Ok, I hand calculate everything and track it all with fuelly. So this is a 2000 7.3 with 250k. When we first bought it we were getting 22mpg. It then fell to about 19 give or take. I figured winter fuel. It has one of the flashpaq programmers and had the performance tune and I switched it to the Mileage XS. However it kept falling. 17, 16, 15 all the way to 13. Replaced the air filter. 17mpg, then back to 15, now on the latest tank I'm back down to 13.5. Keep in mind this is by the tank, not referencing the information center which on mine someone had removed to put boost/egt gauges in) My last oil change (and first since owning the vehicle) was with 5w-40 Mobil 1 Delvac. Blackstone said everything looked good on my analysis. I haven't towed more than my little utility trailer which at most with the load was 2k.

I changed the air filter (with a motorcraft) I have not changed the fuel filter or cleaned the back pressure pipe. I did run some Power Service injector cleaner in the last tank.

*Now I'm fairly sure this is the EBPV but figure I would add it.* In a possible related event, recently when first started in the mornings, it will bog down and it sounds like someone turns on a huge fan under the truck. It will surge back and forth sounding like you're covering a shop vac hose with your hand and then releasing. You can feel it while driving. It will drop a couple hundred rpm. After 10-15 minutes it won't do it at all. It has been below freezing but nothing lower than 20ish. It's audible from the inside and outside.

The exhaust is broken above the rear axle and I haven't gotten that fixed yet. None of the shops around me had 4" so I had to order it myself and just haven't put it on. Could the tail pipe be moving around and causing a restriction?

Anyone have any thoughts?

 

Having "too little" back pressure will actually affect fuel economy and performance. Based on the miles you may want to have your injectors tested.

HPOP, ICP and IPR can also affect MPG.

 

Ok, so I figured this out finally. My brakes were dragging. Eventually warped my rotor. Replace the rotor, caliper and new brakes and I'm back to 18 in mixed driving (hand calculation). I've had two tanks at 18 so it looks like it's stabilized. Hopefully once spring rolls around that number will improve a little more back to 19/20.

 

Well done for figuring it out. I'm wondering whether my EX is doing the same as I'm seeing some pretty crappy MPG. Got a few hurdles to jump through first before the brakes though. Did you remove all wheels to diagnose?

 

It got to the point where it start shaking (all the time, not just when pressing the brakes). Looked through the wheel and could see the rotor was discolored. Before it got bad, I could feel it pull some but not all the time and on the highway it seemed fine, just that it might have needed an alignment. And then suddenly one day it started shaking. It got worse. I've experienced lots of warped rotors over the years but this felt different. Checked ball joints, tie rod ends, u joints... all manner of things. Brake or no brake it shook sometimes and then others it was just fine. Then it got to be really bad. Bad enough we parked it for a couple weeks while I was traveling for work. Once I got back I replaced the brake components and it drove just fine and the MPG is back where its supposed to be so far after two tanks.

 

Interesting. I don't have symptoms that severe, but glad you caught it. Sounds like it was suffering!