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Thread: Help Request - L59 Suburban Wont Stay Running

  1. #1
    Tuner in Training
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    Help Request - L59 Suburban Wont Stay Running

    Hello all, I'm a novice with HPTuners/tuning and hoping someone might be able to spot an inconsistency that'll get me running.

    Vehicle:
    - This is a 2003 Suburban L59 (Flex Fuel 5.3) that I bought knowing it barely ran. Hoping to fix & use the powertrain in a buggy (100% off road use).
    - It's got ~195k on the chassis & motor. Transmission was rebuilt ~12k miles ago.
    - It is stock except for an intake and removed cats. Previous owner was concerned about clogged cats so I removed them, welded in test pipes, and disabled the secondary O2 sensors (one cat was about 50% gone).
    - It was throwing a MIL for the fuel composition sensor, but nothing else. I disabled reporting for that but the sensor is still in line.
    - I replaced the fuel filter.
    - I replaced the MAF upon inspection finding a cooked resistor. Reset ECM and no change.
    - *UPDATE 13 AUG* Replaced plugs & wires with OE spec Denso parts. Also did updated intake manifold gaskets per GM TSB. No operational change.

    Current Issue:
    - Recently, I've disabled the Flex Fuel (toggle switch in HPTuners), and changed the AFR to 14.68 across the board as another thread recommended.
    - It starts fine every time, but will slow idle as it warms up, then starts to chug, and will eventually die.
    - Responds roughly to throttle blips. With aggressive throttle stabs it will occasionally throw a 'random misfire' code (blinking MIL).
    - Can move under its own power but runs weak.

    I've got the tune file and 2 log files attached; the first was startup and warmup, the 2nd was as it started chugging and I had to blip the throttle to keep it alive.

    I appreciate anyone's time who has the ability to help. Thanks.
    Attached Files Attached Files
    Last edited by lost honda pro; 08-13-2020 at 04:23 PM.
    '78 C10 - LY6/T56
    '03 Suburban - L59/4L60 soon to be on tons

  2. #2
    Tuning Addict 5FDP's Avatar
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    Get a pressure gauge on the fuel rail, see what your fuel pressure is.

    The o2 sensors are pegged at over 1000mv and the trims are removing fuel all the time, something seems off there.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  3. #3
    Tuner in Training
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    Copy that, I'll get one on it tomorrow and report back results. Thanks for the quick reply.
    '78 C10 - LY6/T56
    '03 Suburban - L59/4L60 soon to be on tons

  4. #4
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    Alright, grabbed a pressure gauge and got some results...

    Sequence & Rail Pressure:
    1. Key On, Engine Off: during prime ~50spig; after prime ~42 psig
    2. Start Engine
    3. Idle: ~42 psig, fluttering 3-4 psig
    4. Throttle blips: pressure rises, up to ~50 psig

    Also: I disconnected the FPR to confirm operation and pressure bumped up to ~50 psig. No fuel found in the nipple (aka FPR appears to operating nominally).

    Link to YouTube video: https://youtu.be/yZEaVR_svM0

    Based on the conversation on this link, I believe the fuel pump is within spec: https://ls1tech.com/forums/conversio...-question.html
    '78 C10 - LY6/T56
    '03 Suburban - L59/4L60 soon to be on tons

  5. #5
    Tuning Addict 5FDP's Avatar
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    I just find it odd that both sensors are reporting the same voltage at 1,107mv. Maybe it is possible that both upstream o2 sensors are bad.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  6. #6
    Tuner in Training
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    When I got the vehicle it had one upstream sensor that appeared to be operating normally, and one that was dead. I pulled a log with all 4 (before disabling the secondary sensors) and moved a secondary to the upstream location that appeared to bounce around like a functioning sensor would.

    That being said, I?m not familiar enough with how these should look to be confident with that decision. I thought they oscillated normally once they were warmed up, but if replacing them both is a prudent choice I?ll go that route. Would another log show more useful info?

  7. #7
    Tuning Addict 5FDP's Avatar
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    The o2 sensors should always be switching lean and rich of 450mv when in closed loop fuel control. The 2nd log shows it in closed loop with them stuck at 1,107mv.

    Another data log to post here isn't really needed if they keep doing that. Something is wrong if they are that way.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  8. #8
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    But since you mention it, I agree that it?s curious both sensors report the same value. I?m going to take a look at the wiring and maybe run a test with each sensor unplugged to see if they show something unusual.

    I wonder if the PO wired them in tandem or something screwy to cheat another problem... will update with logs tomorrow.

    EDIT: posted at the same time. Your comment is noted and I?ll see what I find with them tomorrow. Thanks again.

  9. #9
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    Quote Originally Posted by 5FDP View Post
    I just find it odd that both sensors are reporting the same voltage at 1,107mv. Maybe it is possible that both upstream o2 sensors are bad.
    I ordered 2 new Denso O2 sensors; will have them tomorrow AM and will update then. Thanks again.
    '78 C10 - LY6/T56
    '03 Suburban - L59/4L60 soon to be on tons

  10. #10
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    Alright, got the sensors, installed them, and its a world of difference. Not sure its running right, but it idles and I was able to drive it a few blocks to get some more fuel in the tank. I've attached a log hoping you can confirm that the fuel trims looks closer to normal. Will have to let it warm up and drive it more later in the week to evaluate further.

    I also attached a pic of the 2 upstream O2 sensors I pulled out. No markings/branding at all. The new Denso's were $26/ea. I don't understand the logic behind trying to save a few bucks for such a critical application...
    Attached Images Attached Images
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    '78 C10 - LY6/T56
    '03 Suburban - L59/4L60 soon to be on tons

  11. #11
    Tuning Addict 5FDP's Avatar
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    Yes, that is way better. The sensors are actually moving and the trims are responding as they should.

    Drive it more and see how things settle.
    2016 Silverado CCSB 5.3/6L80e, not as slow but still heavy.

    If you don't post your tune and logs when you have questions you aren't helping yourself.

  12. #12
    Tuner in Training
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    I can't thank you enough 5FDP - saving me the time & energy of towing this thing to a shop is so awesome.

    Will update this thread in a few days once I've gotten some miles on it. Thanks again.
    '78 C10 - LY6/T56
    '03 Suburban - L59/4L60 soon to be on tons