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Thread: Fuel trims what makes them max out at idle?

  1. #21
    Davezj's Avatar

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    We'll I did the tests and blocked up the PCV and idle adjust screw on the throttle body and guess what........




    It reduced the STFT fuel trim by about 1%.
    So that is not the mason cause of the odd operation.

    Bye for Now!

  2. #22
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    I have been having a think ( it takes a long time no I am getting older, a bit like tree beard LOTR)
    And I am going to flash a standard rom to my car and recheck the fuel trims just in case it is the modified rom file I amusing that is not quite right. I don't think it will be but it is work checking.

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    I don't think it will change anything significantly. Dave, please note that the LTFT-Low trim is capped at around 5%, but the STFT can go quite a bit higher. More modern engines set a fault if the trims sit at or exceed 20%. In your case the LTFT + STFT sum up to about 10% (LTFT 4.7 and STFT 5.1). In my case about 7-8% (LTFT 4.68, STFT 2-3%), and the -Mid and -High trims seem not implemented in the Mitsi firmware at all.

    I do believe trying to nail this down to theoretical "ideal" levels is a bit of an OCD exercise. The MAF may be old and inaccurate, fuel pump underpowered, injectors clogged and with wrong pattern. Even the injector drivers may be underpowered due to capacitors exceeding their lifetime design specifications (consequently the effective injector pulse width is less than the ECU would like). Or you can have a combination of a vacuum leak and leaky injector offsetting one problem with another. Or one injector clogged, another one leaky in which case one cylinder is rich, another one is lean and we have only one O2 sensor to rule them all.

    My head is spinning from all the possibilities, sorry if I am muddying up the waters

  4. #24
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    there are lots of possibilities,

    but i went through the exersize of,

    swapping the injectors for a known good cleaned and flow tested set.
    fuel pump is 255Lph not very old.
    no significant air leaks detected (boost pipes tested to 20-25psi).
    O2 sensor swapped for 3 different sensors and a good working wideband with narrowband emulation.
    Maf swapped for known good working MAF.

    so this why i was questioning the fuel trims and why i was not having any success sorting them out.
    i am not worried by the at idle fuel trim not being as i would expect it to be but i really do not like not knowing why something is happening on my car.

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    Quote Originally Posted by Davezj View Post
    Maf swapped for known good working MAF.

    so this why i was questioning the fuel trims and why i was not having any success sorting them out.
    i am not worried by the at idle fuel trim not being as i would expect it to be but i really do not like not knowing why something is happening on my car.
    OK fair enough, lots of possibilities excluded. However I don't quite trust the "known good working MAF" unless it was somehow airflow tested or brand new. These age same as everything else in our cars, and the ultrasound piezos can weaken with time. It may work but may have developed an offset with time (that's what the glued trim screw is for, I would assume they are checked and calibrated at the factory).

    Also the ECU injector driver circuit is a possible culprit. It could be tested with an oscilloscope, but I don't think I would be competent enough to do it. Somebody can compare a known good injector waveform to a suspected one.

  6. #26
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    the Maf came from a car that was happily runing LTFT on petrol at 1.2% and STFT was switching above and below zero. so would expect the same result on my car it was my original MAF causing the fuel trims to max out.

    injector driver issue can be tested and measured as i have an osiloscope to do the job and as i have a few spare ecu's and can swap out the one from the car running 1.2 LTFT and zero STFT and see if there is a difference.

    but that might take a bit of time setup and do. i have a lot of other more pressing stuff on my schedule, butit is something i would like to get to the bottom of.

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    Very interested to see how the tests turn out

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    Dave what you experience that car jumps out of closed loop is due to lambda feeding the ECU with a static (or near static) for too long time. ECU then assumes the sensor is not working and jumps to open loop (without check engine fault light).
    If you touch the throttle so that AFR gets to the range the lambda can measure, the ECU picks it up and returns to closed loop. This happens if the trims are a bit too far out.
    However STFT jumping is normal and the car wont use more fuel even if the trims are almost maxed out positive, it would just run too lean without the correction. I would say if the oxygen feedback stays within +-10% it is fine.

