[bad_assr]

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I have a 2003 duramax with 112,000 on it. It started out getting 18-20 mpg empty at interstate speed, 12-16 pulling at interstate speed. About 4 months ago (at roughly 85,000) I noticed a significant decrease in fuel economy, now I'm lucky to get 10-12 empty on the highway, and only 5-7 pulling. My local dealer has ran test after test with no real answer to my problem. Oil and filter gets changed every 3-4000 miles, fuel filter at least every 15000, transmission fluid and filters (internal and external) get serviced regularly. It is in the shop again getting looked at. Dealer has suggested bad thermostats?????? I've replace MAF sensor and turbo senser. Engine has good compression on all cylinders, good balance and no leakage on injectors. I'm running the Edge module with juice, cold air intake, 265 tires.
Anyone have any ideas???

 

[Tom Horne]

A couple possibilites you do not mention

Intake restriction, downstream of filter possibly? Exhaust restriction? Exhaust smoke level?

 

[05_LLY]

Winter blend fuel will net lower mpg, but shouldent be as low as you are saying! but usually winter fuel starts showing up around oct. Dose you truck have a cat on it ( some 03's did)? it could be sooted up pluging it! It seems like you had all the most common problems checked out! Also if you are running the ultra low sulfer diesel fuel (ulsd), it will lower mpg a bit also from the old fuel.

Good luck,
Caleb

 

[bad_assr]

intake is clear, exhaust is stock, no cat, no restriction as far as I can tell, after a long pull at idle I get noticable smoke and a strong exhaust smell. Dealer insists injectors are ok.