2016 5.3 Rough Idle?

whatsthestory · October 16, 2021

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???

I am sill having issues. Here is what I have done since the last post.

As I mentioned, I had a lot of hopping/vibration at low RPM with the new trans/converter. To help narrow down, I disconnected the converter from the flex plate (just 3 bolts) and pushed it back. All of the below was done in this state and I have not reconnected it yet.

As for the trans swap, I did the swap. It is pretty straightforward with only the TEHCM plug to the trans and 3 plugs to the transfer case. No ground wires or vacuum hoses. Actually, I can’t locate any engine vacuum hoses on this engine to test. There is the external vac pump that drives the brakes but that would not impact engine runability. I cannot locate anything near the trans that would be involved in the swap that would impact runability like this. This is why I focus on running at 3-4K RPMS (maybe higher) for ½ hour and how that might have caused the runability issue. Note, at the time I asked and based on my wife reading the gauges, I do not believe that either the coolant or the trans fluid overheated.

Without the trans connected, it is still running rather rough but I am only getting the P0300 code. It feels a little different at 500-1000 RPM but that could be the change in resistance.

· I changed all 8 plugs and wires.

o Plugs are AC Delco iridium (OEM) and wires are NAPA Beldin (OEM clone). Old plugs actually looked pretty good. I tested all old wires to between 3,810 and 3,940 OHMS. New wires all tested ~ 4,800 OHMS. I was a little disappointed that the new wires had higher OHMS but this should be within acceptable range.

· Ran compression check

o All 8 cylinders 190-200 psi

· Removed battery overnight

· Replaced battery

· Did the following re-learn

o Start, idle 3 minutes

o Off 3 minutes

o Start, idle 3 minutes

· Ran some testing with Torque on the Android including idling and revving to 4K RPMs

Again, the only code I get is P0300 and I am not sure why the following number of misfires is considered ‘PASS’ as they seem high to me. I also noted that fuel rail pressure was ~500 psi at idle and stayed fairly consistent at high RPM (3K to 4K) with spikes related to the increase in RPMs but pressure dropped to stead state when holding RPMs steady. This was testing to see if running at 3-4K RPMs for ½ hour would create extra stress on the fuel system. It appears not.

MID:$a2 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 1 Data
Max: 65,535Count   Min: 0Count
Test result value: 7Count
PASS
----
MID:$a2 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 1 Data
Max: 65,535Count   Min: 0Count
Test result value: 238Count
PASS
----
MID:$a3 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 2 Data
Max: 65,535Count   Min: 0Count
Test result value: 5Count
PASS
----
MID:$a3 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 2 Data
Max: 65,535Count   Min: 0Count
Test result value: 149Count
PASS
----
MID:$a4 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 3 Data
Max: 65,535Count   Min: 0Count
Test result value: 11Count
PASS
----
MID:$a4 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 3 Data
Max: 65,535Count   Min: 0Count
Test result value: 240Count
PASS
----
MID:$a5 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 4 Data
Max: 65,535Count   Min: 0Count
Test result value: 5Count
PASS
----
MID:$a5 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 4 Data
Max: 65,535Count   Min: 0Count
Test result value: 130Count
PASS
----
MID:$a6 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 5 Data
Max: 65,535Count   Min: 0Count
Test result value: 8Count
PASS
----
MID:$a6 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 5 Data
Max: 65,535Count   Min: 0Count
Test result value: 182Count
PASS
----
MID:$a7 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 6 Data
Max: 65,535Count   Min: 0Count
Test result value: 10Count
PASS
----
MID:$a7 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 6 Data
Max: 65,535Count   Min: 0Count
Test result value: 221Count
PASS
----
MID:$a8 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 7 Data
Max: 65,535Count   Min: 0Count
Test result value: 6Count
PASS
----
MID:$a8 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 7 Data
Max: 65,535Count   Min: 0Count
Test result value: 138Count
PASS
----
MID:$a9 TID:$0b
EWMA Misfire counts (average) for last 10 drive cycles - Misfire Cylinder 8 Data
Max: 65,535Count   Min: 0Count
Test result value: 6Count
PASS
----
MID:$a9 TID:$0c
Misfire counts for last/current driving cycles(calculated) - Misfire Cylinder 8 Data
Max: 65,535Count   Min: 0Count
Test result value: 124Count
PASS
----

So, I reconnected the torque converter. The misfires went way down, the vibration/shudder @ idle to 1K RPM returned. I am now convinced I have warped flex plate (drive plate if you ask GM), off balance torque converter, or a bent input shaft on the trans.

I can't explain the misfires, or the reduction by having the converter attached, but this is reality.

The one small hope I have left for not having to drop the trans again is several references to shimming the torque converter. I am going to measure how far the torque converter can slide and see if it is excessive. I found a reference to this clearing up similar behavior. However, I have found no reference of anyone having to shim a 6L80 torque converter under OEM install specs (not an upgrade/swap). I don't recall if the spacing was too much (supposed to be 1/8 - 3/16 ish) and I didn't read about this until after reconnecting the converter.

Edited October 20, 2021 by whatsthestory
Added Info