    To set up the trims there is 3 things to setup:
    Injector latency (dead time) affects AFR at idle and low load
    Injector scaling affects at all loads.
    Individual cells in fuel map

    If your car runs leaner on boost than target maps, decrease injector scaling. If it runs richer, increase.
    If it runs as the target maps on boost but runs lean on idle/ really light load, increase injector latency. If rich, decrease latency.
    Log STFT and check if adjustments needs to be made to individual cells

    In your case I would just decrease injector scaling a bit, almost all vr4 max out the fuel trims in stock form. If you need to change latency the address was faulty in my vr4 base definition, at least in my rom (2003E013) the address is 115a2

  9. #29
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    Dave, what Anderz says there might also be what Nick was talking about at the weekend - try adjusting the narrowband output so that the ECU doesn't see that static value.

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    Thanks anderz
    That is the sort of information I was looking for.
    I think from what you are saying it is the injector latency I need to adjust.
    But I will have a bit more of a read about and do some testing
    Thanks again

  11. #31
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    If it is possible to adjust the simulation so that it gives a changing signal even if it is outside the measurable range of the sensor, then that can help the problem. Or get the other parameters set up so that the engine stays in the measurable range, then it won't happen This also helps when the car is just started before lambda is used.
    Have done programmable narrowband simulator on my cars using Arduino micro controller, running AFR target 15.5 on E85 with 1000cc EV14 injectors

    Good luck with the testing

  12. #32
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    Quote Originally Posted by Confused View Post
    Dave, what Anderz says there might also be what Nick was talking about at the weekend - try adjusting the narrowband output so that the ECU doesn't see that static value.
    yes i remember now we did have that conversation, i completely forgot about it, there was so much stuff going on on saturday i forgot to look at yours and nicks fuel trims as well, just ran out of time and battery in my laptop.

    i will have a look at the over the weekend, i can make the switching point more vertical so the narrow band will do the switching from lean to rich and back to lean quicker the switch speed from lean to rich and from rich to lean will not change but there will be less of a wait at each state before it switches back.
    that is an easy thing to check.

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    Quote Originally Posted by Anderz View Post
    If it is possible to adjust the simulation so that it gives a changing signal even if it is outside the measurable range of the sensor, then that can help the problem. Or get the other parameters set up so that the engine stays in the measurable range, then it won't happen This also helps when the car is just started before lambda is used.
    Have done programmable narrowband simulator on my cars using Arduino micro controller, running AFR target 15.5 on E85 with 1000cc EV14 injectors

    Good luck with the testing
    i don't think i will be needing to go down the arduino route although i have used them before and they are great little devices. it is an interesting thought.

  14. #34
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    Quote Originally Posted by Anderz View Post
    Have done programmable narrowband simulator on my cars using Arduino micro controller, running AFR target 15.5 on E85 with 1000cc EV14 injectors
    I would be really interested in seeing the source code for that - my PLX control unit has died, if I could simply replace that with the Arduino I have sat in my cupboard, then that might be another option!!

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    Quote Originally Posted by Confused View Post
    I would be really interested in seeing the source code for that - my PLX control unit has died, if I could simply replace that with the Arduino I have sat in my cupboard, then that might be another option!!
    My impementation is not that advanced that it controls the wideband sensor itself. It reads the wideband 0-5V output, and outputs 0-1V narrowband at the desired AFR target. Arduino also does Hz->Voltage conversion for the ethanol sensor.
    Made narrowband with arduino beacuse you can not change the target AFR with PLX

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    Hi all,

    Any update on this fuel trim issue?

    Actually, I may have the similar problem too, ie, the fuel trims is at max with idling, and running too lean at idle and fast idle.

    Let me explain what I have tested so far.

    When at idle, car not running:
    fuel trim low: ~4.7
    fuel trim in use: ~4.7
    oxygen feedback trim: ~5.1
    AFR: around 16-17

    When fast idle, above 2000rpm, car not running:
    fuel trim low: ~4.7
    fuel trim in use: 0
    oxygen feedback trim: ~5.1
    AFR: around 16-17

    When cruising, above 70km/h, around 2000rpm
    fuel trim low: ~4.7
    fuel trim in use: ~4.7
    oxygen feedback trim: anything 0-10
    AFR: 14.5-15.5


    Now, as you can see, at idle(650rpm) and fast idle(>2000rpm), it's running too lean, but when driving/cruising, the AFR is normal at around 14.5-15.5. In this situation, I assume this is no vacuum leak, am I right?

