Engine will not idle in gear

Discussion in 'Troubleshooting & Diagnosis' started by usmc6122, Sep 13, 2019.

  1. usmc6122

    usmc6122 New Member

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    I’m looking for input on what to do to get my car to idle in gear so I can move on with the build and tune for drivability and reliability. Motor starts and idles fine but dies as soon as it is put in gear. I’ve listed the details of knowns and unknowns about the engine/trans combo. So as not to discourage any input I haven’t listed what we have already tried. Long term goals for the car are for a streetable, pump gas driver, with A/C. Car will probably never see a drag strip or line up for a drag race.
    1. Motor is 496 ci big block, iron truck heads, unknown compression ratio. Cam is believed to be hydraulic flat tappet. Cam utilizes gear drive and has a very aggressive idle sound but is of unknown specs. Motor was built by professional speed shop. Greg Scott racing out of California while I was deployed. I failed to get the specs.
    2. Intake and carb combo is Edelbrock RPM Air Gap, Carb is Quick Fuel mechanical secondary, four corner idle, believed to be 750 cfm but part number is unknown. Carb has been rebuilt, do not know if additional holes have been drilled in butterflies.
    3. Carb has PCV hooked to valve cover and base of carb. PCV is of unknown rating.
    4. Trans is stock truck Turbo 400 with stock converter.
    5. Ignition is HEI with vacuum advance hooked to manifold vacuum and MSD 6AL part number 6425.
    6. Car produces just over 9” of vacuum when idling in park or neutral at about 900 rpm. Idle mixture screws have been set using vacuum gauge.
    7. Initial timing with vacuum advance disconnected is set to 20 degrees at 900 rpm. With vacuum advance connected timing is about 32 degrees.
    8. Car dies almost immediately after being put in gear with idle set at 900 rpm. With rpm set to 1100 rpm, car barks the tires when put in gear and dies in about 2 seconds as rpm drops to below 900. Car behaves this way with vacuum advance connected or disconnected.
    Best Regards
     
  2. usmc6122

    usmc6122 New Member

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    IDLE not IDEL I know..
     
  3. Gary S

    Gary S Administrator Lifetime Gold Member

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    There is a good reason that Chevy didn't offer automatic trannys behind engines with agressive cams. Getting an engine with an agressive cam to idle under the load of an automatic tranny is much more of a challenge than idling easily with a manual tranny.
    So, it takes you back to all the basics. It appears that you have already checked and rechecked the basics and haven't found the combination that works for you.

    My fix would be to dump the TH400 and put in a Muncie. That would solve multiple problems for me because I don't know how to drive an automatic. :)
     
  4. RS1979

    RS1979 Veteran Member Lifetime Gold Member

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    Depending what cam is in it you might need a stall for it
     
    PBZ28 likes this.
  5. 80sz

    80sz Veteran Member

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    Lock out the timing.
     
  6. bourbon_scotch

    bourbon_scotch Veteran Member Lifetime Gold Member

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    Lift the back end up by putting jack stands under each side of the axle (wheels on or off) and try putting it in gear. See if you can get it to keep running, maybe you can find out more that way
     
  7. 71flh

    71flh Veteran Member

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    Vacuum leak?
     
  8. G72Zed

    G72Zed Veteran Member

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    In reading your post/specs, try to toss in an extra 4* initial, use a limiter plate on the vacuum rod to limit it to an additional 8* so your the same "all in"

    Next, assuming the transfer slots are adjusted correctly, and the idle mix screws are all equal at the 4 corners, try turning them out an additional 1/8 of a turn each to see if this helps, if it does, go another 1/8 of a turn out, this assumes it's being done at full temp, and the carb has been rebuild correctly.

    Did you, or have you done a cranking compression test, if it's on the low side, and from your comments on the aggressive sounding engine idle with 9" of vacuum, you may have a "to late of a closing intake" (duration) for the given SCR being used.
     
    2ndGenCrazy likes this.
  9. 2ndGenCrazy

    2ndGenCrazy Veteran Member

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    Remove the carb, flip it upside down and take a picture of the primary throttle blades in order to show how much of the transfer slots are exposed. 9" of vac on auto will be tedious to tune, but it can be done. A tight converter will fight you every step of the way for sure. G72Zed makes good points.
     
  10. 80sz

    80sz Veteran Member

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    Hows the diagnosis going?
     

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