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Thread: New EFI user trying to diagnose intermittent low power on LB7

  1. #1
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    Default New EFI user trying to diagnose intermittent low power on LB7

    Last night I logged data using EFI for the first time on my LB7. I'm trying to diagnose an intermittent low power issue on it. My initial thought was that the suction side of my cp3 might be getting weak, so in an attempt to diagnose it, I logged RPM, throttle position, desired fuel rail pressure, actual fuel rail pressure, fuel rail pressure (I'm not sure what the significance of this parameter is compared to the other FRP's), and boost. I was surprised to see that my desired and actual fuel rail pressure fell right on top of each other, so I believe that would rule out a fuel starvation issue.

    One thing I did note though, was that I see a big difference in boost between a run when the truck pulled stronger versus a "weak" run. On the weak runs, I was only seeing a few pounds of boost over atmospheric.

    I'll post the .efi log files tonight from home. In the meantime, does anyone have any thoughts on what would be a likely cause for a low boost situation? Leaking boost line? Bad wastegate actuator?

    Thanks in advance

  2. #2
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    Hopefully these file attachments will work... Here are the log files -- two runs that were very poor accereration, and two that were moderately better.

    Dmax_Run1_sluggish_accel.efi
    Dmax_Run2_sluggish_accel.efi
    Dmax_Run3_better_accel.efi
    Dmax_Run4_better_accel.efi
    2004 LB7 Auto CCSB 4WD

    1971 Chevelle - LS1 / T56

  3. #3
    Member Big D's Avatar
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    Any mods? Stock Tune?
    Need more data to be logged to get the full story.
    log these pids:
    CALC.ACTBOOST
    GM.FRPACT
    SAE.MAF
    GM.BARO2
    GM.BOOST
    SAE.ECT
    SAE.RPM
    SAE.IAT
    SAE.MAP
    GM.FUELQ_MAIN_DMA
    GM.MAINBPW
    GM.MNINJTIM
    GM.DMAX_TRQ_DMA
    GM.BOOST_DMA
    GM.TP
    SAE.VSS

    Log the data using the pids above and get back to us. There are too many variables to make a attempt at a low power situation without more information especially data.
    Making tires smoke since 1994
    Grand Junction CO

  4. #4
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    Only mods are are a 4" downpipe-back MBRP exhaust and a Banks hot-side boost tube. I'm currently running the stock tune. Thanks for the recommendation on which PID's to log. I'll capture more data and post back again.
    2004 LB7 Auto CCSB 4WD

    1971 Chevelle - LS1 / T56

  5. #5
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    I logged a little more data this morning, so hopefully this will help. I wasn't able to capture the PID calc.actboost as it was showing that it was unsupported when I created the PID selection file, but for some reason it is showing as supported now. Anyway, here are the logs. Thanks again.

    Dmax_Run10_weak.efi
    Dmax_Run9_initially_weak.efi
    Dmax_Run6_better.efi
    2004 LB7 Auto CCSB 4WD

    1971 Chevelle - LS1 / T56

  6. #6
    Member Big D's Avatar
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    check for a boost leak, there maybe a ripped boot. make sure the waste gate acuator is linkage is connected. use some pliers and try to actuate the wastegate. you should not be able to move it.
    Your fueling is being limited proably becuase of a boost which limits the MAF therefore the airflow values are not being achieved.
    Making tires smoke since 1994
    Grand Junction CO

  7. #7
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    Quote Originally Posted by Big D View Post
    check for a boost leak, there maybe a ripped boot. make sure the waste gate acuator is linkage is connected. use some pliers and try to actuate the wastegate. you should not be able to move it.
    Your fueling is being limited proably becuase of a boost which limits the MAF therefore the airflow values are not being achieved.
    Big D - thanks for the recommendations. I'll check those items tomorrow and see what I find.
    2004 LB7 Auto CCSB 4WD

    1971 Chevelle - LS1 / T56

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    I just got done looking all of the boost tube connections over. The clamps are tight and the sleeves appear to be properly engaged over the boost tubes and intercooler ports. I noticed a small bit of damage to the outer surface of the sleeve on the lower end of the hot side boost tube, but it appears to just be in the outer layer of the wrap material on the sleeve. I don't know how sensitive these sleeves are in terms of nicks / scratches to know whether it's bad enough to cause a leak.

    I tried to move the wastegate actuator linkage, but it feels solid, and the linkage appears to be hooked up properly. It looks like the rubber line/hose that feeds boost to the wastegate actuator is only about 6"-8" long running from the actuator to the outlet of the compressor side of the turbo. I read some other posts on Duramax Forum that were talking about the wastegate actuator being run by vacuum rather than boost, and that the computer altered the duty cycle of a solenoid to control how much vacuum got fed to the wastegate actuator, allowing the computer to control boost. Does my LB7 use a more basic boost-actuated wastegate without computer control?



    Last edited by kst8engineer; March 19th, 2011 at 06:00 AM. Reason: Added photos
    2004 LB7 Auto CCSB 4WD

    1971 Chevelle - LS1 / T56

  9. #9
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    I had a spare sleeve, so I decided to replace the damaged one and see if that was the problem. After I got the damaged sleeve off, I could see that the damage was considerably deeper than you could see on the outside. I also found another crack all the way through to the inside of the sleeve, so I installed the new one. I took it for a drive after church today and found out that replacing the sleeve didn't cure the problem. I haven't had a chance to log data after replacing the sleeve, but it's on the list of things to do.
    2004 LB7 Auto CCSB 4WD

    1971 Chevelle - LS1 / T56

  10. #10
    Member Big D's Avatar
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    The wastegate is boost controlled. It uses exhaust pressure and boost psi to open the internal gate to limit boost on stock fueling to around 22 psi. Do a trouble code check with the scan tool and tell us what codes there are. If you can download the tune in the ecm and post it up, there maybe something different. There's always a chance of the charge air cooler leaking.
    Making tires smoke since 1994
    Grand Junction CO

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