My 8-speed Auto Transmission is a Stumbling Drunk

InVision · September 21, 2018

Hi Members. We have a 2018 Sierra Denali 5.3L 4x4. When the truck is driven easily (<25mph) it seems to stumble all over. When motor is cold it seems worst. Also if driven aggressively problem is not as apparent. Trouble is we have 4 miles of 15-20 mph zones to get to highway system. Sometimes it is almost embarrassing when riders say WTH, isn't this a $50k + brand new truck. So I read some of the posts on the forum and read about clunks when coming to a stop. Ours doesn't do that. We had a Tahoe LTZ that did that and the fix was to grease the transmission yoke and re-install. I don't see it being the Range Active Fuel Management system as it stays in V8 when it stumbles. Of course dealer says operation is normal.

Any ideas would be greatly appreciated.

M

Nasty · September 21, 2018

4 hours ago, InVision said:

Hi Members. We have a 2018 Sierra Denali 5.3L 4x4. When the truck is driven easily (<25mph) it seems to stumble all over. When motor is cold it seems worst. Also if driven aggressively problem is not as apparent. Trouble is we have 4 miles of 15-20 mph zones to get to highway system. Sometimes it is almost embarrassing when riders say WTH, isn't this a $50k + brand new truck. So I read some of the posts on the forum and read about clunks when coming to a stop. Ours doesn't do that. We had a Tahoe LTZ that did that and the fix was to grease the transmission yoke and re-install. I don't see it being the Range Active Fuel Management system as it stays in V8 when it stumbles. Of course dealer says operation is normal.

Any ideas would be greatly appreciated.

M

For some reason the 5.3 with 8spd are horrible. I have a6.2 with 8spd it ended up being the converter and some adjustments

InVision · September 21, 2018

Oh boy. It is irritating enough it might get sold with 2K miles on it.

M

hizzo3 · September 22, 2018

My 17 5.3 8speed is the same. It's worse under low throttle/coast. Took it to the dealer, they said it's normal. One day I'll see if it can be tuned out.

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InVision · September 23, 2018

Well I limited the truck to v8 mode and it helped a bunch!!

M

Pearl2017 · September 24, 2018

Go to a different dealer. There are a lot of threads about Fluid and filter change and TC replacements fixing the issues. Mine fully resolved with fluid and filter. I have V4 disabled, I find the constant switching in and out of V4 for a few hundred feet at a time more annoying than the 3 drops of gas i may saved leaving it on. WAYYY Smoother with it off.

InVision · September 25, 2018

How exactly is a change of fluid and filter fixing the stumbling?

Thank you

M

scott.moody.9401 · September 26, 2018

My 17 5.3 8speed is the same. It's worse under low throttle/coast. Took it to the dealer, they said it's normal. One day I'll see if it can be tuned out.

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When the idiots at the dealerships say the truck operates as designed, we need to remind them that it did not act this way when we bought it. And that something has changed to cause this degraded operating mode.

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Pearl2017 · September 27, 2018

On 9/25/2018 at 11:24 AM, InVision said:

How exactly is a change of fluid and filter fixing the stumbling?

Thank you

M

There are many posts that describe the problems people are seeing. Having had the problem myself I can say it is correct to call it a chuggle or momentary vibration ...and it can be felt in the steering wheel as well as if a front wheel hit a small bump. Mine started out as random events then ~25k miles started to happen continuously. From everything I have read and my experience the issue is within the torque converter. There is a clutch that is meant to SLIP to maximize engine torque and save fuel. Once the Clutch begins to wear and/or the fluid begins to breakdown or become contaminated the operation of the TCC is no longer smooth. Changing the fluid can resolve the problem by restoring the spec needed to operate the system correctly. It is not always the only fix but just the first thing to try to resolve the problem. Second step is to replace torque convertor as it was defective or gone bad.

I will also add that having looked at the ECU using Hp-tuners after service they did update the operating system so it is not just the trans that is better. And the truck now drives and operates almost perfectly. All this they did without once lowering my expectations or telling me garbage function was normal. Find another dealer that has a good mechanic that keeps up on current GM fixes and they should fix it. Sounds like your existing dealer is lacking that good mechanic.

InVision · September 27, 2018

Not sure why changing the fluid on a brand new truck with 1239 miles on it improves the spec. But I will ask them about it.

M

pronstar · September 27, 2018

Not sure why changing the fluid on a brand new truck with 1239 miles on it improves the spec. But I will ask them about it.

M

Unless the factory fill wasn’t correct [emoji15]

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mjm-1957 · September 27, 2018

The fluid change is only for TCC shudder.TSB 16-NA-175. It will do nothing for the horrible upshifts and downshifts.I am the transmission guy at a Buick GMC dealer and am very familiar with the M5U transmission. Garbage. The ten speed is light years ahead of the eight speed. I can't even begin to explain the amount of time I spend trying to make the eight speed at least driveable.I just did a flush for a shudder concern this morning on a 16 Yukon with 30,000 miles and am finishing up a complete transmission replacement on another 16 Yukon with 46,000 miles after it already had a convertor 18,000 miles ago.GM said don't even bother going inside on this one just replace it.Lets see how long this one lasts.

Edited September 27, 2018 by mjm-1957
spelling

InVision · September 27, 2018

I also heard that the new 10 speed is a much better transmission than 8 speed. Oh well I see a 2019 in my future. I like the new tailgate anyway...

mjm in your opinion should I also look at a 6.2L when sourcing a new GMC Truck?

Wondering if a CVT type transmission may be even better.

M

mjm-1957 · September 27, 2018

The 6.2 with the ten speed is a rocket ship, you will love it.Not a fan of the CVT trans. I like to feel the gear change and see the engine slow down on the gear change.

AgDoctor · September 28, 2018

The 6.2 with the ten speed is a rocket ship, you will love it.Not a fan of the CVT trans. I like to feel the gear change and see the engine slow down on the gear change.

Is there any possibility of swapping a 10 speed into our trucks?

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