2020 5.3L 8spd rpm fluctuation at 60mph while towing

grampus · June 1, 2021

This is a recent occurrence for my 1 yr old truck with 8500 miles on it. When towing our 3900# travel trailer, on flat level road going 60-65mph set on cruise, the engine varies between about 1800rpm/15mpg and 2700rpm/5mpg. It will hold at one end or the other for maybe 30-45 seconds then go back the other way. It does this continuously. Net result is a huge drop in fuel consumption and an angry owner. Prior to this trip, I had no issues, even when driving up and down the Appalachians with my travel trailer, or when towing my 5000# boat. When driving around town with nothing in tow, everything is fine.

I took it into the dealer, but since no codes show up, they shrug their shoulders. From what I've been able to ascertain reading online, it seems most likely a torque converter issue? Does this sound right? I've posted two screen shots of a video I shot of the instrument cluster showing the extremes of behavior

Silverado4x4 · June 1, 2021

1 hour ago, grampus said:

This is a recent occurrence for my 1 yr old truck with 8500 miles on it. When towing our 3900# travel trailer, on flat level road going 60-65mph set on cruise, the engine varies between about 1800rpm/15mpg and 2700rpm/5mpg. It will hold at one end or the other for maybe 30-45 seconds then go back the other way. It does this continuously. Net result is a huge drop in fuel consumption and an angry owner. Prior to this trip, I had no issues, even when driving up and down the Appalachians with my travel trailer, or when towing my 5000# boat. When driving around town with nothing in tow, everything is fine.

I took it into the dealer, but since no codes show up, they shrug their shoulders. From what I've been able to ascertain reading online, it seems most likely a torque converter issue? Does this sound right? I've posted two screen shots of a video I shot of the instrument cluster showing the extremes of behavior

Pulling almost 4000lbs your truck is downshifting to 7th gear maybe down to 6th to maintain speed with cruise. I never use cruise when towing as it will over react shifting to maintain speed (within 2-3 mph) with a pulling load. Also are you using tow/haul mode? This will change the shift patterns when towing also lock out the torque converter that's what tow/haul is for. Travel trailers have alot of wind load at 65mph vs a boat.

Edited June 1, 2021 by Silverado4x4

grampus · June 1, 2021

56 minutes ago, Silverado4x4 said:

Pulling almost 4000lbs your truck is downshifting to 7th gear maybe down to 6th to maintain speed with cruise. I never use cruise when towing as it will over react shifting to maintain speed (within 2-3 mph) with a pulling load. Also are you using tow/haul mode? This will change the shift patterns when towing. Travel trailers have alot of wind load at 65mph vs a boat.

Maybe so, but it didn't behave this way before. I pulled my travel trailer to Atlanta and back from Detroit three weeks earlier, lots of flat road and lots of mountainous, with no issues. At 65mph on flat road and no head wind it looks like my second photo. Something changed during our last trip. Sometimes I use tow/haul mode, sometimes not. If it's flat and level with no head wind I typically won't use it. Towing my boat, I always use it.

Edited June 1, 2021 by grampus
tow/haul mode clarification

Vigilante375 · June 1, 2021

I noticed you don't have the truck in tow mode. I suggest using it at all times when towing, they have that feature for a reason.

I towed my ZL1 to and from racing events and the rpms hold steady.

If you have that tow mode feature, I suggest using it when towing. I use it a often when driving without a trailer on the intestate since it'll make the blinker blink 6 times when you slightly press the turn signal.

Edited June 1, 2021 by Vigilante375

It's Tim · June 2, 2021

18 hours ago, grampus said:

I've posted two screen shots of a video I shot of the instrument cluster showing the extremes of behavior

In the end it's probably best to try to concentrate on your driving and not be concerned. More than likely the vehicle is operating as intended. Without doubt, put down the phone before you kill somebody.

grampus · June 2, 2021

1 hour ago, It's Tim said:

In the end it's probably best to try to concentrate on your driving and not be concerned. More than likely the vehicle is operating as intended.

Wrong. It's a new truck that isn't operating as it is supposed to.

Adamace1 · June 3, 2021

10 hours ago, grampus said:

Wrong. It's a new truck that isn't operating as it is supposed to.

