Diagnosing RPM Fluctuations & Shudder Under Light Load (6L80)

[sean882]

It started the day before Christmas Eve - noticeable but minor rpm fluctuations (a few hundred rpms) while driving on the highway after being warmed up.  About 5 minutes later, I felt the truck stuttering under light loading moving ~60mph - engine rpms below 1500.  I checked the fluid and it was perhaps a tad low (very bottom of 'hot' zone) but was definitely not red/pink.  I'm not sure if the previous owner had changed the fluid or not, but it didn't seem it.  Over the next day, the shuddering got worse and entirely repeatable.  Once the transmission fluid was warm, it would reliably shudder under light loading at slow highway speeds.  Occasionally I could get it to do it on a hill at slower speeds too.  It never happens under aggressive acceleration/heavy engine loading - this seems to work fine with no issues.  I scheduled an appointment at the dealer for next Tuesday, the soonest they had available.  In the mean time, I'm hoping to make sure I'm going down the right path and understand my best options.

 

I found PIP5504 (attached) and related Torque Converter Diagnosis SI (attached) and tried to work through them as best as possible without the specialty GM tools.  

Diagnostics

• Occurs most obviously when transmission fluid is warm-hot
• Occurs in M5 mode as well as Drive
  • Occurs both when in V8 and V4 (with and without AFM)
• Occurs with and without Tow/Haul Mode enabled
• Occurs with and without grade braking disabled
• Occurs under light load conditions
• Does not occur under heavy load conditions
• Using torque app, a couple misfires (less than 20 after an hour of driving, distributed across all cylinders) are noted, but the instances do not remotely line up with when problems are seen. 
• Using torque app, spikes of TC slip is observed.
• Using torque app, RPMs go UP when TC slip is observed - these so probably not a misfire

 

Working through the PIP, the symptoms seem to indicate either 1 (fishbite) or 2 (Torque Converter Shudder).  I'm not entirely sure what fishbite is, but the service procedures are more or less similar.

 

I don't have a GDS2 tool to command TC engagement, unfortunately, so hopefully the dealer is willing to do this properly.

 

The original PIP5504 (not the C revision) had service procedures included that all seemed to point back to a fluid flush or replacing the torque converter.  Knowing that my fluid was pretty black (didn't smell burnt though), I went ahead and got a new transmission filter and performed a fluid flush.  20 Quarts of Valvoline MaxLife fluid were flushed through (not here to debate fluid brands/types - it works with Dex VI and it's what was in stock at the store open on Christmas Eve).

 

Observations

• Fluid was black
• Transmission pan magnet had fine metal debris (felt smooth between fingers) on it
  • Photo included - what is a lot?  Truck has 178,000 miles on it, this is probably the first time it was opened.
• One small piece of flexible (rubber?) debris in pan
• Flow through and out of cooler seemed OK from the start
• No signs of leaks

 

Changes after Flush

• Shuddering is much reduced - but still faintly there.  Doesn't feel like truck is going to come apart though.
• RPM fluctuations are still there - and perhaps more noticeable than before (probably because the shudder isn't overshadowing them).
• Using torque app, Torque Converter Slip is still present when RPMs fluctuate.  It appears there is no change to the frequency or amplitude of slip.

 

Questions

1. For those familiar, does the amount of metal on the magnet look typical, or does this look like a sign of an internal failure?
2. Any other ideas for what the issue could be?  
3. Any other diagnostic tests that would be helpful to zero in on the root cause?
4. The PIP points to replacing the torque converter as the next step.  Is there any way that with all this identified, the transmission itself could be the culprit?  Living in MA in the winter without a garage, I'm almost certainly going to have to have the shop perform any replacements and don't want to pay for labor twice.
   1. If so, thoughts on a reman unit vs. having a shop repair it?
5. I'm well outside of any warranty and see a lot of people going back and forth with the dealers while they throw parts at a problem. Any tips for reducing bad service and getting it right the first time with them?
6. Am I doing more damage if I keep driving on it for the next week?
7. I'm open to thoughts/suggestions at identifying the cause and next steps.

 

Vehicle Information

• 2016 Silverado 1500 LT Z71 4x4
• 6-speed auto (6L80, MYC)
• 5.3L V8 
• 178,000 miles
• NOT tuned in any way
• Daily Driver - mostly highway miles
• Used for occasional light towing (~12,000 miles spent towing an 850 pound teardrop trailer)
• Generally driven gently (boring, I know)

 

Thanks,

-Sean

 

MC-10118277-9999.pdf PIP5504C.pdf Torque Converter Diagnosis SI.pdf

[JimCost2014]

49 minutes ago, sean882 said:

It started the day before Christmas Eve - noticeable but minor rpm fluctuations (a few hundred rpms) while driving on the highway after being warmed up.  About 5 minutes later, I felt the truck stuttering under light loading moving ~60mph - engine rpms below 1500.  I checked the fluid and it was perhaps a tad low (very bottom of 'hot' zone) but was definitely not red/pink.  I'm not sure if the previous owner had changed the fluid or not, but it didn't seem it.  Over the next day, the shuddering got worse and entirely repeatable.  Once the transmission fluid was warm, it would reliably shudder under light loading at slow highway speeds.  Occasionally I could get it to do it on a hill at slower speeds too.  It never happens under aggressive acceleration/heavy engine loading - this seems to work fine with no issues.  I scheduled an appointment at the dealer for next Tuesday, the soonest they had available.  In the mean time, I'm hoping to make sure I'm going down the right path and understand my best options.

