Engine/Transmission (1998.5 - 2002) Sudden change in fuel mileage

[snowman]

I have a 2000 4X4 QC 5sp (stock) that has been rock steady at 18 to 19mpg in mixed driving. The truck, for the last 2 months, is doing good to get 17mpg with an average decrease in fuel mileage of 10%. I do not think they use a winter fuel down here (north Atlanta), but even if they did it has not affected me in the past.



I have changed all the filters and oil. I do have an exhaust brake, but it has been on long enough for me not to have considered it a s a problem. Driving style/environment has not changed. Does anybody have an idea what might be causing the drop?

 

[Graphic man]

Map sensor can cause that problem and get no codes

 

[rkressg]

A failing MAP and IAT sensor can cause poor mileage. For what they cost, I would replace them (Get them from Cummins, the will cost much less than the dealer).

 

[Rattlin_Ram]

My Vp-44 was the cause of my poor fuel mileage... ... Replaced now under the 100k warr.