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  1. #1
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    Default My split blm problem

    Hey guys Iíve been having split blms across the board for a long time now and Iím trying to finally get it figured out.
    I have tried everything, novacuum leaks, no leaky injectors, exhaust leaks, good iac counts etc. they always end up around 108/130. The engine seems to run fine, just not when you look at the data.

    yesterday I was playing with eehack, and I found that if I cut fuel to either cylinder 1 or cylinder 7, the blms start to even out! My question is, why? Weak spark? Valve issues?
    thanks!

    if it will help I will upload a log.
    Last edited by Fastbird; 06-02-2020 at 05:53 PM.
    1991 Z28
    Engine: '94 355 LT1, GM hot cam, 1.6 RR's, Ported Heads, Dyno Don's headers, #30 SVO injectors, stock internals with skirted pistons.
    Drivetrain: '95 T56, pro 5.0 shifter, hayes clutch, Stock 10 bolt with 3.42's




  2. #2
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    Default

    Do you have BLM data for all 18 cells? If so, post it and we can take a look at it. Would also be helpful to have a list of engine mods, since the engine obviously isn’t stock. Particularly the injectors. Are they new, old, ever been cleaned and flow matched? Also, who did the tune?

  3. #3
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    Default

    Thanks for the reply. I updated my sig to include mods. I was working with Solomon at PCM for less for the tune, this was 7 years ago. He said he wasn't sure if the problem was mechanical or with the tune, but he was leaning toward a mechanical issue. Ive recently got the urge to figure it out, but would like to try and get this resolved before I send him another log.

    I just went out and did a quick drive. I tried to get some Idle at different points and some PE. I also did a separate log where I deactivated cylinders 1 at a time 1,7,5, and then 3. The results are less convincing today. I did a cylinder test at the end as well.

    Im having trouble uploading the .eedata file ?
    1991 Z28
    Engine: '94 355 LT1, GM hot cam, 1.6 RR's, Ported Heads, Dyno Don's headers, #30 SVO injectors, stock internals with skirted pistons.
    Drivetrain: '95 T56, pro 5.0 shifter, hayes clutch, Stock 10 bolt with 3.42's




  4. #4
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    If it's eeHack, I'd prefer you just scrolled through it and listed the left and right BLM's for each of the 18 cells that you drove in, plus O2 sensor readings in PE mode. I have reviewed a couple eeHack logs and they are just too cumbersome to work with. Scan9495 is so much easier and user friendly. A made this comment a day of two ago, and I wasn't the only one who had issues with the eeHack logs.

    94 Formula A3+1: 381ci forged stroker - Callies Stealth, Oliver 5.85 billet rods, BME nitrous pistons / CNC LT4 heads / Comp Cams solid roller / TH400+GearVendors OD / 4.11's/Strange 12-bolt / 300-shot (dry) N2O / Spohn Suspension / 6-pt roll bar /AutoMeter instrumentation / MoTeC M48 Pro engine management system /a few other odds 'n ends.

  5. #5
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    Default

    Ok. I will give you some snapshots, I will try and take another log with that software if you would like.

    Cell BLM INT O2
    2 124/129 109/151 .852/.044
    3 118/147 109/147 .839/.209
    6 114/125 118/130 .777/.755
    18 108/129 137/128 .115/.053
    16 108/135 126/130 .213/.106
    17 108/142 130/122 .400/.715
    10 120/119 124/122 .511/.839
    15 128/128 128/128 .950/.941
    11 125/125 128/133 .142/.164
    7 122/122 121/119 .719/.875

    It might be worth saying that all of this was reset since the last time this car was driven. All of these numbers were taken from later in the 15 minute drive.

    Thanks

  6. #6
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    Default

    Quote Originally Posted by BCdawg57 View Post
    Ok. I will give you some snapshots, I will try and take another log with that software if you would like.

    Cell BLM INT O2
    2 124/129 109/151 .852/.044
    3 118/147 109/147 .839/.209
    6 114/125 118/130 .777/.755
    18 108/129 137/128 .115/.053
    16 108/135 126/130 .213/.106
    17 108/142 130/122 .400/.715
    10 120/119 124/122 .511/.839
    15 128/128 128/128 .950/.941
    11 125/125 128/133 .142/.164
    7 122/122 121/119 .719/.875

    It might be worth saying that all of this was reset since the last time this car was driven. All of these numbers were taken from later in the 15 minute drive.

