Engine shaking/jerkyness while at steady speed

Discussion in 'GMC Truck Forum' started by sixteenacrewood, Mar 12, 2019.

  1. sixteenacrewood

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    I have an older work truck and I am hoping someone can offer advise other than replacing the truck
    Here are her stats;
    93 GMC C1500 5.7 L 415,000 miles original motor. I plan to replace the motor but I am hoping to wait until January when business slows down.
    Starts up just fine, good idle, decent power for the miles she's gone, runs at 75 mph on interstate no problem, can still pull a loaded trailer.

    I'm not sure how to describe what my truck is doing, but here it goes.

    while at or above 40 mph, while holding a constant speed, the engine jerks/ misfires/ vibrates? the dash will shake.

    It goes away completely when I accelerate or let off the gas, it only does this when trying to hold a constant speed at 40-75 mph. On another forum it was described as "trailer hitching" but no solution was posted.
    Advise was egr valve causing a vacuum leak at higher speeds, this did not make sense to me.

    Another was a worn distributer gear or failing pickup coil in distributer. This seemed to make better sense and I am thinking about putting in a new distributer.

    I have tested, with digital miltimeter, the TPS, MAP sensor, etc. she has a new fuel pump assy. injectors, fuel filter, fuel regulator diaphragm, spark control module, cap rotor, plugs.

    Any advise would be appreciated

    Thanks
    Steve
     
  2. dirtrider73068

    dirtrider73068 Senior Moderator
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    Are you sure its its coming from the engine, and not a tire out of balance or driveshaft, or even the carrier bearing could be bad, or a u-joint. Can check for engine misfires in park, start it up I would let it warm up to temp, apply brake put in gear hold brake down hard would also apply parking brake and accelerate, this puts it under load will feel a misfire. Just don't go too long or hard it also puts stress on the tranny clutches and will get them hot. It won't much or too long to feel the misfire if its engine related.
     
  3. leolkfrm

    leolkfrm Full Member

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    vac leak or injector flooding???
     
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  4. sixteenacrewood

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    I had another mechanic on another forum suggested a transmission problem with the TCC solenoid.

    Now I feel a bit dumb, haven't checked trans fluid in a month or so!

    Two quarts low and a slow leak, I topped it up and checked it after going about 15 miles,
    it's black as coffee!

    No slipping or hard shifting, but the mid throttle shacking has gotten slightly worse.

    With the fluid being black is it worth having the leak fixed and fixing the TCC solenoid

    $1800 for the trans rebuild
    $2350 for factory remanufactured.

    Haven't looked in to fixing the leak yet.
     
  5. sixteenacrewood

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    I did a smoke test, no vac leaks

    fuel pump, filter, injectors, regulator diaphram, spark control module, plugs, wires, rotor and cap, all new or near new.
     
  6. dirtrider73068

    dirtrider73068 Senior Moderator
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    If its that bad on the fluid color it could be the clutches has been overheated it will burn the fluid to a black color. Also on auto tranny's if you don't ever change the fluid don't change it, I have heard when you do you flush out small clutch disc particles and when you do the tranny slips and will fail. So if its never been changed keep the old fluid in it, just top it off.

    If it has alot of miles, never been touched as in rebuilt and the fluid is that dark, I would suggest look into rebuilding it if you want to keep, also DON'T get a used tranny to put in it unless you can test it to test the shifting of the tranny its a high risk when buying used tranny's you could end up with a bad one or somebody else's problem.
     
  7. sixteenacrewood

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    I puller out some fluid and put in the Lucas Trans fluid supposed to stop leaks and slipping. I ran it around 40 miles yesterday and again today, it looks like the leak has stopped.
    The fluid looks dark but does not smell burned.
    The is a mild shuddering/vibration above 45 mph
    I'll see how this goes, tomorrow I have a 200 mile round trip!

    Thanks again!
     
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