SOLVED: Shudder from full stop

[alvocado]

I'm getting a light shudder noticeable through the steering wheel when accelerating from a full stop. There is no issue when accelerating once the truck is moving and no vibration when slowing or coming to a complete stop. There is also no shudder when very gently accelerating from a full stop.

I had the drive shafts out two weeks ago for a separate repair and the ujoints didn't show any signs of binding. Note: they are original and the truck has 226k miles on it.

Trans was rebuilt in Jan with a new TC.

I'm leaning towards a ujoint that didn't like being coaxed out of the yoke with the mileage. Remote chance it could be a transmission mount but that mount looked fine when off recently. Drivers side engine mount was replaced yesterday so that's not a factor.

What do the driveline experts recommend?

 

[Fless]

Based on the forum section posted, this is on a 2021+?

 

[alvocado]

Based on the forum section posted, this is on a 2021+?

Shoot, Fless. My mistake. Should be with the 2011 section. Can the @moderator move the post?

 

[rdezs]

You can't really judge the mounts by just looking. Place a jack under the engine, and then under the transmission and watch the mounts while you jack up slowly. (Not the whole weight of the vehicle, if a mount is bad you'll notice it pretty quick)

Double check that you're u-joints are fully seated in the yokes. But at that mileage, they're pretty much beyond their life expectancy. I'd recommend replacing them regardless.

Check your harmonic balancer..... They've been known to have the rubber deteriorate and the outer part rotates throwing everything off balance, although I wouldn't expect it to only be noticeable just when taking off from a stop.

Rule all that stuff out, what really comes to mind for me is internal in the transmission or more likely the torque converter itself. @NickTransmissions would be the guy to tell you if that's a common symptom with the transmission.....

 

[alvocado]

Thanks for the troubleshooting steps. What additional symptoms would present if the harmonic balancer was the issue?

 

[rdezs]

I would expect the idle to be slightly rough, would probably feel some noticeable shaking or vibration depending on RPM

 

[alvocado]

The idle is rougher than usual. It was particularly noticeable after I changed the drivers side engine mount this week and was surprised to feel so much movement at idle.

It's unusually coincidental that this has developed after doing the rear main seal replacement but at this mileage, I suppose I can expect a string of wear items to present.

I'll dig in further and report back.

 

[Joseph Garcia]

My guess is that something is going on in the TC or transmission.

 

[mikez71]

The idle is rougher than usual. It was particularly noticeable after I changed the drivers side engine mount this week and was surprised to feel so much movement at idle.

I installed some H3 mounts, and there was slight vibration at idle..
Ends up, I didn't really tighten one side engine mount to block bolts.

Odd that you notice rougher idle after the motor mount swap.
Possible the other engine mount got damaged in the process?

 

[solli5pack]

When's the last time you cleaned the throttle body and M.A.F? That's the first place I go when dealing with a rough idle.

 

[Charlie207]

I would expect the idle to be slightly rough, would probably feel some noticeable shaking or vibration depending on RPM

You would feel it at higher RPMs vs. idle.

 

[Charlie207]

The idle is rougher than usual. It was particularly noticeable after I changed the drivers side engine mount this week and was surprised to feel so much movement at idle.

It's unusually coincidental that this has developed after doing the rear main seal replacement but at this mileage, I suppose I can expect a string of wear items to present.

I'll dig in further and report back.

Clean your MAF sensor, clean your MAP sensor, make sure all your spark plugs are tight (#1 loves to loosen for some reason), make sure your plug wire terminals aren't getting corroded.

Could be bad gas, or bad engine mounts... but, you'd feel a clunk more than a shudder.

 

[alvocado]

Here's an update. I took the truck to a shop to evacuate and refill the AC system with UV dye since I have a slight leak. I had the mechanic check for a shudder and he found some intermittent engine misfires reading live data but no codes. My plugs are Delco Iridium with 75k miles which in the past hasn't been an issue. I change them every 100k. He pulled the cylinder 3 plug and noted signs of a potential valve seat leak. Here's a pic from the diagnostic report.

I'll swap all the plugs tomorrow and inspect them since it's an easy first step. Has anyone familiar with valve seat leaks seen similar signs on the plugs?

 

[Charlie207]

Is that gap huge, or just an iridium thing?

I've never seen the justification for iridium plugs, except for longer change intervals. Fresh OEM standard plugs are the best, IMO.

 

[Lil Timmy Shy]

The old torque converter quiver, I wonder?

 

[alvocado]

I dropped in new AC Delco plugs and wires yesterday and cleared the misfire codes. Runs well but the shudder persists. I will say I don't have much confidence in the shop that thought misfires were causing a take off shudder when it feels driveline related.

It's not presenting when I gradually take off, only when there's firm acceleration. No issues at any other speed or when slowing down. I have travel coming up so will have to resume troubleshooting in a few days.

 

[Gearz]

I would also lean toward the Torque convertor. I had a 1985 GMC pickup with a shudder from a dead stop and accelerating and that was the issue. The transmission shop didn't even diagnose right they installed a shift kit which ruined the truck so I put in a GM reman and it fixed the problem. It is a unique feeling when it happens that doesn't feel like U-joints or a miss fire. I will always install a GM reman unit for this reason because all the updates will be fixed and updated.

 

[Dustin Jackson]

@alvocado In troubleshooting it's good to build a timeline and collect the facts. Is it true that this started immediately after the rear main seal replacement?

 

[alvocado]

@alvocado In troubleshooting it's good to build a timeline and collect the facts. Is it true that this started immediately after the rear main seal replacement?

Yes, it started after the rear main seal job. I’m taking it to the shop that rebuilt the transmission to have the shop owner drive it tomorrow. The torque converter wasn’t touched when the transmission was as out for the rear main seal but timing is highly coincidental.

 

[alvocado]

I have narrowed this down to what I believe is the drivers side engine mount. I replaced it last week which eliminated most of the symptoms (primarily knocking and clunking over bumps) but I can reproduce the shudder in park while power braking the truck.

The new mount is an AC Delco unit, fasteners torqued to spec. I double checked and confirmed all the bolts are tight.

Not sure if there's anything installation related that would be a factor since there's an alignment dowel on the bottom to seat into the frame. If not, maybe there's a chance it's a bad unit and I'll have to request a replacement.