Then it is time to start eliminating some things. My vote is to go after easy ones like the source of fuel and the fueling method.

Consider another fuel station that also has high turnover at the diesel pump. Preference is on the name-brand station as the fuel is more likely to come from within that company and have a consistent chemistry based on the season's blend (Summer versus Winter).

For filling the tank, once the pump clicks-off the first time, consider dribbling in the remaining fuel until it is visible in the filler neck. You want to see actual fuel and not foam. If the tank mod was not done, the last 3 - 5 gallons will take about as long as the first ~30.

Try these two things for a few tanks and see if mileage numbers settle-down.

Another factor is the A/C. I have noticed about a 10% tax on the fuel burn when I want cold air.

 

Mill give that a try. I’m at 1/4 tank and planning on a run to town in a bit. I when I get home the tanks getting dropped. I’ll try what you mentioned.

 

That's true. It can be tricky to get the tank filled to the same exact level. If you short fill it, it will make it look like you got fantastic MPG. If you sneak every last drop in there until it is dribbling out of the filler neck it will make it look like you did horrible.

After the pump clicks off I can dribble in another 5-6 gallons if I stand there patiently enough. Diesel foams up so badly that it will stop the pump well short of getting the tank full-full. On this last trip of mine I carefully topped off the tank twice in a row and calculated around 12.75 MPG pulling a 5500 lb travel trailer with good Montana fuel. I don't complain at all with those numbers pulling an RV. This is with a steel 36 gallon cab and chassis tank...no harpooning needed.

That being said there are other factors. Even from tank to tank at the same place there could be some quality variation. I usually get good MPG from the fuel I buy in Indiana and Illinois, awful mpg from fuel I buy in MN and ND, and very good MPG from the Montana fuel. I think there actually is a difference between the stuff, but there is also usually a light headwind running west across ND that could be part of it, so it is hard to pinpoint what exactly is responsible for a 2-3 MPG variation.

Might have been my imagination but it seemed like I got slightly better economy this last trip after changing the oil from Rotella T6 to Schaeffer 9000. A significantly more expensive oil but I was calculating some pretty interesting numbers running bobtail that I hadn't seen any time recently. Up in the high 19's and even low 20's on Montana fuel. Then it sank into the 18's on Iowa fuel. Prior to that I was only seeing mid 17's running the truck at 65 on the cruise control. Not the most scientific thing ever (I'm a trucker not an SAE engineer) but I noticed a significant improvement.

 

Yep. All good points. I figure the A/C kills a good .5 mpg but that is kind of a WAG. Never done anything scientific. I will run with the A/C off and a window cracked for as long as I can stand it in the AM.

I notice at Pilot and Flying J they will shut the pump off if you click the nozzle off too many times trying to dribble lol.

 

Well,, dropped the tank and found the pick-up foot broken. Replaced it! Did the Harpoon while I was in there. Would have done the Hutch if I had all the parts ready. I’ll run the tank down again at a later time then do it.

Mixing valve screens were still clean.

I payed attn to the coolant temp. It was lower than I thought it had been 165 deg on a 75!deg day. Only drove it 15 or so miles. May not have heated up enough. I’ll take it on a 20-25 mile run to see what it does before I pull the thermostat. In the event I have to change it, any suggestions on beat brand? Motorcraft, Mishimoto, NAPA, etc in the 195 deg, not the 203 deg.

thanks for all the help!

 

That temp could come from a partially locked fan clutch too. If the clutch has some age on it, might add it to the list of things to do. And if the clutch has gone wonky where the internal valve is not working well, that might add to the mileage variation a bit.

 

That might not have been enough time to warm it up all the way depending on what type of driving it was. There is no way to directly monitor the coolant temp but it will closely track the oil temp. Use your phone or laptop to run ForScan or Torque Pro and take it for a longish drive. Oil temp should get up into the 190's or even up over 200*F running at highway speed and drop into the 180's in easy driving and stop and go. Colder than that you might have a thermostat stuck open or like Jay said a fan clutch that is stuck on. The fan clutch is not the easiest to diagnose. There should be a significant amount of viscous fluid resistance when it is cold if you try to move the fan blades, but not too much. It can be removed and you can run the truck without the fan for troubleshooting purposes. Some guys remove it during the winter anyway to save a little drag on the motor since you do not need it forcing air past the radiator or air conditioner condenser when it is 30-40 degrees out.

If the truck is running too cold it will hurt power and MPG. Diesels need heat to operate at peak efficiency. Putting cooler thermostats in is gasoline engine stuff to avoid pinging and detonation in the summer with cheap gas. International originally put a 203* F one in and I believe the only reason Ford dropped it to 195* was for emissions reasons. Not to the make the engine actually run better. Mishimoto is 205 I think.

 

Don’t believe the fan clutch is stuck. I can hear it kick in at lower speeds during town driving when it’s hot out. It will stay engaged when accelerating until enough air flow cools everything down, then kick off. Unless it’s partially engaging..

 

Good chance you'll break the 3 bolts holding the T-stat housing to the water pump, so get ready to either drill those out or just get a new water pump. One of those billet aluminum t-stat housings is not a bad idea.

 

Have had them out before. Drove the truck and got it hot,, and I may have even heated them with a torch. I’m pretty sure I used anti-seize on them when I replaced everything.