Chevy and GMC Duramax Diesel Forum banner
  • Hey Everyone! Enter your ride HERE to be a part of this months Ride of the Month Challenge!

21 - 40 of 49 Posts

·
Registered
Joined
·
17 Posts
Discussion Starter · #22 ·
How did it perform BEFORE you started doing modifications?….
Refer back to my response to Viper8315:


Long story short, I bought the truck brand new. At about 500 miles I hooked up the trailer for the first time. Noticed this insufficient power on the highway so I initially thought it was still going through it’s “break in period”. I’d never bought a brand new truck before so I thought that was the culprit.
Few months down the road I put 35’s on stock wheels and put a cognito leveling kit on. (Upper arms, torsion keys, fox shocks)……..”

Still performed sub par while towing at highway speeds. Did not improve after what I thought was an adequate amount of time for a “break in period”

Rough mileage on the truck when mods were added:
2,000 - 35” tires on stock wheels
7,000 - derringer, S&b intake, wehrli intake horn
 

·
Registered
Joined
·
1,764 Posts
Stock tires on this were 34.2 (275/65r20) and it now has 35’s. Some increase, but very little change.
When it's at 65mph and refusing to accelerate, what gear is it in? If it's refusing to downshift then it may not be an engine power problem. You could try running manual mode to force a downshift. I know that's not a long term solution but if forcing the "correct" gear gets you the commanded power then at least we've narrowed the problem down a bit.

I have the older generation L5P which came with 32.6" tires the ones I have on currently are 33.6". I am about 27,500 lbs GCW and I can easily maintain 65 MPH up a 6-pct grade, so I agree that your truck should have no problem exceeding 65 MPH under the conditions you described.
 

·
Moderator
2017 GMC Denali 2500HD, 3.5" Rough Country Lift, 305/55R20 Yokohama Geolandar A/T
Joined
·
1,869 Posts
These trucks have a lot of safeguards built in to prevent damage to the driveline and motor, and I'm willing to bet that since the truck has no issues while you're on flat ground, 65mph or so on an incline is where the truck is able to settle at an EGT that it feels is safe. The ECM and the Derringer both will back off the fueling rate if the EGT's are getting too high for comfort.

Since you have the Derringer do you also have the iDash? I'd be interested to see what your EGT's are doing in the situations you're describing.

Also, 80 mph is pretty fast for towing 10k lbs. Make sure the tires on your trailer are rated for that speed, or you might be setting yourself up for a bad time.
 

·
Registered
Joined
·
1,060 Posts
I hope you find an answer to help the truck reach your expectations. We certainly invest a lot in these trucks with high expectations.

I know right where you're coming from. I traded a "well used" LML for a lot newer L5P with just 121k miles so virtually still new:) I was a little disappointed the first time I had the newer truck in the mountains with a load. The LML had a little tune on it but I expected the L5P to be in the same ballpark. It wasn't. The '12 would have pulled circles around the '18 and got better mileage doing it. If your LML was like mine, completely devoid of emissions issues, you might not get the newer truck up to that standard. My '18 had other issues but I believe it ran right when I got it.

Good luck with it though, Hopefully it just turns out to need a simple part or flash or something like that.
 

·
Premium Member
2018 Silverado 3500 LTZ Z71 L5P CCSB
Joined
·
144 Posts
I'm sure this is not your issue, but its a crazy thought. I had a friend who thought he was having a similar situation. His issue ended up being a floor mat that wouldn't let him push the pedal down past about 75%.
 

·
Registered
Joined
·
23 Posts
Almost sounds like you don’t have the right gearing for towing. I would check your vin and see if you can come up with what ratio your rear end is


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
159 Posts
As I was reading the responses, I too was wondering about what gear the tranny was in like JD Warren asked? Is it downshifting? I have the LML, so I'm not familiar with the 10 speed tranny, but when I'm towing our 10K trailer, I select manual mode and toggle up to 5 to lock out 6th. No issues on flat ground, no issues in the hills. It downshifts to maintain the speed I want. I'll even use the passing lanes on occasion. EGTs, motor, and tranny temps have always remained happy.

I too am running 35" equivalent tires on the stock 20" wheels with a leveling kit.
 

