2003 Jetta TDI with 204k+ miles and recently the mileage has decreased (last 10 fill ups). 5 years ago, I was averaging a consistent 46.2 MPG all the time and now I rarely hit 40MPG anymore. What made me write in is that I just put 600 miles on the car, all highway which normally results in the best mileage I see. Tonight it averaged out to 38.4. Is this just age?
In case it's not age, here's what I've looked at hoping there's some maintenance I've missed
1) The intake is not all coked up with carbon
2) The air filter was just replaced
3) The MAF was out of spec last year and causing the car to go into limp mode. I replaced the MAF last year and recently found it's still within specs
4) Tires are properly inflated
5) I switched diesel stations in case there is a difference in fuel now from a few years ago. I've seen no change in mileage.
6) No engine codes or engine light
7) The timing is within specs
The car can breathe and the fuel was eliminated as a factor, what else could it be? aging injectors?