DEF when towing

[Toolguy5]

What is your usage of DEF when towing. I have a 2020 GMC 3500 sierra 6.6 Duramax diesel.

I have 17000 miles on it now. Most of the miles are regular driving hardly use any DEF.
Had my truck serviced oil and filter change. Fuel filter changed as well as air filter changed. Dealer topped off with DEF. 3 days after service temperature got down around zero with a brisk wind. Received an message bad DEF 100 miles to slow down. Message disappeared on way to dealer when the truck warmed up. Dealer checked for codes and nothing. Next several weeks temperatures were in the teens. We left for Florida going through West Virginia and Virginia. Right before we got to Savannah received another message 1000 miles to reduced speed DEF. I had 2 1/2 gallons with me i filled it up. When we got to Florida I bought another 2 1/2 gallons and it took almost all of that. So I had just under 900 miles and that tank of DEF. On the way home I received the message again 998 miles i put 2 1/2 gallons in the gauge showed full. About 700 miles. Went another 375 miles and received message again 998 miles to slow down. Checked the gauge and still 3/4 full. I have an appointment for dealer to look at.

Towing a 14,000 lb fifth wheel speeds of 60 to 70 MPH.

[Toolguy5]

just picked up truck from Dealer. Could not find anything wrong with my DEF system. GM did issue an article revised February 3rd 2022 on Excessive Diesel Exhaust Fluid. The gauge is only a reference and may need several key cycles to show actual amount in tank. The rate of DEF will vary on Temperature, Altitude, speed and load. So the harder engine works the more DEF.
They Also stated as the NOx reduction requirements increase the more the newer vehicles will use. I guess 2020 was a reduction requirement year.
Article also states that if you receive message of XXX amount of miles that may not be accurate as well. It could decrease or increase depending on conditions and load of engine.

[ksss]

I would agree that the '20 uses more DEF than the older trucks use. Same L5P engine, but they clearly dialed up the DEF.

[mkirsch]

DEF freezes when it's extremely cold. So I would expect some unexpected results when the DEF is frozen which clear up after the DEF thaws out.

[Toolguy5]

Huntindog wrote:
I am now on my third Duramax.
My current one is a 2020. I do not use them for daily driving. They have all been 90%+ towing. I can say that this one uses a lot more DEF than the others.
GM obviously expected that, as this one has a larger tank.
It is what it is.

Having fun costs money. How much fun do you want to have?

That is what I am noticing but i will find out if i have an issue

[Grit_dog]

We’re now almost 1/4 of the way through the 21st century.
Self driving vehicles actually exist, and the big 3 can’t figure out how to incorporate an accurate gauge for a consumable fluid, that is slightly more important than, say, windshield washer fluid!
Maybe glass sight gauges shouldn’t be a thing of the past! Lol

On a different note, I wish the def gauge in our truck was only part of the information screen and not an actual gauge. Because it looks like I have a broken gauge in the cluster since it’s been sitting below E since 2016….

[Huntindog]

I am now on my third Duramax.
My current one is a 2020. I do not use them for daily driving. They have all been 90%+ towing. I can say that this one uses a lot more DEF than the others.
GM obviously expected that, as this one has a larger tank.
It is what it is.

Having fun costs money. How much fun do you want to have?

[Cummins12V98]

FishOnOne wrote:
With recent fuel prices skyrocketing, not sure if DEF consumption is a really big deal.

At least they include lube.

[FishOnOne]

With recent fuel prices skyrocketing, not sure if DEF consumption is a really big deal.

[Toolguy5]

I also carry jugs of DEF when towing so I have it.

But when your gauge is showing almost a full tank and you get a warning low DEF something else is going on.

Hardly use any DEF when not towing. Usually top it off at oil changes.

[Cummins12V98]

Lantley said: "If lack of DEF gauge is the biggest negative of my truck than I'm doing OK!"


I agree! Really can't be that big of a deal to simply add to it based on the type of driving you have been doing. Always have some fresh water to rinse away any spilled DEF. If using a funnel or reusing the fill tube provided be sure to rinse thoroughly.

[Lantley]

I think we are on the same page. My lack of a DEF gauge is not the crisis of the century. There are work arounds "Like be sure you have DEF jug with you in case you run out"...LOL.
If lack of DEF gauge is the biggest negative of my truck than I'm doing OK!

[ksss]

I agree that the effort it took to put in the system that they did, they could have given it a full gauge. Does that require "some work arounds" well maybe. I am not sure if the issue is not a full gauge or the algorithym not being correct regardless of whether you can see the amount that it indicates. I too have seen the issues you mention about the rate of burn not being consistent. I just periodically top it off. I guess that is my work around. I have other equipment that burns DEF so I just empty my partial containers in my pickups and I always have it with me. So for me it is just a habit I have developed. Periodically topping it at every other fill up would probably be similar. DEF itself is also ridiculous, but they don't run without it so...

[Lantley]

ksss wrote:
I am not sure why they didn't put a full gauge in the older trucks, but it really isn't the deal people make it out to be. When it kicks off at 30% you still have plenty of time to get DEF,

Well that's the catch. I now carry a jug of DEF in the RV during camping season. My first experience with the DEF warning came when the display said 500 miles until limp mode.
I thought fine I'm 200 miles from home I'm good.
About 20 miles later display said 150 miles until limp mode.
At that point I could not consider the milage accurate. Nor could I trust it. So I left the interstate to find DEF.
I found it at a little gas station in congested suburbia in northern VA. THe whole expeince was a huge pain in the A@$%*.
So now I keep a jug of DEF on hand.Maybe a good idea anyway,but I don't carry a jug of diesel for just in case! However if you don't have a gauge, just in case can happen very easily.
My dually is also my daily driver. I'm not towing 100% of the time.
MY DEF usage is not consistant. Whatever strategy or algorithym is being used to trigger the warning light is not accurate or foolproof.
With that in mind the truck needs a gauge plain and simple. All the work arounds are ridicoulous.
Even more ridicoulous is that it took them until 2020 to figure out they need a DEF gauge...that's quite a few model years with no gauge.

[Cummins12V98]

I recently filled mine to the top then towed my 35k combined load from NW WA to So cal. I filled to the top at a Truck Stop and calculated at 347mpg. My fuel consumption was around 123 gallons 9mpg.

I also can easily add a 2.5 gallon DEF at 1/2 tank.