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Thread: Rapid timing oscillation, will pay for a solution

  1. #1

    Rapid timing oscillation, will pay for a solution

    2008 LS3 Corvette M6, BTR 225/238 cam 50k miles E50

    Fuel trims have been tuned in VVE and combination VVE MAF.


    I have a rapid timing oscillation under light load, level surface driving, in the 800-1,300 RPM range where timing jumps between 35 and 5 degrees many times in one second. I've had this issue with other aftermarket LS cam swaps. In the past I attributed the problem to reversion due to valve overlap but I noticed recently the timing swinging wildly and that swinging is what's causing the rapid jerking of the car. I'd gladly pay for someone's assistance in resolving this issue.


    Included is a troubleshooting tune and log with all of the timing everywhere set at 24 degrees with the exception of idle under speed and final minimum Spark. Even with a timing lock at 24 degrees why is the ECM dropping to 5 degrees?


    I have a second troubleshooting tune and log with final minimum spark set at 24 that does prevent the timing from dipping down to 5 degrees but the oscillation is still there.


    This is a manual transmission and this jerking issue is very annoying in my driveway and in parking lots so please don't criticize the circumstances of the log it's just an example
    Last edited by ericwilloughby; 07-26-2020 at 05:26 AM.

  2. #2
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    Looks like its cycling between coastdown idle overspeed spark and main spark with that low throttle position at cruise. Zero out the coastdown idle overspeed table and see if that doesn't calm it down.

  3. #3
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    I had a similar but not exactly the same problem. I made it better by...

    1) disabling spark smoothing

    2) setting the main spark table in the cells in which the engine idles to the corresponding idle spark value plus the values in the adaptative spark overspeed cells.
    Hib Halverson
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  4. #4
    Quote Originally Posted by smokeshow View Post
    Looks like its cycling between coastdown idle overspeed spark and main spark with that low throttle position at cruise. Zero out the coastdown idle overspeed table and see if that doesn't calm it down.
    You mean like this.? overspeed is zero and idle costdown is 24. EVERYTHING is set to 24. and its going down to 5 somehow.coastdown idle.jpg

  5. #5
    Quote Originally Posted by Hib Halverson View Post
    I had a similar but not exactly the same problem. I made it better by...

    1) disabling spark smoothing

    2) setting the main spark table in the cells in which the engine idles to the corresponding idle spark value plus the values in the adaptative spark overspeed cells.

    EVERYTHING, I think, is set to 24 degrees. I will disable spark smoothing but 24 smoothed to 24 shouldnt be a problem, hahaha

    15958037306484917246547741606238.jpg
    Last edited by ericwilloughby; 07-26-2020 at 05:49 PM.

  6. #6
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    Nevermind. I think I know what this is. Disable catalyst diagnostic.

  7. #7
    Senior Tuner Ben Charles's Avatar
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    Don’t set that table to 24 degrees... hadn’t looked at your file, but if that’s a precursor to what the rest of it looks like it needs some work!
    Sounds like airflow is off.. if airflow becomes right spark doesn’t have to work very hard

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  8. #8

    We have a winner

    Quote Originally Posted by Hib Halverson View Post
    I had a similar but not exactly the same problem. I made it better by...

    1) disabling spark smoothing

    2) setting the main spark table in the cells in which the engine idles to the corresponding idle spark value plus the values in the adaptative spark overspeed cells.
    Disabling spark smoothing dramatically reduced the rather violent jerking I've been enduring for years. It's still there but only a fifth of what it used to be which makes it unnoticeable most of the time. I can't thank u enough. Where can I send you the grand prize?
    Last edited by ericwilloughby; 07-28-2020 at 05:23 AM.

  9. #9
    i know this issue from my LHU, the real reason for it should be troque management, you should log torque source.
    the ecm thinks it's making too much torque in this region and try to pull it down by timing reduction.

    depending on the rest of your tune (maybe messed up trq management or something else) (haven't even looked at it, because last beta isnt starting on my laptop) it may helps make the ecu think it is making less torque there, easiest way to do is modifying EQ ratio based trq coefficients, this has no effect on anything else, besides ecu's actual torque calculation.
    also make sure your gear dependent torque limits are increased, these could cause that also

  10. #10
    Please correct me if I'm wrong here but this is a manual transmission and commanded throttle is only 11-13% when this oscillation takes place. Does torque management do anything on these low throttle positions?

  11. #11
    it does!
    here is a log of my car, check the 3 minute mark there you can see timing oscillation, then check torque mgt advance...its exactly that. road speed is 48km/h there and throttle at 8%
    street_08102018_tobi.hpl

  12. #12
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    The cam is a touch big for MAF use. I'd go strickly SD.

    I'd go back to stock idle timing tables (all of it) Try to shoot for about 18 degrees in the base tables to allow for some reserve torque. That is if timing is already 24 degrees it doesn't have much extra advance ready to save an idle drop. You want 18 normal, dip it can shoot up to 28ish+ to save idle. If you already at 24 you don't have much reserve left.
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  13. #13
    Quote Originally Posted by Alvin View Post
    The cam is a touch big for MAF use. I'd go strickly SD.

    I'd go back to stock idle timing tables (all of it) Try to shoot for about 18 degrees in the base tables to allow for some reserve torque. That is if timing is already 24 degrees it doesn't have much extra advance ready to save an idle drop. You want 18 normal, dip it can shoot up to 28ish+ to save idle. If you already at 24 you don't have much reserve left.
    Thanks. I do use the OEM idle base spark. 13 degrees. That 2nd tune above is the actual daily driving tune, except for 24 degree minimum final spark. the 24 degree everywhere tune was just a troubleshooting tool.
    I have noticed the oscillation seamed better when I was tuning the SD only portion.
    question 1. Can I make a tune that uses SD only below 2500 RPM and switches to MAF/SD above?
    question 2. What about the TQ management suggestion? I dont have any unwanted advance. My problem is 20 degrees of retard at 11% commanded throttle. And I have no idea what to change in the TQ MGT tabs.