With all things remaining the same, what are some of the things that can cause a change in fuel mileage in a diesel besides adirty air filter? For no particular reason that I can find, mine has really gone down the last couple of months. At first I thought it was the large bird nest I found in the air box. I am down from 18-19 on the highway to around 14-14.5 and around 11-12 in town, down from around 15. I buy my fuel from the same station unless I am out of town.

Heavy right foot usually causes my problems!!! Seriously I don't know if everything on the truck is the same. I am sure someone more knowledgeable will chime in. I know mine doesn't do quite as well when it's real hot out but certainly nothing like your experiencing.

Just air temps? When its hot, engine won't run as efficiently. Air conditioning running. Cooling fan? (will draw power, using more fuel)

Probably won't cause that much of a drop, though.

Before my injecor problem, I was getting about 16-17 mpg. My truck is still waiting at the shop for new injectprs. I have a 2001 2500HD D/A. Will I get better mileage when my new upgraded ingectors get replaced? I have 100k miles on my truck. I want at least 300k before I replace the motor or rebilld, what should I have the dealership do to the truck while it is in the shop to achiieve this?

I'm getting about the same mileage with the new injectors. I get about 18-19MPG around town. Upwards of 19-20 MPG 50/50 city/interstate. On sustained interstate driving at 70-75MPH I can get upwards of 22MPG. Anything over 75MPH really kills it, though. BTW I'm running the Predator @ 40HP. Edited by: 1BADDMAX

Hey Raptorrider:


We saw the same effect on our 2003 2500HD Duramax with Ali. Only 14 Months old but with 35K miles. Fuel economy went from 18 to 15 to 12 now below 10, within a 2-month period. K&N air filter, changed fuel filter 2X, no engine lights or codes. Got the dealer to inspect the injectors and sure "nuf they're bad. On back order as of today. Really need the truck for a Search and Recovery Mission this weekend too!

I have posted a while back about this problem with my truck. The dealer says no codes, no problem. Still bad milage. I have complained enough that the dealer is getting the GM rep involved. Just today started to get a occassional rough idle and more of a exhoust smell.