·
Registered
Joined
·
23 Posts
I have a 2020 as well. And I don’t have any issues towing my camper. My foot is a little heavy but no issues with towing at all. That’s why I was thinking he might have the incorrect gear ratio in the rear end to tow. I’ve got larger tires as well just don’t know the size right offhand. But that 10 speed transmission is a dream to tow with. Seems like the engine never struggles


Sent from my iPhone using Tapatalk
 

·
Registered
Joined
·
17 Posts
Discussion Starter · #32 ·
Had a friends truck a few years back that had a similar problem, turned out the throttle wasn't opening to FULL throttle adjusted it and made a big difference felt like a different truck.
Great idea, I’ll give that a try.
Had a friends truck a few years back that had a similar problem, turned out the throttle wasn't opening to FULL throttle adjusted it and made a big difference felt like a different truck.
Never heard of this! Sweet. I need to look into this for sure, not sure how to adjust that or check. But I’ll do some digging.
 

·
Registered
Joined
·
17 Posts
Discussion Starter · #33 ·
These trucks have a lot of safeguards built in to prevent damage to the driveline and motor, and I'm willing to bet that since the truck has no issues while you're on flat ground, 65mph or so on an incline is where the truck is able to settle at an EGT that it feels is safe. The ECM and the Derringer both will back off the fueling rate if the EGT's are getting too high for comfort.

Since you have the Derringer do you also have the iDash? I'd be interested to see what your EGT's are doing in the situations you're describing.

Also, 80 mph is pretty fast for towing 10k lbs. Make sure the tires on your trailer are rated for that speed, or you might be setting yourself up for a bad time.
Next time I hook up the trailer I’ll be able to get exact EGT’s off the idash and post when I do 👍🏼
 

·
Registered
Joined
·
17 Posts
Discussion Starter · #34 ·
Almost sounds like you don’t have the right gearing for towing. I would check your vin and see if you can come up with what ratio your rear end is


Sent from my iPhone using Tapatalk
Yeah the gearing was a discussion I had with my mechanic/dealership when I bought the truck. All these 10 speeds came with 1 gear ratio option for the duramax engine. Would have elected for the lower of the two if I had the choice.
 

·
Registered
Joined
·
471 Posts
Not sure about the 10 speed but more than likely your top two gears are basically over drives. I didn't see it mentioned it you were using the manual gear lockout but I would be if your were towing anything above 5k.
 

·
Premium Member
Joined
·
1,871 Posts
I would also like to know if OP is locking out overdrive when towing with manual mode. If not, that may be the lugging issue.
 

·
Registered
Joined
·
1,079 Posts
I would put it back to stone stock, especially if it goes to dealer!! Change the fuel filter and possibly air filter but use all OEM parts , JMHO but Never never never use aftermarket anything except cosmetics. My 19 is different but when it was new it would pull a hill at speed with the cruise but was a dog with me driving it, it got much better when it got to around 15,000 or more. Good luck with it ,it should pull any hill at 65 + but also altitude and air density are a factor but it should never be a weak dog!!
 

·
Registered
Joined
·
36 Posts
I to installed a Derringer and they do require a "break in routine." If you read the manual it recommends multiple up shifts and down shifts in each level, including full throttle and hard breaking. It allows the ECM to retrain and recognize the new module. You might try uninstalling and reinstalling with this break in routine. Not sure if it will help since you had the problem before installing the derringer, but worth a shot. Good luck.
 

·
Registered
Joined
·
17 Posts
Discussion Starter · #39 ·
I would put it back to stone stock, especially if it goes to dealer!! Change the fuel filter and possibly air filter but use all OEM parts , JMHO but Never never never use aftermarket anything except cosmetics. My 19 is different but when it was new it would pull a hill at speed with the cruise but was a dog with me driving it, it got much better when it got to around 15,000 or more. Good luck with it ,it should pull any hill at 65 + but also altitude and air density are a factor but it should never be a weak dog!!
Yeah I wasn’t planning on adding anything to it. This is my 4th duramax so I know what these trucks are capable of, wasn’t impressed few months after I bought it when I hooked the trailer up (hence the mods). I’ll go back to the drawing board
 

·
Registered
Joined
·
727 Posts
Without towing. Disconnect both batteries and let sit over night. Tie them back in in the morning and drive a few days bone stock. After those few days, drive with the Derringer back on for a few days and complete their relearn period. Then, hook it up and see if it feels any different

Software could have changed / not changed and you just need a reset. Tooooooooo many computers and sensors on these things now - so many chances for error
 
21 - 40 of 49 Posts
Top