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Thread: LS LQ4 Twin Turbo. First time no start. Crank sensor?

  1. #1
    Join Date
    May 2020
    Location
    South East USA
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    Unhappy LS LQ4 Twin Turbo. First time no start. Crank sensor?

    Good Evening All,

    I've been working on my friend's new LQ4 build for the past few days. This is our first time with the Holley EFI HP, but I've worked with other EFI systems before on less custom builds. We've been a little more than frustrated with Holley's tech support thus far, so we're turning to all of you. I think we've narrowed it down to the crank sensor after a lot of tech data reading, time on the forum, and several datalog and system log reviews.

    Symptoms:
    Engine cranks. Intermittent pops through the exhaust and intake. No start. Seems like a typical mis-timed engine. All engine sensors seem to be working fine per the datalogs, but the crank sensor reading looks "noisy". You can see it in the screenshot. Definitely not right. The RPM traces look strange as well, maybe as a result of the sensor. MAP, datalog, and system logs attached.

    Engine Configuration:
    378 cid, .060" over, 6.2L LQ4
    Fuel Injectors, Holley 66 lb/hr
    Pistons - Diamond
    Rods - Lunati Voodoo
    Turbos - dual Borg Warner 300SX
    BOVs - internet special
    Wastegates - twin Turbo Smart Comp Gate 40s
    Holley EFI HP
    Holley engine wiring harness
    Holley crank sensor harness extender
    Fuel Pump - dual Walbro 255 knockoffs
    Camshaft - Brian Tooley Stage 2 Turbo
    AC Delco Cam sensor 1/rev
    AC Delco Crank sensor 24 tooth

    More data:
    We verified proper coil wiring. The ignition coils are mounted in the frame rails, so the coils and wires are fairly close to the crank sensor wiring harness (around 6"?). We tried dropping the harness as far away from the ignition wires and coils as we could during troubleshooting, no change to the trace. The Holley crank sensor extension harness is NOT shielded, and does NOT have twisted pairs. I think that's strange. We have not yet verified crank sensor clearance.

    If someone/anyone can take a look at the sensor data and has a course of action, we'd appreciate it. We planned to buy a new crank sensor and possibly shorten and shield the sensor extension cable. If anyone has any vendors for crank sensors that you trust, we'd appreciate that as well. Thanks in advance for the help. Jason

    Click image for larger version. 

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    Last edited by Jason_The_Angry; 05-27-2020 at 08:09 PM.

  2. #2
    Join Date
    May 2020
    Location
    South East USA
    Posts
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    Default

    UPDATE #1: Replaced the crank sensor, and shortened & shielded the sensor extension harness. Engine starts, but dies about three seconds after. Working on adjusting the Base Fuel Table.

    UPDATE #2: Got it started! Had to increase the Base Fuel Table idle settings by about 10% past the original setting. Engine is hunting at idle (800-1000 RPM) and it seems to get worse as engine temp increases. I'll try and smooth the Fuel Graph in the region per some of the other posts.
    Last edited by Jason_The_Angry; 05-29-2020 at 01:19 PM.
    ___________________________________________
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  3. #3
    Join Date
    Nov 2014
    Location
    Ont. Canada
    Posts
    1,583

    Default

    Good to see you got it running. I looked at your datalog file, the RPM was dropping out, but not losing sync. Must have been the crank sensor.

  4. #4
    Join Date
    May 2020
    Location
    South East USA
    Posts
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    Default

    It's possible. We didn't independently verify there was no EMI due to the ignition and the unshielded harness. However, we've learned how variable the quality is on some of these LS sensors on the market. It's not the first one we've had issues with on this build that needed to be replaced. We did find some minor coolant and oil leaks that we need to address before we go at it again, but hearing it come to life is a major step.

    Here's little picture of what we're working on. It's a little outdated since it's before we had to run all the wiring harnesses and add the intake sensors.
    Click image for larger version. 

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    ___________________________________________
    "Seven times down, eight times up."

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