Lurching or jerking feeling at 55-70mph?

[McFizzl]

This is my first time asking this here. Just figured I might get new opinions. So it starts between 50-55. maybe before but i cant feel it. And all the way up until around 70mph then its gone. I have no idea what this is. If i take it out of OD into 3rd it will start jerking like crazy. Tranny fluid and filter just replaced, new TPS, new intake gaskets, new plugs, wires, cap, rotor, dizzy inspected and tested. When doing tranny fluid change the pan showed nothing out of the ordinary. I also get a random misfire occasionally under heavy to wide open throttle. Havent had that lately but the truck is still doing this. It especially does it using cruise control. Any ideas? Im thinking maybe ignition coil, EGR valve, or my screaming fuel pump. I have a very low idle on startup and then it idles up after a few seconds if that could be related to my issue.

 

[VorTecxas]

TBI or Vortec? Miles?

 

[98chevyz71]

when you say wires do you mean plug wires?

 

[VorTecxas]

I assumed that since it was thrown in with plugs and cap and rotor

 

[McFizzl]

350 Vortec. New plugs, wires, cap, and rotor in january.

 

[VorTecxas]

Ok. How many miles. You know it's typically not a good idea to flush and fill your trans with new fluid if it wasn't changed before right? There's a few things that could cause the random misfire at WOT. Improper spark gap (may have bumped one or two putting it in), plug wires touching each other or anything metal, bad gas, there's quite a few factors. As far as your immediate problem, that sounds like a trans issue. Your low idle, did you mess with the idle screw at all? Mine started doing that when I started experimenting with different throttle plates and idle screw positions

 

[McFizzl]

Truck has 226k on it. motor..60-70k tranny around 110k

 

[twomanymontes]

It sounds like you have a torque convertor slipping. When it's doing this, Try touching the brake enough to just turn the brake lights on. If the jerking stops, it's the torque convertor. If not then you may have an ignition issue.

 

[McFizzl]

Tried the brake thing and didnt notice anything. checking rpm and everything. The flex plate was replaced recently as well so all that was taken out and looked at.

 

[twodrhoe]

Just found your post, having the same issue. 1996 350 vortec, all new delco plugs, wire, cap, rotor, intake (upper and lower) gaskets injectors and new gm trans installed by dealer. Truck has a 6in lift and a bit of driveline wind up on acceleration. same range as you described. Will only happen while keeping steady highway speeds even worse on a slight downgraded road. Did you ever figure yours out??? Thanks!