Def Consumption

Philiparellano · August 14

The Vehicle:

2024 sierra 3500 Denali Ultimate (9130 miles on the odometer)

The Problem:

When I tow I use 93 gallons of fuel to one tank of def. Is this Normal. Total gross weight is 34,640 lbs, I wen’t to a truck spot to get weighed on their scale. I average 6.2 mpg(576 miles before I need more def). I have been back and forth with GM and the dealership. I am pursuing a vehicle repurchase.

Please let me know if this is reasonable def consumption.

This only happens when I tow. I can go hundreds of gallons of fuel before needing def when I am driving without a load.

CamGTP · August 14

That is 100% normal.

You are now 4x the weight of the truck damn near towing that combined weight, you are making the engine work way harder and comsume way more fuel. So it must use way more def to keep the system clean.

My work truck is a 6.7 cummins in a big box truck style, I am 14,000lbs dry and will go about 800-1000 miles per 4-5 gallons of def. It's about 1 gallon per 250-300 miles give or take a little.

I have no idea what the dealership has told you so far but they should have told you that DEF consumption will skyrocket while towing heavy loads and their is nothing they can do about it.

Chadw90 · August 15

When I pull my 30’ deck over enclosed on our mountain trip, over the 1650 miles, i burn 165 gal of fuel and use about 7.5 gal of DEF.

How much do these DEF tanks hold?
i put 1 jug in when I get to 1/4 tank. I always run on the bottom half just because Im from Saskatchewan and subject to 6 or more months of freezing temps. Half a tank is easier to thaw out than a full tank.

6.2 mpg out of nearly 35,000lbs running down the highway is quite impressive i must say.
I still dont agree with the towing capacity of a pickup towing an electric brake trailer.
Electric brakes fail “off”, so in the event of an emergency and brakes are not working, that truck with 4 disc brakes must be able to stop that load…its not happening. At least with a semi, failure on the brake circuit locks brakes on. Just my opinion though.

But by the looks of it, my DEF usage is similar to yours? Im not concerned in the least bit. I prefer to use DEF and less frequent hard regens.
i have a Banks I Dash monitor and when towing, i get 700 miles between active regens. When empty, im getting 500 miles between active regens. The truck isnt getting hot enough to keep a clean filter with DEF injection when empty.

Edited August 15 by Chadw90

redwngr · August 28

More fuel burned means more def required to process the NOx

The fuel to DEF ratio should stay at about 2%.

100 gal fuel to 2 gallons of DEF

If it's using a whole tank -- 7 gallons -- for 93 gal fuel,

that would be 7.5% --

From 2024 6.6 diesel supplement

The 3.0 has a smaller DEF tank

Edited August 28 by redwngr

Chadw90 · August 29

On 8/27/2024 at 6:08 PM, redwngr said:

More fuel burned means more def required to process the NOx

The fuel to DEF ratio should stay at about 2%.

100 gal fuel to 2 gallons of DEF

If it's using a whole tank -- 7 gallons -- for 93 gal fuel,

that would be 7.5% --

From 2024 6.6 diesel supplement

The 3.0 has a smaller DEF tank

I would have to disagree to some extent here.
In R&D yes you are correct, but R&D does not replicate real world customer applications. Reality is that there is a large volume of customers that should not own a FT4 diesel.
Short commutes, lots of stop and go, idling, aborted regens due to short commutes etc will contribute to more DEF consumption than light-medium load long distance driving.
New FT4 diesel like to be hot and worked. When I am trying to explain to someone how the adverse effects of city driving has on a diesel, i reference a tuned up 24v pre emissions Cummins. Every green light, every corner, every start up, that thing gives a huff of black smoke. Now add an exhaust filter in there that has a job to trap that soot, every time you accelerate, it is soot loading that filter. In the SCR, more DEF is now required to keep the chemical reaction up to keep the exiting nox levels down.

if you are cruising down the highway with a light to medium load and achieving 14mpg, you are going to burn less DEF and experience fewer passive regens and active regens than you will driving around town or those short commutes and achieving that same 14mpg.

BUT in the R&D world where they fold the mirrors in on road tests for optimal fuel economy, yes you are very correct. More diesel = more def. But hell, park the truck in the garage and you will use zero diesel, zero def. It’s not much of a theory.

