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!
1 - 7 of 7 Posts

·
Registered
Joined
·
4 Posts
Discussion Starter · #1 ·
Bought a 2015 duramax and decided to tune and delete the dpf for mpg purposes and a longer life. Also added an s&b cold air intake. Was getting great mileage of 16-17 in town for about 5 days then for whatever reason it just dropped to 14.5. My brother and friended noticed my tranny was acting lazy and used an edge insight or whatever and used tranny relearn and it’s up to 15 ish now but still. Anyone have any idea?
 

·
Registered
Joined
·
7,363 Posts
Bought a 2015 duramax and decided to tune and delete the dpf for mpg purposes and a longer life. Also added an s&b cold air intake. Was getting great mileage of 16-17 in town for about 5 days then for whatever reason it just dropped to 14.5. My brother and friended noticed my tranny was acting lazy and used an edge insight or whatever and used tranny relearn and it’s up to 15 ish now but still. Anyone have any idea?
Are you using any diesel fuel additives, it's possible you gots a bad load of fuel or the fuel quality has gone down where your located.
Using additives will help a little, you really should use additives anyway.
Possible CP-4 fueling issues, when was the last time your fuel filter was changed. 'dunno;

The Edge Insight is pretty useless for trans re-learn on a Allison from my experiences.
Search the Allison re-learn procedures and try that, keep a eye on your RPM's making sure they don't jump way up in-between shifts.
If they do then you might be in for some trans work.

Your talking about 14.5 vs 15, mileage differences with this small amount is pretty normal for any driving.
What terrain, the same exact driving conditions, highway, grades, stop and go driving differences, all play a part with mileage.
 

·
Registered
Joined
·
7,363 Posts
One more thing to check is your tires psi.
In general pretty much everyone's outside temps have dropped, tire psi goes down when the temps drop, mine went from 60-61 fronts to 55-56 front, dropped the same psi amount on the rears, which affected the mileage, until I aired them back up.
Lower tire psi will affect mileage.
 

·
Registered
Joined
·
4 Posts
Discussion Starter · #4 ·
Same tank of fuel so it shouldn’t be that. I drove home one night and was getting great mileage (and the same with previous days) and then noticed it was down the next day and it never went back up. Fuel filter was changed about 1000 miles ago. I cleaned the MAP and MAF sensor even though they looked clean. My average before was about 16.5 and now it’s maybe 15.1 or so. Not big but I just don’t understand why it would dip down all of a sudden and stay that way. I don’t have any engine codes either. Maybe I will try to find a way to figure out the transmission deal.
 

·
Registered
Joined
·
7,363 Posts
It appears your going by the ave mpg on your dic, not actual tank to tank.
If your constantly resetting the ave mpg, it will jump up and down quickly and will NOT be accurate.
One short period in stop and go traffic, highway, even the slightest grade up and/or down the mpg will change quickly, even climate change can affect mileage, even more when your constantly resetting the dic.
I think you might be worrying about something that in your in control causing this.
You need about 150-200 miles on since you last reset the dic, for it to respond slower to changes with driving conditions.
Or
It could be a fueling issue with the CP-4.

Leave the dic ave mpg alone for a while, or at least when you want to figure your mileage stop and fuel up topping off the tank every time and hand calculate, everyday if that’s how you want to monitor your mileage.
 

·
Registered
Joined
·
4 Posts
Discussion Starter · #7 ·
I was using the dic since I haven’t been driving but 5-10 miles a day and waiting to drain my tank would be awhile. I kept it in that back of my mind that the dic might not always be accurate but I don’t know. How would I know if the cp4 was giving me problems?
 
1 - 7 of 7 Posts
Top