Vibration under acceleration over 50mph

KenMathisHD · April 13, 2020

I've got a 2000 Sierra Z71 with the 5.3, 108k miles. Noticed I'm getting some decent vibration hitting 50+, with it most noticeable at about 55. It's significant when on the throttle, coasting reduces vibration significantly. Can feel it mostly through the seat and steering wheel. Thinking it's either U-Joints or motor mounts. I remember there being some slight play in the U-Joints when I checked them for the same issue when I borrowed it about a year ago. However, I also get some noticeable delay in power being applied in my 1-2 and 2-3 shifts - you can feel where it shifts, and then about 3/4-1 second later the power sort of silently thunks into place. The way it feels makes me think there's play in the mounts - shift, pause, and then power kicks in. It doesn't happen with the 3-4 shift, and I can usually prevent it from feeling like it's doing this in the 1-2, 2-3 shifts if I let off the throttle right around when I know it's going to shift for just as long as it takes it to shift - usually no more than a second or so. Truck was a ranch truck before I got it and usually only went short distances, so not sure if the delay is actually motor mounts moving significantly at lower speeds when there's greater torque being applied, or if it's clutches slipping because it's seen so many short 1-2, 2-3 trips. I know 2-3 clutches had some slipping issues when I got it, though it seems to have lessened with more driving - but maybe that's just cause the clutch material in the fluid is being circulated around more often now.

Still need to get the wheels and tires rotated and balanced to be safe, but considering it's most significant when applying throttle I don't think the significant vibration is related to wheels being unbalanced. Is there anything else I'm not considering? Thinking I should pop the jack under it and check wheel bearings too. Bushings don't seem bad, don't see much cracking if any.

sefiroxx · April 14, 2020

Maybe time for some valve work.

Did you drop the pan?, material in the pan?

There are 1-2 and 2-3 known shift issues. 1-2 has a plastic accumulator that is prone to leaks. The 3 valve has a design issue. With time, both of these can weaken the system leading to excess load on clutches etc.

High accelerating shifts might mask the problems. Does it work better under WOT?

KenMathisHD · April 15, 2020

Haven't dropped the pan to check if there was material in there. Wasn't sure if that would be a great idea at 108k miles with the 2-3 slippage, not sure how much material I might lose.

Haven't tried WOT, but after reading a sticky about 4L60E issues where the TPS could be the cause of the delayed or odd shifting, I started messing around trying to see if throttle position changed the delayed shifting I'm getting. More throttle definitely makes it go away for the 2-3 shift, though I am still getting noticeable slippage in that shift. The 1-2 shift seems to do better when giving it more throttle in first gear. Don't think I ever went over maybe 30-40% throttle while messing with it. I still get the delayed shift most of the time, but not as much. And it still can be avoided if you let off the throttle right before it shifts. I'm ordering a new TPS since they're fairly cheap and I have order brakes for it anyways.

Out of curiosity, I put it in tow/haul mode to see how that affected things. The delayed 1-2 shift went away, but my shifts became noticeably harder. I don't recall it slipping going from 2-3, but it slipped when I got out of tow/haul mode - had to get off the throttle to let it upshift.