TRNW-2: Chevy Active Fuel Management Killing 6L80's?

[Chris and Cathy Kershaw]

I’m of the opinion the engine is just as raggedy as the trans in gm trucks
for about the last 15 years. I advise customers not to buy them if I’m
asked. To me it’s often a 20k truck that needs 15k worth of work. Just like
when 6.0 diesels started running out of warranty there’s an entire industry
popped up just trying to keep a not so great engine on the road.

As far as evaluating the engine on a “no move” there a few things I check
on all the late model gm trucks.  Long term fuel trims should be no
more/less than 10, I think tranibob said 8 can cause issues? Check the
engine oil level and feel how it runs at idle on a cold start. Many have no
codes but run rough and have fuel trims that are way off. These tend to run
worse than average in afm. Also not unusual to find many of these late
model engines several quarts low on oil, fords too.

I use all the available clues to inform the customer of possible upcoming
issues and recommend a shop if asked.

Given the choice many customers figure “it’s not worth anything with a bad
trans” and figure they’ll cross the next bridge when they get there.

 I can’t recall any customer complaints about an afm issue after oh. John
(builder) doesn’t even like to road test them, says none of them work right
lol. Gotta figure it’s been running rough and rumbling a while before it
stopped moving.

On Tue, Jun 17, 2025 at 8:52 PM Tony's Autoworks via trnw2 <trnw2 at trnw.net>
wrote:

> Hello group,
>
> Wanted to get an opinion of what I have seen over the last 3 months. I
> have rebuilt six 6L80/6L90 and have had a few other vehicles come in
> the door with similar problems that I will explain in a moment. Of the
> 6 I rebuilt, all had 4-5-6 clutch stacks completely burnt and 3 of the
> 6 had pumps that were not reusable. Of the 6, 2 of them were towed in
> with no movement. Just finished the 2nd of the two yesterday and
> completed a test drive for the new techem to adapt. But noticed both
> towed in units having some issues but could not determine the cause
> until now. After a certain amount of time test driving I'm noticing
> rpm fluctuations and an intermittent shudder noise on the most recent
> unit. Checking scan tool data the TCC is applying and no TCC RPM slip
> is occurring when the shudder happens. I noticed the same issues on
> the last unit but NO shudder until today the customer told me he
> experienced it and I just test drove to confirm.
>
> I also have a customer with a 2015 Suburban that had a transmission
> installed a few weeks prior from a different city and started
> experiencing the shudder issue. He thought maybe he had a defective
> torque converter. I noticed the shudder in the Suburban sounded
> different from the shudder in the silverados so I thought maybe the
> converter was defective. The suburban customer took the vehicle to the
> dealership and they diagnosed the Active Fuel Management causing the
> shudder when the system is activated and recommended a new engine or
> complete replacement of all AFM components.
>
> I took my recent rebuild on another test drive and with the traction
> control off (a friend from the chevy dealer advised this test) and now
> there are no RPM fluctuations and no shudder. Same with the other unit
> with similar issues.
>
> So now I have 2 units that have been rebuilt that looks like the
> Cylinder displacement may have been the cause of failure? My friend at
> the Chevy dealer says they are seeing a lot of these units come in and
> if they catch them in time they can save the transmissions. And most
> are replacing the engine rather than components.
>
> My question is this: Unless there are engine codes to know that the
> Cylinder displacement is an issue, how do we sell the build?
> Especially if the vehicle can not be test driven. I have 2 customers
> that have spent money on rebuilt units but now are looking at twice as
> much to fix the engine issues.
>
> I'm sure this topic has come up before and I guess I missed the memo.
> Just curious of your opinion and what have you done to diagnose this
> issue before selling a build.
>
> Thanks for reading,
> Tony
> This message was sent from the TRNW Technical Discussion Forum (trnw2).
> You were sent this message because you have subscribed to our mailing list.
> If you don't want to receive these emails from TRNW in the future, please
> unsubscribe here:
> https://www.trnw.net/unsubscribe.php?forum=2
> TRNW Inc., 2825 Walnut Terrace, Modesto, CA 95355
> <https://www.google.com/maps/search/2825+Walnut+Terrace,+Modesto,+CA+95355?entry=gmail&source=g>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.trnw.net/pipermail/trnw2/attachments/20250617/09f36f9d/attachment.html>