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Help Me Understand This Vacuum Issue

Discussion in 'Early CJ5 and CJ6 Tech' started by garage gnome, Mar 19, 2019.

  1. Mar 19, 2019
    garage gnome

    garage gnome ECJ5 welder

    Western MA
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    I have been messing with the ‘56 cj5 trying to get it running right. It has always had a stumble just off idle and idled rough. I rebuilt an original vacuum and fuel pump for it and put that on. After hooking up the vacuum side, it idles perfect and no more stumble.

    I am not understanding how the addition of the vacuum pump to it helped it out. It is hooked up like the factory would have.
     
  2. Mar 19, 2019
    Rick Whitson

    Rick Whitson Detroit Area 2024 Sponsor 2022 Sponsor

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    Maybe, just a guess, but maybe the vacuum pump is pulling away crankcase gasses from the PCV valve? Just a thought. Neither of my 134 F heads has a vacuum pump on them, so I have no idea how it all works. One of my motors has the PCV valve on the intake, the other has the PCV on the side cover? I thought the vacuum pump on the fuel pump just went to the wipers? Good Luck
     
  3. Mar 19, 2019
    garage gnome

    garage gnome ECJ5 welder

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    The pcv valve is on the valve cover and then there is a tee that goes to the intake and then to the vacuum pump. The wiper goes to the other fitting on the pump.
     
  4. Mar 19, 2019
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    Sounds perhaps like you had an vacuum leak (sucking air to the intake manifold) until the pump was installed.

    A cracked hose, or a loose fitting, etc?
     
  5. Mar 19, 2019
    colojeepguy

    colojeepguy Colorado Springs

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    Why would you have a vacuum pump hooked up to manifold vacuum??
     
  6. Mar 19, 2019
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    The manifold vacuum is supplemented by the pump vacuum to help operate the wipers, when engine vacuum is diminished during acceleration, etc.
     
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  7. Mar 20, 2019
    garage gnome

    garage gnome ECJ5 welder

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    Replaced all the hoses.
     
  8. Mar 20, 2019
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    Before or after the improvement?
     
  9. Mar 20, 2019
    garage gnome

    garage gnome ECJ5 welder

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    There were no hoses before adding the vacuum pump.
     
  10. Mar 20, 2019
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    Crazy. Is the manifold mixture somehow benefitting from pulling PCV air, or air leaking through the pump diaphragm? Or maybe the PCV, if it was jammed before, is now working right?
     
  11. Mar 20, 2019
    garage gnome

    garage gnome ECJ5 welder

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    Not sure. Maybe there is a vacuum leak somewhere and the vacuum pump is compensating for it.

    It’s running decent now and starts right up, so I dont think I’ll mess with it. The wiper doesn’t operate smoothly. It jerks across the windshield. That could be due to a bad wiper motor though.
     
  12. Mar 20, 2019
    PeteL

    PeteL If it wasn't for physics, and law enforcement... 2024 Sponsor 2023 Sponsor 2022 Sponsor

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    Except I believe the pump normally exhausts into the manifold line, which would introduce more air. But only when the wipers are operating and calling for vacuum.

    Got the pump plumbed correctly?
     
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  13. Mar 20, 2019
    RATTYFLATTY

    RATTYFLATTY I think you need a little more throttle

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  14. Mar 20, 2019
    ITLKSEZ

    ITLKSEZ Hope for the best, prepare for the worst

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    Your OP stated that you rebuilt the fuel pump at the same time. Any chance it was a fuel issue rather than vacuum?
     
  15. Mar 20, 2019
    Glenn

    Glenn Kinda grumpy old man Staff Member

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    I'm assuming the carb is a YF?
     
  16. Mar 20, 2019
    garage gnome

    garage gnome ECJ5 welder

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    Yes, first time I hooked it up, I had pressure going to the wiper and not vacuum.

    Yes it is.

    I suppose that could be possible. The pump I took off looked like a good quality carter from Rock Auto. The fuel pump and vacuum pump is a combination unit.
     
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