MPG cut in half

[Preston]

I’m suffering from some power loss and terrible fuel mileage.
My 2015, 2500 series P/U, 150k, lml with Kennedy lift pump, Kennedy tune, mega filter and air intake.
I just returned from a short trip 300 miles. I noticed some power loss and my fuel mileage is down to 6-7 mpg.
Last trip was around 9 mpg. I’m use to the mileage being around 12-14.
After last trip I thought it might dirty fuel filter so changed fuel filter (Baldwin) and oil (T6) just to see declining results. The only thing I’ve changed is I went back to stock tire size from a basic step up.
I can’t imagine down sizeing tires would cut mpg in half.
I don’t know where to start to diagnose the problem.
Have you got any suggestions or can you point me in a direction.

 

[juddski88]

Do you have someone nearby who is an experienced mechanic who works on a lot of duramax? The suggestions of things to check are all going to require diagnostic equipment.

 

[Dean E]

I’m suffering from some power loss and terrible fuel mileage.
My 2015, 2500 series P/U, 150k, lml with Kennedy lift pump, Kennedy tune, mega filter and air intake.
I just returned from a short trip 300 miles. I noticed some power loss and my fuel mileage is down to 6-7 mpg.
Last trip was around 9 mpg. I’m use to the mileage being around 12-14.
After last trip I thought it might dirty fuel filter so changed fuel filter (Baldwin) and oil (T6) just to see declining results. The only thing I’ve changed is I went back to stock tire size from a basic step up.
I can’t imagine down sizeing tires would cut mpg in half.
I don’t know where to start to diagnose the problem.
Have you got any suggestions or can you point me in a direction.

Sounds like you might have an air leak in your boost side. Might want to take a look at your boots coming off of the turbo compressor, your intercooler and around your air valve. Are you getting any black smoke when you get into it? Here is a good video that shows how to pressure test your boost side. Good luck, Dean

Source: https://www.youtube.com/watch?v=hEQBj4uhAwo

 

[Chevy1925]

Do you have someone nearby who is an experienced mechanic who works on a lot of duramax? The suggestions of things to check are all going to require diagnostic equipment.

pretty much this.

if all the basic stuff is checking out, its going to be hard to diag over the internet if you have no diag equipment. What do you have as far as scanner? or is it just an autocal to flash the tune in?

 

[Preston]

Finally off work, 16hr days suck. BUT, I’ve got a friend who is diesel savy. Mostly fords because that’s what the power company buys. He has equipment and knowledge to help me run through a check list of possibilities.
All I’ve got is the edge programmer, my efi live tune equipment from Kennedy. A scan tool from Oreilly’s and my fuel psi/vacuum guage from Kennedy from the twin pump install.
I have not noticed any smoke on kick down or take off. That was the first thing I looked for when I felt something is f-n up.
2 things are bothering me. I didn’t bleed system when I changed fuel filter. I tell you the filter looked fairly unused. Hell I didn’t have 4k miles on it. The second is, tried the exhaust brake idiot button on my 137 mile trip 3 weeks ago. I used it to get out of town. Once on highway I felt down on power. I figured that damn button is killing this thing and I reached down and cut it off. No real noticeable difference. Wondering if turbo pinch is hung up.
I’m listing all possibilities. I got 2 more days of hell with power Co. Before I get time to focus on issues.
I appreciate you. Your knowledge and input is invaluable. I’m in NW alabama.stop bye, dinner is on me.

 

[2004LB7]

deleted or stock exhaust?

 

[Preston]

Stock

 

[Chevy1925]

if you damn near never use the turbo brake, its possible the vanes are stuck but you should be getting a code for overboost (unless the tune shut it off).

put the stock tune back in and see if the issue stays the same and if you get any codes. you will need to put some miles on it, not just a drive around the block.

 

[2004LB7]

also possible that the cat (DOC or SCR) or dpf is partially clogged and creating a lot of back pressure.

 

[Chevy1925]

^^ sure could.

I dont think Kennedy is a code killer kinda tuner but ya never know...

 

[2004LB7]

I don't think there is any codes to cover clogged DOC or SCR. there is no pressure sensor to monitor those. maybe if one of them gets too bad there may be a performance code

 

[08lmm72mm]

Log with efi live desired frp and frp then go for a drive. Low fuel pressure from the cp3 will cause horrible mpgs and if you drive like normal it won't set any codes.

 

[Preston]

I’ll do that Thursday night. Then get my buddy to check actual sand demands with his scan tool. Maybe we’ll find something low, loose or stuck. Like the fuel primer sucking air.

 

[Preston]

A year ago the dogs ate the external def sensors. Fayette dealership repaired it. I’ll go by and have him check his repair and check regen operations.

 

[JoshH]

The biggest thing I see affect fuel mileage is a dirty air filter, and LMLs are quick to plug them up sometimes. Almost everything else will set a light (especially on a truck that isn't deleted).

 

[dzimbric]

A year ago the dogs ate the external def sensors. Fayette dealership repaired it. I’ll go by and have him check his repair and check regen operations.

LOL, that is almost too funny.
I feel your pain and will be following this thread because the mileage on LMM has dropped, but never as low as yours. I still got 10 mpg pulling a loaded 7' all cargo trailer at 80 mph las weekend. 6 mpg would suck terribly bad.

 

[Preston]

Cool, I’ve got access to a high end diagnostic scan tool. I go off call Thursday and will start looking into this condition. I hope I can find a problem.