19 PM
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  6. Utel67

    Utel67 Active Member

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    Because you reset the pcm give it time for the fuel trims to adjust.
     
    Last edited: May 23, 2020 at 5:27 PM
  7. PappaYeet

    PappaYeet New Member

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    That's what I was thinking, but the idle feels smooth and exhaust doesn't smell/look rich. Are there any other numbers I could be looking at to find what's causing this?

    Another issue I have is a small stutter when accelerating while the engine is cold. Issue goes away after around 5 minutes of running. I guess this could be any sort of fuel/ignition problem, but strange that it's only while cold. I guess I'll do a full fuel system check this weekend.

    Probably correct, but I remember when I had a look at some codes a while ago the LTFT was still around -9 which is probably still a bit high.
     
  8. 07GTS

    07GTS Well-Known Member

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    ive seen almost new cars running 10% errors on the trims they are not dialled in from factory so it can be totally normal that's why getting a tune helps as it cleans it all up to closer to zero to suit your engine, -11 at least is safe as its removing 11% fuel to reach stoich
     
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  9. mechanic

    mechanic Active Member

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    Autocorrect error. I meant to type rich.
     
  10. mechanic

    mechanic Active Member

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    That’s not how fuel trims work. LTFT adjusts to engine conditions every few minutes. Other than initial emissions programming the factory has nothing to do with it. Nothing is dialed in.
     
  11. mechanic

    mechanic Active Member

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    If nothing was changed, they will adjust back to the current engine running conditions, which will likely be exactly the same as before.
     
  12. 07GTS

    07GTS Well-Known Member

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    don't understand what ur meaning as i say factory isn't dialled in and u say trims don't work that way nothing is dialled in ? i know how fuel trims work base fueling is set in stone so STFT and LTFT operate together to keep CL fueling at stoich within there airflow boundarys, i had my engine dialled in so close all year round i could stay in constant open loop fueling with minimal error
     
  13. mechanic

    mechanic Active Member

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    Could you be more specific about what you mean by ‘dialled in?’
     
  14. 07GTS

    07GTS Well-Known Member

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    i adjusted the VE table since im SD tuned, transient fuel tables, injector data offsets and other fuel multiplier tables so throughout the year my fueling stays within a small error percentage under any situation without the use of the 02's feedback
     
  15. mechanic

    mechanic Active Member

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    So you’ve reduced the size of the window of tolerance for each cell in the fuel map. How is this going to benefit you?

    if anything you’ve made your engine less capable of dealing with outlier conditions. Are you sure you haven’t dumbed-down your ECU without realising it.

    Maybe I still don’t get it? I’m not sure.
    Can you give me a specific example of what I’ve done? (Not in programming language, but real-world terms).
     
  16. 07GTS

    07GTS Well-Known Member

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    with my ecu there are a bit over 1000 cells (rpm v map) to adjust in the VE table so the closer u get that to spot on the better ( less error is less calculations the ecu has to consistently make from 02 feedback so the better the engine runs ) it also involves the transient fueling sorta like a pump shot on a carby setup, and all the multipliers based on various temps iat, ect, ivt, itt, as they all affect it under different conditions especially if ur modified

    ecu is not dumbed down as it corrects more accurately for any outside temperature changes throughout the year so it is in a way smarter then it came out from the factory as its not relying on feedback for fueling it just knows now from my changes, only way i can simplify it is this think of it as a maths equation and the fuel trims are u having to write down all your working out to get to the answer (more fuel trims error is more working out) but if ur dialled in in the first place u just write down the answer so both do get to the same end product just one is alot less work
     
  17. shane_3800

    shane_3800 Active Member

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    Before everyone gets too carried away. To actually know if the fuel is out or the sensors are faulty and over correcting you really need to use a gas anyliser to check lambda. If you haven't checked the actual lambda with an external sensor you have absolutely no idea if the readings the O2 sensors are giving the computer are correct.
    you have to assume they're incorrect untill verfied otherwise you're just pissing up the creek.
     
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  18. mechanic

    mechanic Active Member

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    If you’re only after increased power output, you may as well toss the O2 sensor out the window. It’s not difficult to make power at the expense of fuel, emissions, longevity etc. Stoichiometry is of little importance in this instance.

    What I don’t get is how you think your calculations are better than the O2 sensor and standard fuel maps. The O2 sensor adjusts for things you can’t possibly detect; worn compression rings and increased blow by, a partial blockage of an injector pintle, a minute gasket leak, small changes in oil viscosity etc.

    Fuel maps contain millions of lines of data and thousands of hours of lab testing and analysis. There are fuel map adjustments made for every degree of coolant temperature, air temperature, engine load, fuel type, knock signal, battery voltage etc ... the list of parameters is almost endless.

    You don’t type the way I’d expect an engineer to type, so I don’t think you understand that it’s not as simple as you describe.

    Anyway, getting way off track.
    O2 sensors are your friend. They help you diagnose engine faults and tell you faster than anything else when there’s a problem. You’ll never do a better job by manually overriding them.
     
  19. delcowizzid

    delcowizzid on holiday

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    He is not overriding them he's tuning the actual ecu maps based off the o2 sensor feedback so the maps are more refined and the o2 are correcting less variation the closer you get it the better the economy as the ecu swings rich and lean over and over when in closed loop and watches the o2 feedback to add or remove fuel where needed.
     
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  20. 07GTS

    07GTS Well-Known Member

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    ur thinking that factory is way more dialled in then they actually are they are not spot on, the fuel maps u think have millions of data is my VE table i have 1089 cells to adjust which is more then the engine will ever actually use (MAF sensor only has 66 cells to adjust), then i have multiplier tables for ECT, IAT, IVT, ITT and more then the injector data has all the offsets for delta pressure/flow, short pulse, min pulse, voltage/map offset, IAT offset, VOLTS offset, INJ tip temp offset its all there to adjust, fuel type flex setup and knock sensors all have there own settings u can adjust.

    im not an engineer i just like to tune and go through as much as i can to see exactly what it adjusts and how it affects all other tables its the only way u gain knowledge about it, i have no issue with 02 sensors when they are running good but i also know that if u disconnect the 02's on a factory tune the engine will run like garbage because its not that dialled in it needs them where as after a proper tune like mine if i was using the 02's and disconnect them mine will still run the same no issues, even without the 02's i can still dial in stoich fueling and only use enrichment when power is needed so economy is better then factory (i have a wideband also for tuning), its why so many people do there own tuning as u can get it to run so much better then factory even using the 02's as factory has to suit hundreds of thousands of vehicles where as tuning can refine it to what your engine likes
     

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