 

I found PIP5504 (attached) and related Torque Converter Diagnosis SI (attached) and tried to work through them as best as possible without the specialty GM tools.  

Diagnostics

• Occurs most obviously when transmission fluid is warm-hot
• Occurs in M5 mode as well as Drive
  • Occurs both when in V8 and V4 (with and without AFM)
• Occurs with and without Tow/Haul Mode enabled
• Occurs with and without grade braking disabled
• Occurs under light load conditions
• Does not occur under heavy load conditions
• Using torque app, a couple misfires (less than 20 after an hour of driving, distributed across all cylinders) are noted, but the instances do not remotely line up with when problems are seen. 
• Using torque app, spikes of TC slip is observed.
• Using torque app, RPMs go UP when TC slip is observed - these so probably not a misfire

 

Working through the PIP, the symptoms seem to indicate either 1 (fishbite) or 2 (Torque Converter Shudder).  I'm not entirely sure what fishbite is, but the service procedures are more or less similar.

 

I don't have a GDS2 tool to command TC engagement, unfortunately, so hopefully the dealer is willing to do this properly.

 

The original PIP5504 (not the C revision) had service procedures included that all seemed to point back to a fluid flush or replacing the torque converter.  Knowing that my fluid was pretty black (didn't smell burnt though), I went ahead and got a new transmission filter and performed a fluid flush.  20 Quarts of Valvoline MaxLife fluid were flushed through (not here to debate fluid brands/types - it works with Dex VI and it's what was in stock at the store open on Christmas Eve).

 

Observations

• Fluid was black
• Transmission pan magnet had fine metal debris (felt smooth between fingers) on it
  • Photo included - what is a lot?  Truck has 178,000 miles on it, this is probably the first time it was opened.
• One small piece of flexible (rubber?) debris in pan
• Flow through and out of cooler seemed OK from the start
• No signs of leaks

 

Changes after Flush

• Shuddering is much reduced - but still faintly there.  Doesn't feel like truck is going to come apart though.
• RPM fluctuations are still there - and perhaps more noticeable than before (probably because the shudder isn't overshadowing them).
• Using torque app, Torque Converter Slip is still present when RPMs fluctuate.  It appears there is no change to the frequency or amplitude of slip.

 

Questions

1. For those familiar, does the amount of metal on the magnet look typical, or does this look like a sign of an internal failure?
2. Any other ideas for what the issue could be?  
3. Any other diagnostic tests that would be helpful to zero in on the root cause?
4. The PIP points to replacing the torque converter as the next step.  Is there any way that with all this identified, the transmission itself could be the culprit?  Living in MA in the winter without a garage, I'm almost certainly going to have to have the shop perform any replacements and don't want to pay for labor twice.
   1. If so, thoughts on a reman unit vs. having a shop repair it?
5. I'm well outside of any warranty and see a lot of people going back and forth with the dealers while they throw parts at a problem. Any tips for reducing bad service and getting it right the first time with them?
6. Am I doing more damage if I keep driving on it for the next week?
7. I'm open to thoughts/suggestions at identifying the cause and next steps.

 

Vehicle Information

• 2016 Silverado 1500 LT Z71 4x4
• 6-speed auto (6L80, MYC)
• 5.3L V8 
• 178,000 miles
• NOT tuned in any way
• Daily Driver - mostly highway miles
• Used for occasional light towing (~12,000 miles spent towing an 850 pound teardrop trailer)
• Generally driven gently (boring, I know)

 

Thanks,

-Sean

 

MC-10118277-9999.pdf 166.94 kB · 0 downloads PIP5504C.pdf 157.72 kB · 0 downloads Torque Converter Diagnosis SI.pdf 229.39 kB · 0 downloads

My guess is with the shudder it is your torque converter.

[sean882]

25 minutes ago, JimCost2014 said:

My guess is with the shudder it is your torque converter.

Thanks - do I risk further damage to the torque converter/transmission by continuing to drive it for a week or so?  Or should I be looking for a rental?

[Truckboattruck]

4 minutes ago, sean882 said:

Thanks - do I risk further damage to the torque converter/transmission by continuing to drive it for a week or so?  Or should I be looking for a rental?

Yup.

[JimCost2014]

2 minutes ago, sean882 said:

Thanks - do I risk further damage to the torque converter/transmission by continuing to drive it for a week or so?  Or should I be looking for a rental?

Tough call, with the mileage it is probaly gone, especially with your description of what it is doing.

 

I would be more afraid of being stranded, then having to tow it to the dealership if it fails completely.