    However, I have tested 1 time, which the fast idle is normal, around 15.5, much better than 17, and the oxygen feedback trim could changed anything 0-10. One day, I was driving my car, and then I pulled-over and immediately took the log with fast idle(>2000rpm), and the AFR is around 15.5, no more lean, and that's surprising me.

    I have already flashed back using stock ROM, stock fuel pump. Also, I have changed to a spare MAF sensor and no different. Used stock O2 sensor, and also I used PLX simulated narrowband, both have no different in AFR reading.

    I still don't know why it is running lean when idle and fast idle, but I found something interesting is whenever I was driving/cruising, the AFR was perfectly around 14.5-15.5, once I completely stopped the car, eg, traffic light, the AFR was good(around 15) for the first 20-30 second, after that, the AFR changed leaner and leaner, around >16, or even 17. This is repeatable! If you have this lean problem, have you noticed that?

    As you can see, when in cruising, the AFR is perfect, so, I assume there is no air leak, no misfire, etc. There are few things that I suspect: the fuel pressure regulator, or the fuel pressure regulator soleniod, or weak fuel pump, as you all know our fuel pump only has 7-8v when in idle, but I have hot-wire to 12v, but no different, still lean at idle.

  17. #37
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    Quote Originally Posted by kc427 View Post

    Now, as you can see, at idle(650rpm) and fast idle(>2000rpm), it's running too lean, but when driving/cruising, the AFR is normal at around 14.5-15.5. In this situation, I assume this is no vacuum leak, am I right?
    Nope, you still can have vacuum leak between maf and turbo.
    Ex: Galant VR4
    Running 268 HP ATW and 443 Nm torque at 0.9 bar
    Now: Lancer Evolution 8 FQ-300
    Running 325 HP ATW and 510 Nm torque at 1.6 bar

  18. #38
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    Quote Originally Posted by Anderz View Post
    My impementation is not that advanced that it controls the wideband sensor itself. It reads the wideband 0-5V output, and outputs 0-1V narrowband at the desired AFR target. Arduino also does Hz->Voltage conversion for the ethanol sensor.
    Made narrowband with arduino beacuse you can not change the target AFR with PLX
    Hi anderz
    Can you post up the hz to voltage formula as this could be used as a base for a voltage to hz formula for a MAF to map conversion could it not.

    Or am I think along the wrong lines.

    If you have data point for the hz with respect to the volts, could it be used.
    You know the boost pressure , and the corresponding voltage level could this be converted to a frequency for the into of the MAF signal to the ecu.

  19. #39

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    Quote Originally Posted by Davezj View Post
    Hi anderz
    Can you post up the hz to voltage formula as this could be used as a base for a voltage to hz formula for a MAF to map conversion could it not.

    Or am I think along the wrong lines.

    If you have data point for the hz with respect to the volts, could it be used.
    You know the boost pressure , and the corresponding voltage level could this be converted to a frequency for the into of the MAF signal to the ecu.
    MAF directly measures airflow. MAP can be used to infer airflow from the pressure signal. With respect to technical means the MAF-MAP conversion indeed relies on a Hz-Volts mapping. However the transfer function needs to be adjusted to engine internals. Ie. you could develop a MAF-MAP mapping for a stock engine and then it should more or less match other VR-4s with stock internals. Any modification with regards to intake or exhaust will require that mapping to be adjusted. In that respect the MAF is "safer" as it adapts to intake/exhaust modifications by itself.

    The MAP-ECU has an auto learn mode specifically for this purpose, where it memorises airflow in a RPM vs Pressure table. Then it can "pretend" to be the MAF (any type).
    The more you know, the faster you can go. And I still don't know enough.

  20. #40
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    Makes sense.
    A td03 at 1.0 bar will be flowing much less air than a td04 at 1.0 bar.
    How do the evos cope with all the mods that are thrown at them, are they not MAP sensed rather than MAF or is that one of the mods.

    Swinks mentioned about an SD (is it speed density or something like that) mods I presume it is this.

    Would be nice to get rid of all that pipe work under the bonnet.

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