    Thanks
    split blms at idle dont matter that much. i chased my tail for a long time. split blm's matter more when cruising or when you transition into power enrichment mode and dont have your blm's locked to 128 wide open throttle. cams cause reversion. reversion can excacerbate split blm's at idle.

    are you running an open breather on the engine? or an open catch can?
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  7. #7
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    Default

    Are you referring to the intake? I so here is a picIMG_20200602_141604.jpg
    I have ran open maf and it didn't change anything.

    The tuner made it sound like this should be corrected.
    Last edited by BCdawg57; 06-02-2020 at 02:32 PM.
    1991 Z28
    Engine: '94 355 LT1, GM hot cam, 1.6 RR's, Ported Heads, Dyno Don's headers, #30 SVO injectors, stock internals with skirted pistons.
    Drivetrain: '95 T56, pro 5.0 shifter, hayes clutch, Stock 10 bolt with 3.42's




  8. #8
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    Default

    If it was reset, there's no way to know how much time you spent in each one of the cells. The longer you drive after a reset, and the more you can vary the combinations of RPM and MAP that define the cells, the more meaningful it becomes. Looking at the data, the only PE data would be Cell 15. If that Cell was subtracting fuel in closed loop/learning enabled, it defaults to 128 for both the long term fuel trims (LTFT = BLM) and the short term fuel trims (STFT = INT). The fact the O2 values are so high confirms it is in PE, meeting that mode's rich A/F ratio. Fuel flow seems balanced, since Bank 1 isn’t leaner than Bank 2 (within the limited accuracy of narrow band sensors at anything other than 14.7:1).

    In all other Cells, the INT and O2 values are meaningless, since the are changing about 9 times per second, as the PCM toggles the A/F ratio back and forth from slightly rich to slightly lean, to make the cat(s) work. The value of a log over several minutes is that you see a lot more of that data, at least at higher scan rates.

    Since the log started after a reset, how fast did Cell 16 (idle Cell) Bank 1 drop to 108? That's the real problem in the limited amount of data. Cells 17 (decel) and 18 (open loop cruise/Accel) are mathematically derived from the closed loop cells. So they are most likely simply reflecting the Bank 1 problem from Cell 16.

    If cutting fuel to #1 or #7 (but not other cylinders) results in a BLM significantly higher than 108, it may be an injector flow problem. You could try swapping the injectors bank-to-Bank to see if the problem follows the injectors.

    What throttle body are you using? Aftermarket TB's seem to be able cause “splits” due to idle air problems.
    Last edited by Injuneer; 06-02-2020 at 02:55 PM.

  9. #9
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    Default

    Stock or aftermarket throttle body? Injuneer touched on it above, but essentially all the aftermarket throttle bodies have a cutout on the back that dumps the IAC passage directly into the plenum, instead of routing it through the manifold IAC passages like it should. That has cause plenty of people split BLM issues based off inadequate air distribution.
    Quote Originally Posted by Ryan Stout View Post
    I would try to work my neck muscles but I'm not invited to the LS guy parties.

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  11. #10
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    Default

    The throttle body is a stock TB ported to 52mm. Once it achieved closed loop after startup I let it idle for a few minutes and bank 1 got down to 114. After 10 minutes of driving, bank 1 was 108. I will get a better log with that other software with more data. I sprung a pinhole leak in one of my brake lines so I will have to fix that before I can drive it again. I will try and switch the fuel injectors and see if anything changes. They were brand new SVO injectors when I bought them 12 years ago...and they have about 10k on them. Only cleaning ive done to them is fuel system cleaner.

    Thanks for your help so far.
    1991 Z28
    Engine: '94 355 LT1, GM hot cam, 1.6 RR's, Ported Heads, Dyno Don's headers, #30 SVO injectors, stock internals with skirted pistons.
    Drivetrain: '95 T56, pro 5.0 shifter, hayes clutch, Stock 10 bolt with 3.42's




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