Had dealer change my fuel filter last time I was in having him check a warranty issue with my brakes. Went from aprox. 17-18 mpg (city and hwy mix, no load) to 15 mpg.
Weather is mid 50's .

Winter diesel change at the stations was probably done back in Oct. and I was still getting better mileage.
nothing else has changed

any ideas?

 

Can try cleaning your IAT sensor and doing an APPS reset. Would do nothing but help.

 

How do you clean the IAT sensor? Where is it? How often should that be done?

 

Colder weather means decreased fuel milage, generally. There are a number of reasons. If you drive a lot of highway miles the big reason is the increased density of the colder air that you have to push the truck through. Plus the viscosity of all your driveline fluids will be greater, things of that nature.

 

Sound like the sometimes feared Re-Flash of your ECM.

Believe I read somewhere it hurts mileage on earlier 24v engines (98.5's & 99's).

RJ

 

I suspect what RJ said. I took mine in back in 2000 to have it reflashed for more power and better fuel mileage. Fuel mileage went down and transmission locked up sooner. Another DC screwup.