Short version is @99K miles my wife was driving and trans got stcuk in 2nd. She could not pull off and drove 30 minutes at 3-4K RPM. Trans (6L80) issues have been resolved but has a rough idle, worst at start up. Seems to run okay down the road, just idles rough. Reminds me of my old '68 V8 when you pull the vacuume hose. No check engine, no codes. What could it be? Looking for where to start looking.

Long version is that my wife was able to get it stopped and I went to her. Was able to limp it to where we were staying using manual about 1 hour drive less than 40 MPH. It was running rough and died a couple of times. Next day took it a couple miles to GM, they said needed new trans. Drove it back to where we were staying a couple of miles. Towed it home (4 hours). Torque converter had failed and took out trans. Replaced trans with rebuilt unit, also replaced AC cond/trans cooler same time. Towed to GM had them program the new trans. Now, trans works great but idle is rough. Idle has been rough each time moved after initial incident. Pulled battery cable for days during trans swap and expect some re-learn but roughness and time seems excessive. What else could have been effected?

Thanks for any thoughts

Just spitballing here… Is it possible the new/rebuilt torque converter is out of balance? I have been playing with it a bit and sitting still it idles pretty good @500 RPM. I feel a slight bounce but very slight. It is on both accelerating to 1000-1500 RPM and then back down from 1000-1500 RPM to 500RPM that I feel the shutter. Maybe I am not feeling rough idle but instead the torque converter?

I was thinking if the old converter had an internal failure (which is what the trans issues appeared to be) then it could have been off balance. Then, if the new converter is also off balance, it could explain the consistency between before and after the trans swap???

Does it log misfires?

I was going to say I did not see any then I checked again and see a P0300 code and the following. I don't think plugs/coils have ever been changed and likely plugs need replaced minimum but this was a sudden drastic change with the transmission issue/replacement so I am looking for something connected to the change.

MID:$a2 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 1 Data
Max: 65,535Count Min: 0Count
Test result value: 20Count
PASS
----
MID:$a2 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 1 Data
Max: 65,535Count Min: 0Count
Test result value: 2Count
PASS
----
MID:$a3 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 2 Data
Max: 65,535Count Min: 0Count
Test result value: 11Count
PASS
----
MID:$a3 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 2 Data
Max: 65,535Count Min: 0Count
Test result value: 2Count
PASS
----
MID:$a4 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 3 Data
Max: 65,535Count Min: 0Count
Test result value: 21Count
PASS
----
MID:$a4 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 3 Data
Max: 65,535Count Min: 0Count
Test result value: 6Count
PASS
----
MID:$a5 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 4 Data
Max: 65,535Count Min: 0Count
Test result value: 11Count
PASS
----
MID:$a5 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 4 Data
Max: 65,535Count Min: 0Count
Test result value: 3Count
PASS
----
MID:$a6 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 5 Data
Max: 65,535Count Min: 0Count
Test result value: 15Count
PASS
----
MID:$a6 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 5 Data
Max: 65,535Count Min: 0Count
Test result value: 1Count
PASS
----
MID:$a7 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 6 Data
Max: 65,535Count Min: 0Count
Test result value: 13Count
PASS
----
MID:$a7 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 6 Data
Max: 65,535Count Min: 0Count
Test result value: 13Count
PASS
----
MID:$a8 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 7 Data
Max: 65,535Count Min: 0Count
Test result value: 10Count
PASS
----
MID:$a8 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 7 Data
Max: 65,535Count Min: 0Count
Test result value: 4Count
PASS
----
MID:$a9 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 8 Data
Max: 65,535Count Min: 0Count
Test result value: 11Count
PASS
----
MID:$a9 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 8 Data
Max: 65,535Count Min: 0Count
Test result value: 1Count
PASS
----

You need to get the engine running right.

P codes would be handy. Hex codes are an extra few steps for us to look up.

My guess is that driving at higher RPMs while the first transmission had its issues might be related. Putting it under a more extreme extended duty cycle might have knocked some carbon loose, extra fuel flow maybe brought a bit of junk to an injector, a coil got a little warmer than usual with the extended heightened duty cycle, or something else related. My advice: Start with what you know. At 99,000 miles it'* due for spark plugs.

Regarding the torque converters: I've never heard of one out of balance in recent vehicles, like within the last thirty years, let alone two, let alone on the same vehicle. Even if one was out of balance, it would likely be so minor that it wouldn't be felt in such a way as to be mistaken for a misfire. Most of the weight of a torque converter is transmission fluid, so unless there is a lobe of fluid sticking out from the otherwise round converter, it is pretty well balanced by default.

Thank you Captain Obvious... LOL

Seriously, that is what I am trying to do but without throwing plugs, wires, coils, injectors, etc. at it I am trying to understand what is the most likely culprit based on the experience. It was running great, then had trans issues, now runs like crap. Those 2 don't add up so I am trying to understand what running @ 3-4K RPM for 30 minutes could likely foul up to cause this. My current plan is to replace plugs first since it needs it any way and see if that narrows down the issues. I doubt just plugs will solve all but a good first step. Otherwise, was just looking for additional insight. I will let y'all know what the plugs do for me...

THANKS! The P0300 is the only code I get. This is the kind of info I was looking for but still leads to new plugs. My current plan is to replace plugs first since it needs it any way and see if that narrows down the issues. I doubt just plugs will solve all but a good first step. Otherwise, was just looking for additional insight. I will let y'all know what the plugs do for me...

Oops, I meant this reply to this post

Thank you Captain Obvious... LOL

Seriously, that is what I am trying to do but without throwing plugs, wires, coils, injectors, etc. at it I am trying to understand what is the most likely culprit based on the experience. It was running great, then had trans issues, now runs like crap. Those 2 don't add up so I am trying to understand what running @ 3-4K RPM for 30 minutes could likely foul up to cause this. My current plan is to replace plugs first since it needs it any way and see if that narrows down the issues. I doubt just plugs will solve all but a good first step. Otherwise, was just looking for additional insight. I will let y'all know what the plugs do for me...

If the trans swap was done correctly the engine problem is a coincidence. If they missed a ground or a plug is not tight during swap then. Well?