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Thread: LS7 Hot Start Issue

  1. #1
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    Default LS7 Hot Start Issue

    I have a cam'ed C6Z that has some transient idle instability immediately after a hot start. Cold starts don't have this idle oscillation.

    The ECM has not been AutoVE'ed as yet - I'm waiting patiently for BB flashing to be finished since I don't have a Windows laptop. I've been borrowing one for occasional flashing. This ECM has been flashed with a generic cam'ed tune.

    I've attached an image that shows the hot startup to and just past the transition from open to closed loop. I've also attached the log this came from.

    One issue with 7.5 logging of fuel status is that it displays closed loop fault status as 'unknown'. BBLogging shows this as 'CL-fault' This car has headers and I've noticed that sometimes closed loop fuel status is logged as CL and sometimes as CL-fault. I'm assuming this is due to the ECM getting confused about the status of the post cat O2 sensors. I have the MIL for both of these DICs turned off. I've logged both pre cat O2 sensors and these are working fine and closed loop operation seems fine so the fact fuel status is reporting 'unknown' doesn't seem to be an issue.

    The car has dual LC1's installed and these are both logged as serial PIDs.

    The attached image shows that RPMS oscillate several times before stabilizing. It shows the commanded AFR oscillating more or less in sync with RPMS. MAP oscillates out of sync with RPM. I'm not clear what is going on here. Any clue's would be appreciated.
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  2. #2
    Lifetime Member swingtan's Avatar
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    1. Post the tune.....
    2. Commanded AFR is changing due to other factors changing ( RPM and MAP )
    3. MAP is changing due to spark and throttle changes
    4. spark is changing due to RPM and MAP changes.
    5. Basically, everything is changing trying to stop the hunting and most likely it's not causing the hunting.
    6. Once the idle stabilises, you are commanding more idle air than just after the start, so I'm assuming you don;t have enough idle air set, which is causing the initial hunt.
    7. You also seem to be commanding quite a low idle spark advance, which may be attributing to the problem.
    8. I'm also going to guess from the idle MAP values, that this is a fairly decent cam. Your commanded idle speed may be a little low at around 750 rpm.


    Simon.

  3. #3
    Lifetime Member Bruce Melton's Avatar
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    Don't know what your spark tables look like but you could be jumping around adjacent cells due to cam and getting big swing in spark whch becomes self perpetuating.

    I had that probem (not as bad as yours) -- I put ~ 24* of spark in all cells <1500 rpm which really helped.

    2000 C5 Coupe, 6M, Callies/Mahle stroked LS7 (441), Blackwing, Halltech, LS3 intake, 90mm Shaner TB, ported L92 heads, FAST 50# inj, not too much cam, Kooks 1 7/8" headers , 3" catless mid pipes, Z TIs, track suspension, , 3:90 rear, EFI V2, LM-2, etc.
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    Thanks for taking a look guys.

    Here's the tune. I've also attached a cold start log and an image of the cold start from OL to just past CL so you can see that the oscillation doesn't occur there.

    Idle is set to 800 across the board.

    Minimum idle air has been increased 5.6% over OEM.

    The cam is the Katech Torquer here's the OEM and Torquer specs:

    Stock LS7:
    Duration - 211/230
    Lift - 591/591
    LSA - 120

    Katech Torquer:
    Duration @ .050 - 220/244
    Lift - .615/.648
    LSA - 110
    ICL - 106

    It is interesting to note that the Torquer has less LSA than stock which slightly increases dynamic compression.

    The car also has long tube headers with cats, corsa catback, KillerBee intake, ported throttle body, ported intake, 160 degree tstat.
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  5. #5
    Lifetime Member swingtan's Avatar
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    There seems to be a few things going on here...

    1. I still think there is insufficient min idle air for P/N.
    2. Idle timing is probably a bit low at 16'
    3. High and low idle spark looks very weird indeed. At some places it actually reduces the correction amount as the idle speeds drifts further out.
    4. Cranking air appears a little low for the cam size.
    5. The isle correction tables look to be a tad low as well.


    I've had a quick play, see if this helps......

    Oh, I should also mention that your STFT's indicate that you are lean at idle. So in OL you are probably running lean on a warm start, as you have no ECT enrichment. A lean idle will not help hunting.
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    Last edited by swingtan; July 12th, 2009 at 08:29 PM.

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    Thanks, I'll give this a try and let you know the result.

  7. #7
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    Swingtan,

    The hot start stability is much improved. I'm guessing this is mostly due to the increase in hot cranking idle air.

    There is still a little hot start idle oscillation but that seems minor.

    There is an issue with revs dropping to almost stall the first couple of lifts after putting load on the engine. For instance, on the lift between reverse and 1st. Frame 2270 in the attached log is an example of this. I believe I hit the gas soon after revs dropped past idle so that confuses the log a bit.

    Your tune reduced Idle High RPM Spark -10 degress for >80, the previous tune was -3 degrees >32. Could this be causing this idle undershoot?

    I also get some minor bucking when lifting at <1500. After a faint buck or two the car then coasts smoothly. Frame 18349 and 18720 are examples of low rpm lifts.

    B1845 "Idle Proportional Step Size" -0.4898 "Different from cell: 0,5 to cell: 0,11 inclusive."
    B1829 "Minimum Idle Airflow" 4.4099 "Different from cell: 1,0 to cell: 8,16 inclusive."
    B1832 "Cranking Idle Air" -4.3290 "Different from cell: 0,0 to cell: 16,16 inclusive."
    B5104 "Base Idle Spark Table" 7.0588 "Different from cell: 0,0 to cell: 8,9 inclusive."
    B5128 "Idle High RPM Spark Correction (In Gear)" -5.5086 "Different from row 1 to row 32 inclusive."
    B5129 "Idle Low RPM Spark Correction (In Gear)" 7.8615 "Different from row 1 to row 32 inclusive."
    B5130 "Idle High RPM Spark Correction (In P/N)" -5.5086 "Different from row 1 to row 32 inclusive."
    B5131 "Idle Low RPM Spark Correction (In P/N)" 7.8615 "Different from row 1 to row 32 inclusive."

  8. #8
    Lifetime Member swingtan's Avatar
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    Hi, sorry I missed the last post and only stumbled across it now....


    The additional cranking air and additional idle air will help with the starting and subsequent idle stability. You may want to add an additional 5% of air to the idle RPM cells in B1829. That may help to settle that last bit

    To help with the RPM drop between gears, the fix above may do the trick. If not, then you may need to massage the integral and proportional corrections factors. The spark tables shouldn't cause the drops as they act a lot quicker than the air flow ones. If you get too aggressive in the spark timing tables, the car will sound like it has a much bigger cam than it actually does.

    The slight bucking is something I've been looking at for a very long time now. I now think it's a combination of minimum air flow, AFR and timing. Actually, I think if you were to add 5% min idle air from 600 RPM to 2000 RPM it may help with everything.

    Simon.

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    Thanks for the further info,

    I increased idle air 6% and increased the 0.20 cylinder air timing in a few of the lower rpm cells to match the 40 degree advance in the cells above. These both seemed to help initial idle stability.

    I scanned initial idle after these adjustments and found that cylinder air was nominally around 0.35 so the timing advance likely had little actual affect.

    I still get a one or two rpm oscillations prior to closed loop idle. The hotter the car is the lower the oscillation goes. I also still get mild bucking at 1500 and below cruise.

    Once BB Flashing is available I'll do an auto VE. If I'm still having these minor issues I'll fiddle with it more. Things are good enough now that it isn't worth doing more until VE is properly calibrated.

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