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Thread: Rough idle on cold start up

  1. #1

    Default Rough idle on cold start up

    Recently installed an MSD Atomic EFI along with a ready to run distributor and 6425 ignition control module in my 1977 Pontiac Trans Am. Engine is a stock Pontiac 400.

    The car runs great after start up, accelerating right away and smoothly. However the cold start up has been acting up.

    The car revs up to about 1000 rpm and then bounces between 750 rpm and 1000 for about 20 seconds. I then shift into reverse and the revs drop to 500 rpm, sounds like it's going to stall but manages to catch itself and go back to 750 rpm.

    This behavior has happened 3 days in a row. Does it take a while for the system to learn how to cold idle properly? Or are my settings perhaps off?

    Thanks
    Darryl

  2. #2
    msdtech1955 Guest

    Default

    p/m me a day time phone number

  3. #3

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    Worked with tech support but no luck still fixing the cold start up stall. Idle rpm target is 725 and the Atomic is not controlling timing so I can debug the issue better. I adjusted the throttle blades and IAC count is between 0 and 11 when in gear and at operating temp.

    Pre EFI I had to give this engine 30 seconds to a minute to warm up else it'd stall. Am I expecting too much for EFI to allow me to avoid this wait period?

    I have the MSD ignition control, ready to run distributor and an adjustable rotor but it's not enabled as I mentioned above. Will using its capabilities will that help this issue?

  4. #4

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    I changed the cam type to mild and increased the target idle rpm to 1300. Rpm still drops but it stops closer to 7000 rpm.

    Is it normal to have an idle this high though? Seems like I only need it high for cold start up, but for normal driving I could drop it lower.

  5. #5

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    I decided to put the idle rpm down to 775 and lower the idle AFR to 13 from 14. I set the cam to mild, though it's a completely stock 1977 Pontiac Trans Am L78 400 engine. Hoping for the best tomorrow morning. Also I got the IAC down to 7 and let it idle for 10 minutes.

  6. #6

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    Quote Originally Posted by dstrosbe View Post
    I decided to put the idle rpm down to 775 and lower the idle AFR to 13 from 14. I set the cam to mild, though it's a completely stock 1977 Pontiac Trans Am L78 400 engine. Hoping for the best tomorrow morning. Also I got the IAC down to 7 and let it idle for 10 minutes.
    It stalled out on the first crank today. On the 2nd crank it settled in at the 775 idle well. Seems like we're getting closer. Any other suggestions to avoid the initial stall?

  7. #7
    Join Date
    Jan 2010
    Location
    Brisbane, Australia
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    1,593

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    Quote Originally Posted by dstrosbe View Post
    I have the MSD ignition control, ready to run distributor and an adjustable rotor but it's not enabled as I mentioned above. Will using its capabilities will that help this issue?
    Factory EFI uses IAC and ignition timing to control idle speed.

    You should too, now you have that capability.
    Regards, Gary

  8. #8

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    Quote Originally Posted by Gaz64 View Post
    Factory EFI uses IAC and ignition timing to control idle speed.

    You should too, now you have that capability.
    Fair enough. Does anyone know what I should set all of my settings too when I do enable timing control? Again this is a 1977 Trans Am with an L78 400 engine with over 100k miles.

  9. #9

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    Today I adjusted my idle AFR down to 11.5 from 14 and the car didn't stall when started up cold.
    But warm idle the car runs fine at 14.

    Is there no cold start up adjustment available like the other EFI systems? This seems essential.

    Also having to lower the AFR so low makes the garage fumey as well so it'd be great to have another option.

  10. #10

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    Got the Atomic EFI to control the timing now. I was able to cold start without any issues this morning. My IDLE A/F is back to 14.0 again.

    But now I have a new issue - the ECT was up to 253 degrees when I came home from about a 15 minute drive around the neighborhood. IAT was 151 degrees. It's only about 80 degrees here today in San Jose. I've also never seen the car warm up so fast, which I guess is a good thing.

    Here are all of my settings now - does anyone see anything obvious that could be causing the higher temps?

    Cam type = Mild. This is a stock L78 Cam, perhaps I should set it back to Street/Stock?

    IDLE A/F 14.0
    CRUISE A/F: 14.6
    WOT A/F: 12.8
    IDLE RPM TARGET: 775
    IDLE RPM: 850
    TOTAL RPM: 2200
    Idle advance: 12
    Total advance: 34
    Vacuum advance: 10
    Vacuum type: Unported
    Power valve enrich: 25%
    Pump squirt: 20%

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