I have a customer that is in landscaping and snow removal. The owner runs one skidsteer and son runs another. Identical twin machines. Same parking lots moving snow.
The owner burns less fuel, more DEF and has more frequent parked regens.

Many complaints and service calls and I finally went and pulled load profiles on the units myself thinking we have an issue
Owners unit’s RPM graph is like a roller coaster. He operates with the fixed rpm dial off and utilizes the foot throttle for his RPM and rarely sees 80% RPM or higher. He “baby’s” his unit.
Son runs his fixed throttle WFO all the time. Burns more fuel, less DEF and has almost zero parked regens.
The dad complains that the boy is hard on iron, but he is running that unit in the most efficient way. Hot and hard. Keeps the exhaust system clean as a whistle.

Owner now runs his at 90% ish RPM and has alleviated all of his problems. No more idle to hard acceleration has solves his emissions complaints.

redwngr · August 29

What's an FT4 engine?

What's any of that got to do with the guy towing a and working it hard enough that it gets 10 mpg pulling a camper down the road?

Chadw90 · August 29

Your first line of this comment says enough about your knowledge of the most modern diesel engine.
FT4 = Final Tier 4 Emissions. Aka, DEF
And camper, are you on the right post? Or maybe Im mistaken and the OP camper is full of lead? Combined GVW of 34,640lbs would be like a 26,000lb camper. Keep in mind a run of the mill bumper hitch 35’ camper is 8500lbs.

Anyways. Back to school…

what does it have to do with working a diesel hard?
your claim of more diesel burnt = more DEF.

This is false. DEF consumption isnt a direct correlation to fuel consumption.
A 20mpg truck is not going to burn half as much DEF as a 10mpg truck. It is all relative to load and temp
A truck sitting on site at idle all day long is going to sip fuel and chug DEF.

Theres a measurable line to obtain optimal DEF and fuel consumption. Extreme light load and low exhaust temp will result in extremely high DEF consumption and frequent regens.
Medium load and exhaust temps result in a favourable fuel and DEF consumption with fewer regens
Extremely high load and higher exhaust temps result in moderate DEF consumption and fewer regens.

distance/mpg mean absolutely nothing when measuring DEF consumption.
Exhaust temp and load is all that matters.
Light load is <600F

Medium load >600F but <1050F

High load >1050F

Melting holes in pistons is >1600F

To the OP, your DEF consumption is fine for what you are pulling. You are not using 3x the rated spec of DEF as to what the above post is referencing of 2% and 7.5%. These percentages are dismissible as you are not using the truck in an “as per spec” application. Loaded, you are using EVERYTHING that truck has, which is awesome and very rarely seen. Some would say that is big d!ckin it for sure.
A new vehicle purchase is going to yield you the same results and a pissed off pocket book.
the Ford uses marginally less DEF, but not worth the ick of owning a Ford.

newdude · August 29

OP, sounds normal. Plus DEF dosage requirements have changed since DEF was introduced as well. L5P came out in 2017 which there were changes then, but starting 2020 there were changes made to meet the next round of EPA diesel requirements, and that included new DEF strategy. Even empty they will consume more than say an older truck 2011-2016, and the 1500s are more strict due to their under 8500 GVWR.

Diesel Exhaust Fluid Consumption – TechLink (gm-techlink.com)

Quote

Emissions Requirements

Every year, GM produces diesel engines that continue to meet aggressive federal requirements to reduce Nitrogen Oxides (NOx) in the vehicle exhaust. As DEF is required to reduce the NOx in the exhaust, DEF consumption will increase as NOx reduction requirements increase. Customer trading in an older model year diesel vehicle for a newer model year vehicle will likely see an increase in DEF usage. DEF consumption increases as the newer vehicles meet the more stringent emission requirements for that model year.

The amount of DEF consumed also is a function of how hard the engine is working, or engine load. Due to engine use, it is more representative to compare DEF consumption to the amount of fuel consumed – also a function of engine load – instead of miles traveled. In addition to engine load, other factors that affect the DEF usage rate are the humidity, temperature, and altitude where the vehicle is operating.

Under certain conditions, the ECM will increase or decrease the amount of DEF used based on learning or adaptive algorithms. In the event of a malfunction and Check Engine light, the ECM may double or even eliminate the amount of DEF that is being used. An improper amount of DEF use will continue until the vehicle is repaired and the learned value in the ECM is reset.