Gm is in a class action lawsuit for I belive 2015 to 2019 silverado for transmission issues. Not sure how you have been promised or know how the transmission should operate. Gm says hard shifts, slamming into gear, downshifting so hard you think you got rear ended, tourqe converting locking and unlocking rpm's up and down while going the same speed. Is all normal for their transmissions operating as designed. Gm says this in the class action lawsuit involving millions of vehicles. You need to go after who ever promised you this stuff wouldn't happen. I know GM didn't tell you how their transmission operate with a satisfaction guarantee.

Edited June 3, 2021 by Adamace1

grampus · June 3, 2021

Interesting. I hadn't heard/read that. Makes one ponder.

thank you adamace1

Edited June 3, 2021 by grampus

redwngr · June 3, 2021

Reducing the excessive shifting posted about is one of the main reasons for tow/haul.

Tow/Haul Mode

The Tow/Haul Mode adjusts the transmission shift pattern to reduce shift cycling. This provides
increased performance, vehicle control, and enhanced transmission and engine cooling when driving
down steep hills or mountain grades, when towing, or when hauling heavy loads.

Adamace1 · June 3, 2021

13 hours ago, grampus said:

Interesting. I hadn't heard/read that. Makes one ponder.

thank you adamace1

Yea, I have the hard 2nd to 1st gear hard downshift every 7 days or so then it's fine for the next 6 days. I did know what I was getting into when I bought the truck. As others have suggested try not using cruise and tow haul mode. See what happens then, im thinking alot of the up and down shifting will go away.

grampus · June 3, 2021

I appreciate all the comments about tow/haul mode. I'm well aware of it. I know how to use it and what its effects are. I've been using it for 12 years, with my old 2009 silverado and this 2020. That's not the problem. Nor is using cruise control. If I'm going up and down hills, I never use it. Nor do I when towing my heavier boat.

This behavior came on suddenly during my last trip. Two hours into the trip, all on flat level roadways (other than bridges over rivers etc). A drastic change from how the truck has towed previously. Reread my op.

My intention is to find out what changed suddenly. I have no intention of masking or bandaiding the problem. It the dealer can't fix it and tell me what's going on, my truck will be traded in for something else.

Edited June 3, 2021 by grampus
added info

Vigilante375 · June 3, 2021

1 hour ago, grampus said:

I appreciate all the comments about tow/haul mode. I'm well aware of it. I know how to use it and what its effects are. I've been using it for 12 years, with my old 2009 silverado and this 2020. That's not the problem. Nor is using cruise control. If I'm going up and down hills, I never use it. Nor do I when towing my heavier boat.

This behavior came on suddenly during my last trip. Two hours into the trip, all on flat level roadways (other than bridges over rivers etc). A drastic change from how the truck has towed previously. Reread my op.

My intention is to find out what changed suddenly. I have no intention of masking or bandaiding the problem. It the dealer can't fix it and tell me what's going on, my truck will be traded in for something else.

Yes, we understand what you're saying. It would be better to try and recreate the scenario again but see if it happens with tow/haul mode and without. As I stated, if the vehicle has tow/haul mode then why not use it? To get better MPGs? Then you shouldn't be towing anything. MPG's is the last thing on my mind when I tow.

The dealer isn't going to be much help if you cannot prove to them that the vehicle is doing something strange. If they can't find any strange codes and with GM saying "this is how our transmission's work", you're probably better off going to something else.

You can find lots of topics about the 8 speed issue on here but 99% will be those not towing anything.

I have no issues towing my ZL1 and you didn't have any issues until this 1 time and it could only be a 1 time thing. Only thing that I would really recommend, would be for you to get the fluid flushed and filled back up then try this scenario again. I've heard Amsoil has worked well in the transmission.

Atlas · June 7, 2021

OP....nobody is doubting your ability or experience here. Based on the words and pictures, it appears your truck is shifting between 7th and 8th, so folks are encouraging you to explore that more.. That's obviously different from the torque converter slipping, which would be a defect. If locking the trans in 7th maintains RPM's and also keeps the trans out of 8th, that would indicate something is causing it to shift up and down. Maybe a defect or, could be something like dragging trailer brakes or even a light headwind.

Edited June 7, 2021 by NorthskyblueT1