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Discussion Starter · #1 ·
Transmission: 8L90

I recently purchased a 2019 Sierra 1500 SLE 5.3L (25,000 miles) and I've noticed this issue a few times now. It seems to happen as soon as I leave my driveway and I'm going somwhere around 5-10mph, I'll maintain the same pressure on the accelerator, however the vehicle will stop accelerating for a couple seconds and just coast, then it will resume accelerating without issue. I haven't noticed this happening at stoplights, only when I first leave my house, but i've only been driving it for a week or so. I'm not hearing any noises or any jerking motions etc... when this issue occurs, for all I know it's normal for this vehicle. This is my first GM truck.
 

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I had a 2018 F150 with the 10 speed auto, which I believe is the same transmission that GM use. When it was new, it would find new neutrals between 5th and 6th, and generally drove like a piece of doodoo. I complained to the dealer, and they reset the computer, no idea what they actually did, but that did solve the problem
 

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Discussion Starter · #3 ·
I had a 2018 F150 with the 10 speed auto, which I believe is the same transmission that GM use. When it was new, it would find new neutrals between 5th and 6th, and generally drove like a piece of doodoo. I complained to the dealer, and they reset the computer, no idea what they actually did, but that did solve the problem
Did your RPMs rev up or anything? Normally in neutral with my foot on the gas, it would rev up the RPMs, but with my particular issue it doesn't do that.
 

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The 8-speed is not the same trans. The 8-speeds have been known to shift funky at times. Is it cold outside with this happens? Does this correspond to when it's shifting gears? There is an updated Mobil 1 fluid for the 8-speeds. If you do not have that, it might help.
 

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Discussion Starter · #5 ·
The 8-speed is not the same trans. The 8-speeds have been known to shift funky at times. Is it cold outside with this happens? Does this correspond to when it's shifting gears? There is an updated Mobil 1 fluid for the 8-speeds. If you do not have that, it might help.
I'm in Florida, 75F+ when it happens. I've only really ever noticed the transmission acting weird when I'm going very slow, I assume it's when it shifts from 1st gear to 2nd. I think it's called a "shifting delay". I've seen the articles about using the new fluid, but it seems to be for fixing harsh gear changes and that's not really what I'm experiencing. It's still got the powertrain manufacturer warranty on it, so if it's anything major they should cover it (I hope).
 

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If it's still under warranty, then head on over to dealer to have it documented and investigated. In addition to the harsh shifts, the 8-speeds are known to be very sloppy shifting, just like Gary said. Fluid change and/or software update might do the trick.

And if it does, then get in touch with Lew and get the truck driving and shifting like it should. (y)

 

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Info that might apply....


Delayed Transmission Engagement
September 30, 2020
Some 2018-2019 ATS, CTS, CT6, Corvette; 2018-2020 Express; 2018-2021 Camaro, Colorado, Silverado, Canyon, Savana, Sierra; and 2021 CT4 models equipped with the 8L45 or 8L90 8-speed automatic transmission (RPO M5T, M5N, M5U, M5X, MQD, MQE) may have a delayed engagement condition when the transmission is shifted from Park to Reverse or Park to Drive after the vehicle has been sitting with the engine off. (Fig. 12) The delayed engagement typically occurs after several hours or, more commonly, overnight.

Fig. 12

The condition may seem to be delayed gear engagement, a slipping transmission, or delayed engagement followed by a harsh engagement.
The transmission is designed to allow three seconds to complete a garage shift (shifting from Park to Reverse or Park to Drive). If the engine speed is increased before the transmission has engaged, the garage shift may be harsh.
After the initial shift, transmission operation will be normal for the subsequent engagements. The condition will not occur again until the vehicle sits again with the engine off for several hours or overnight.

Delayed Engagement Test
Use GDS2 to monitor engine RPM and the transmission input speed sensor (ISS) prior to starting the engine. Start the engine with the transmission in Park and shift the vehicle from Park to Drive or Park to Reverse with the service brake applied. The ISS should drop to zero within three seconds, which is the clutch engagement time. Recording the GDS session log can be useful in diagnosing the condition.
Delayed engagement of Park to Reverse or Park to Drive is present if the time difference between the gear selected (Pt. 1) and the transmission input shaft speed reaching 0 RPMs (Pt. 2) exceeds three seconds. (Fig 13)

Fig. 13

In most cases, the transmission will engage in three seconds or less, which is considered an acceptable engagement time and no repairs should be attempted.
TIP: The vehicle should only be evaluated after sitting for a minimum of eight hours but less than 24 hours and at an ambient temperature of 50°F (10°C) or above.
If engagement time exceeds three seconds, attempt to learn the C3 – Drive and C5 Reverse – Clutch using the following steps:
  1. Allow the transmission sump temperature to rise between 20° and 30°C (68° and 86°F). Do not apply the accelerator pedal.
  2. Perform 20 Park to Reverse shifts or 20 Park to Drive shifts releasing the brake pedal with each shift, allowing the vehicle to roll 5-10 feet (1.5-3.0 m) per engagement into gear.

If the learn procedure does not correct the condition, disassemble the transmission and inspect the 1-3-5-6-7 Clutch (C3) for a delay into Drive condition and inspect the 4-5-6-7-8 Reverse Clutch (C5) for a delay into Reverse condition. The respective clutch plates and seals should be inspected for wear and or damage and repaired accordingly.
If the vehicle has less than 2,500 miles (4,000 km), the transmission or valve body has been replaced or a clutch repair has been performed, follow the clutch learn procedure in Bulletin #16-NA-019 to learn the C5 Reverse and C3 Drive Clutch.
For additional information on a delayed transmission engagement condition, refer to Bulletin #20-NA-187.
 

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2003 Silverado 1500 LS
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..... I leave my driveway and I'm going somwhere around 5-10mph, I'll maintain the same pressure on the accelerator, however the vehicle will stop accelerating for a couple seconds and just coast, then it will resume accelerating without issue..
Buddy, this ^^^^^^^^ would drive me absolutely insane. There's no way I would put up with that. First thing is take it back to dealer where you purchased it.

And PLEASE post back here with the results. Good luck.
 

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2004 Sierra 3500 6.0L, auto, extended cab, cab&chassis, upgraded to 4wd
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You've done the basic thing, going through the process described in the owners manual for checking the trans fluid level (it's not just "run the engine and pull out the dipstick").
 

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Discussion Starter · #11 ·
Buddy, this ^^^^^^^^ would drive me absolutely insane. There's no way I would put up with that. First thing is take it back to dealer where you purchased it.

And PLEASE post back here with the results. Good luck.
I bought it from Carvana, so no taking it back now. I drove it today and it didn't act up at all. It's only happened 2-3 times since I've been driving it and I fear it won't exhibit this issue when I bring it to the dealer.

You've done the basic thing, going through the process described in the owners manual for checking the trans fluid level (it's not just "run the engine and pull out the dipstick").
There's no dipstick on the newer models it seems, manual says only the dealer should attempt to check the transmission fluid levels.
 

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It's technically possible for you to check it yourself, you just need to find the right dipstick plug for it and the process for prepping the transmission to check the fluid level, then crawl under the truck to check it. I would expect the full service manual for the truck would include both the tool part number and the process for prepping the transmission (available as a subscription at a site like alldatadiy.com).
 

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It might still be under warranty depending upon the in-service date when the truck was first sold. If so, take it to the dealer ASAP and get the problem documented, as others have mentioned.
 
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