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2018 5.7 Tach fluctuating

Discussion in 'Heavy Duty' started by SouthernBubba, May 3, 2021.

  1. SouthernBubba

    SouthernBubba Junior Member

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    Ram Year:
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    Engine:
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    2018 2500
    5.7 Engine
    6 speed auto
    38K on the clock
    No check engine lights or any other warnings indicated

    I noticed the issue a couple of months back on a return trip from Tn not towing or any load in the vehicle . Driving on an incline 70-75 mps pushing on the accelerator and putting load on the engine it had a slight vibration but wasn't consistent . Figured maybe it was something on the road causing the vibration and didnt think much of it .
    Flash forward to today , Past few weeks I have been paying attention to the tach . Ive noticed while accelerating lightly as the tach approaches 2K needle with fluctuate (bouncing maybe 200 rpms up and down) and I can feel that slight vibration again . Oddly enough if I give put more throttle into it , the vibration and the needle fluctuations will disappear and will accelerate normally with no issues . If I take off medium or full throttle acceleration no issues . Truck pulls strong and doesnt hesitate or anything that would indicate any issues . Tires recently balanced and rotated serviced regularly . Any thoughts or ideas ?
     
  2. csuder99

    csuder99 Senior Member

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    Hard to say without seeing/hearing it. I'd venture torque converter clutch slipping/unlocking which can also be caused by TPS issues. Does it behave differently in tow/haul mode ?
     
  3. SouthernBubba

    SouthernBubba Junior Member

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    I thought the same thing but will also do the same behavior in tow haul mode . I cleaned the throttle body recently hoping it would correct this issue . TB was pretty dirty , maybe clean it again?
     
  4. Richard Kim

    Richard Kim Member

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    Had the same thing happen on my 2016 2500 5.7 except it was the speedometer fluttering when I did 70 -75mph. I wonder if this is related.
     
  5. SouthernBubba

    SouthernBubba Junior Member

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    I did a Throttle position sensor calibration (Supposedly) Turn the key to on and hold the pedal to the floor for 15 seconds ,blah blah blah . It seems to have cleared it up some or reduced it . I will know more when I take the truck to work in the morning . As mentioned earlier its more of an annoyance then it is an issue . Hell I might try cleaning the throttle body one more time . Hell Im even wondering if the tach's behavior has been there the entire time and never noticed it . I want to plug into it and check to see if any codes floating around in the system . Probably wont be able to get around to it till this weekend .
     
    Richard Kim likes this.
  6. SouthTexan

    SouthTexan Senior Member

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    Might be a faulty engine crank position sensor. That is a common issue on 5.7L Hemi's in other vehicles.
     
  7. SouthernBubba

    SouthernBubba Junior Member

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    That had crossed my mind as well . I had started googling it's location as well as places to get it from
     
  8. SouthernBubba

    SouthernBubba Junior Member

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    Still doing it . As I said before if I keep steady pressure on the throttle while going up an incline doing 50 or so and the tach is around 2k will wobble / flutter . Same at higher speeds in upper gears. But romping on it no hesitation or anything. Would a bad crank sensor only screw up at a certain load and rpm ? Or do they just flat out fail ? Speedo is unaffected

    Sidenote: I know tachs aren't the most accurate. Just never seen one wave before at certain rpms and speeds is all. I don't remember my 1500 doing it and had the same engine and 6speed auto in it.
     
    Last edited: May 5, 2021

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