ISX mileage dropped suddenly

[Rawze]

(09-15-2024 )ColdDiesel Wrote:  Hi,
I have a 2014 ISX motor in my RV. When I bought it, it had been deleted and I was not aware of it. So I had the emissions systems all repaired put back on it. So now it’s fully stock in every way.
I am getting no codes at all. But my mileage was around 7.5mpg in an RV weighing around 50K pounds initially. But now that’s dropped to around 5mpg for apparent reason at all. Is there anything anybody could think is the problem? I did get OTR diagnostics so I can monitor more things on it, but didn’t have it delivered before my mileage dropped by 2.5pmg. Any advice would be appreciated.

Fuel mileage issues can be caused by literally anything. It is anyone's guess.

Next. .. are you going by what the dash is telling you ?.. because many, many of the bad delete programming out there will lie to you and tell you that the fuel mileage is waay higher than it actually is.

Fixing bad de-mandate programming that is harmful would have been likely the proper thing to do .. but otherwise, was there a specific reason that you decided to completely 'undo everything'?. Some specific reason to go thru all that expense and now will have to chase its emissions systems + extra yearly maintenance, etc. to constantly keep the cans under the vehicle happy? - just curious.