Loss in mpg

[David Brickey]

Hello all,
I'm kinda new to this site, mostly because I'm normally pretty good at figuring out what is wrong with my tdi's. A little about myself I teach HVAC, but I'm also a 3rd generation auto/diesel mechanic. I'm really good with the older stuff. All that being said 2013 passat, def delete, and tuned by zilla. After tune and delete I was getting 48mpg average speed 70mph (mph was calculated by hand) for the past couple of fill ups I have been getting about 25mpg under same conditions. I have checked tire pressure, brake drag, fuel filter, air filter, no egr, cat,or regen since delete. I did the delete and tune back in January 2024 I noticed the drop in mpg last month July 2024 so I don't think it's the delete or tune that's causing the problem. Any thoughts would be helpful. Thank you

 

[Shoveltrev]

thermostat.

 

[David Brickey]

Motor is warming up normally, but it's sometimes hard to tell when it's over 120°F outside, but will check on that thank you Soveltrev

 

[turbodieseldyke]

When my 98's thermostat was stuck open, my mpg wasn't the greatest but it wasn't cut in half either. Does the car seem to run normally, no smoke or hesitation or anything?

 

[David Brickey]

No smoke, no hesitation, no lights on the dash, no codes, all seems normal. That's what is puzzling to me. My other tdi has the same delete & tune but I'm still getting 52mpg.