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Thread: 408 LS with 4L85E Supermatic seems to "push through brakes" at idle when hot

  1. #1
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    408 LS with 4L85E Supermatic seems to "push through brakes" at idle when hot

    I have a 408" LS with a GMPP 4L85E Supermatic transmission (E38 engine controller and T42 trans controller) that seems to want to push through the brakes at idle when the truck is warm. The transmission is stock (other than vasco input shaft, deep alum pan, large ext cooler) and I have a lockup torque converter. When cold, the trans shifts ok. After the truck warms up, it becomes difficult to keep it stopped a light. The engine doesn't seem to idle down properly and the trans seems "locked". It wants to push through the brakes. I have to shift it into neutral to keep t from rolling through the intersection. Once shifted into neutral, the engine seems to idle down properly. As soon as I put into drive, it wants to push through the brakes again.

    Not sure f it is mechanical or something in the engine/transmission tune. I don't have a log file just yet. Hoping for some general advice or maybe that someone will spot an obvious flaw in the tune -see attached.

    408 LS, HiRam, 102 Nick Williams DBW TB, 235/239 and .610/.610 cam, 88mm turbo @ 14 pounds, 4L85E, Pro Torque 4500 lockup torque converter, 9 inch, 4.11 gears with 32 inch tall tires.
    Attached Files Attached Files
    Last edited by scottt28; 10-20-2019 at 06:16 PM. Reason: typo in title

  2. #2
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    If all of your vacuum leaks are fixed and your sure your converter is working correctly, then you can try lowering the brake and modifying some of your idle settings... That's if it's just not returning to idle now? A high stall converter shouldn't be pulling at all much though?
    2010 Vette Stock Bottom LS3 - LS2 APS Twin Turbo Kit, Trick Flow Heads and Custom Cam - 12psi - 714rwhp and 820rwtq / 100hp Nitrous Shot starting at 3000 rpms - 948rwhp and 1044rwtq still on 93
    2011 Vette Cam Only Internal Mod in stock LS3 -- YSI @ 18psi - 811rwhp on 93 / 926rwhp on E60 & 1008rwhp with a 50 shot of nitrous all through a 6L80

    No Longer with passion performance auto for good reasons

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    Quote Originally Posted by GHuggins View Post
    If all of your vacuum leaks are fixed and your sure your converter is working correctly, then you can try lowering the brake and modifying some of your idle settings... That's if it's just not returning to idle now? A high stall converter shouldn't be pulling at all much though?

    I've done some reading on the subject. During that time, I stumbled upon https://www.sonnax.com/parts/1989-lu...egulator-valve . Sonnax says it "Prevents converter or pump drainback, Improves converter release pressure, and Prevents engine surge at idle in'99-later vehicles" which seems to perfectly describe my problem.

    I'm thinking of installing this part plus their line pressure booster valve and spring https://www.sonnax.com/parts/2170-li...re-booster-kit.

    Does anybody have experience with these parts?

    Nice to see you back online GHuggins.
    Last edited by scottt28; 10-21-2019 at 08:08 AM.

  4. #4
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    I've definitely heard of issues keeping the converter clutch disengaged at idle on the 4l80e, and that there is more than one fix, and that sounds like one of them.
    Sonnax is a well known company

    anyways maybe this is similar issue from 4l80e
    https://ls1tech.com/forums/automatic...l#post18371882
    The problem is gone! Idles fine with the Sonnax valve installed. No amount of tuning would solve my problem. It took several days, and input from others, to figure out it was the TCC staying partially engaged, and this valve solved the problem. It may not help everyone, but it made my life a lot more pleasant. Hope this helps others with the same problem. Now, all I need is a converter that is as loose as this one is supposed to be!

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    UPDATE 408 LS with 4L85E Supermatic seems to "push through brakes" at idle when hot

    Quote Originally Posted by kingtal0n View Post
    I've definitely heard of issues keeping the converter clutch disengaged at idle on the 4l80e, and that there is more than one fix, and that sounds like one of them.
    Sonnax is a well known company

    anyways maybe this is similar issue from 4l80e
    https://ls1tech.com/forums/automatic...l#post18371882
    Code P1810 set today. I am currently researching it. Does anybody have any experience with this code in a 4L85E? Is common? Are there certain things to look for? This seemed to happen infrequently with the prior converter but its actually setting the code with the new converter.