RPMs Flutter and Engine Stumbles

Discussion in 'Gen 1 & 2 - Engine, Exhaust, Drive Line & AC syste' started by Black91SHO, Jan 11, 2017.

  1. Black91SHO

    Black91SHO SHO Member

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    I'm not sure if this is the right forum area to post this, but here goes nothing:

    I was driving my '91 SHO with 115,000 on it today when it started to stumble. Kind of like the current to the spark plugs dropped out completely for a second. It's different than a miss, which is rhythmic. This was more sporadic.

    The tach would accurately read 3,000 rpms, then the needle would start dropping and recovering, but it was doing it really rapidly, like lots of tiny little misses, but not misses. When it misses, the tach needle doesn't usually fall. It seems it's more of an electrical issue where the current drops suddenly, causing the needle not to read the voltage. It would jump back to the correct rpm during these quick flutters of the needle.

    This seemed to happen more readily when the engine was under load. The more load it was under, the more likely it was to happen and the more harsh it was when it did happen (more stumbling, sometimes harder stumbling). So more current equaled more stumbling, which again, I'm guessing, means something electrical is breaking down when it gets too much current or gets too many bursts of current too rapidly.

    It kind of acts like a crank sensor going out, which it very well could be. But before I got breaking the bank to have that replaced, is anyone else thinking it could be something else?

    It's had the crank sensor replaced on it a while back, but there was a short in the wiring harness that was affecting the electrical components, which is why I replaced the crank sensor. But the wiring harness was changed later than the crank sensor, which means the crank sensor was probably getting some bad voltage mojo when the wiring hardness would short out (it would cause the rpms to flutter like what's happening now, sort of). That could've worn it out more quickly and it's about to kill over on me now.

    Any thoughts?

    Thanks guys.

    - Brian
     
  2. Black91SHO

    Black91SHO SHO Member

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    Oh, and no check engine light is on, but I'm guessing there are soft codes that'll show up when I pull the codes next time.
     
  3. rubydist

    rubydist Moderator Staff Member

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    let us know the stored codes, because they are important and will lead you (usually) in the right direction.

    from your symptoms, it could be a cam sensor, a dis, a coil, or possibly a crank sensor, in addition to the possibility it is a pcm issue.
     
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  4. Black91SHO

    Black91SHO SHO Member

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    Good point. This has happened to me with bad PCMs before, too. I considered the cam sensor and the coil pack, but I figured it'd be the crank sensor since I've had that problem most often. I hadn't even thought about the DIS for some reason.

    I'll probably swap out the easy parts one at a time, first, and then move on to the more costly and difficult ones.

    Thanks for the input. That's a big help.

    - Brian
     
  5. NoSlo

    NoSlo GoldMember

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    I think everything that can go wrong has been mentioned in the thread:
    [​IMG]
     
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  6. Toolman

    Toolman Boost it! Staff Member

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    DIS is my guess
     
  7. Black91SHO

    Black91SHO SHO Member

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    Thanks for the chart. That's helpful!

    Yeah, DIS is pretty easy to change out first and test. PCM is, too. I'll start with one of those and see how it goes.

    Thanks.
     
  8. NoSlo

    NoSlo GoldMember

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    Something as simple as a bad PCM ground could cause problems too. The immediate zero reading on the tach makes me think it is the cam sensor. The cam sensor is supposed to only sync the spark and drive the tach, but the same symptom of the engine cutting out has been documented on the forum, and it is actually the cheapest part to replace with new. A 25 year old cam seal past 100k lets the hall effect sensor get oily inside.

    A wiring diagram of the same pic above is in this post: http://shoforum.com/index.php?threads/bad-voltage-at-the-coil-pack.124007/#post-1372824
     
    Last edited: Jan 17, 2017
  9. zak

    zak SHO Member

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    I'll see Toolman's DIS and raise crank sensor as a more likely possiblity:burnout:
     
  10. Devin

    Devin SHO owner

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    Your symptoms sound very similar to mine when my crank sensor failed. In the end it was a sensor wire that shorted against the water pump pulley, but your failures sound very similar.
     
  11. rubydist

    rubydist Moderator Staff Member

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    we can keep guessing, or you can pull the codes...
     
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