Quite a reversal of fortune?

IMO & that of EPA, your "14.3 MPG... all at 70-85 MPH" is NOT "terrible".

 

Let me elaborate. Average speed was about 77-78 MPH... no load at all. Completely empty except me.

The problem I have is the mixed city/hwy. My under 10 MPG number was from no speeds over 70 MPH and I was easing the gas off red lights in the city.

 

"Let me elaborate."

If you got "14.3 MPG" & your "Average speed was about 77-78 MPH" under any circumstances, that is NOT "terrible" gas mileage.

Overdrive cruising on open roads give high MPG. The "mixed city/hwy" driving gives 0 MPG at all "red lights" & that sucking sound every time you accelerate a 5000 lb mass thru the lower gears.

If you went 60-65 MPH, which I think is the EPA estimates are based on, my guess is you'd see their 16-17 MPG. Also note that ethanol has reduced MPG & I'm not sure if that is figured into the 'new' EPA estimates for '98 E250?

Your posted MPG numbers have morphed from atrocious to expected, all but erasing suspected faults.

Makes your "under 10 MPG number" begin to sound more like a driver issue, both in the calculations & in the right foot.

 

I had a friend pull the intake from my 99, same as yours, I removed the inner silencer/baffle from it, it knocks the 3" intake pipe to a 1 1/2" inner diameter, supposedly to make it quiet, I recognize no change in sound, but can feel a difference at wide open throttle. I run a K&N element due to the difficulty in replacing the element, plus the cost of a new paper cone every 3000 miles is murder, the K&N filtration improves as it gets dirty, just pull every now and then to tap out the bugs, not clean.
I found 72 MPH is the sweet spot, it will keep from shifting a lot with the cruise set here, more and holding overdrive is impossible, and anything less and you're liable to be on a hill in 2nd gear, which sounds good at 70 MPH. I have a Superchip Microtuner program in it, which changed the shift points and horse power, high test is now required, it brought it alive, and after 7 years, I'll not be removing it to offset the cost of the higher gas, loss of the extra power on hills sux.
I used to get 15 MPG HWY, got 17 on the way to Florida once, was all down hill, likely the major contributing factor, city used to be 12 MPG. It had 73,000 miles on it and the dealer had just cleaned the throttle body and injectors, not the pour in the gas tank crap either.

A sticking brake eats the rotor, generates heat and loses you mileage, this is what happens when the caliper pins stick, they allow it to get cockeyed and drag. You have to grease them good with high temp grease during assembly, requires a needle injector to get in the rubber sleeve cover. Get some carbon metallic pads, new rotors, and do the front brakes right now, both sides, I'm positive you can't turn those and a new surface is required for a pad upgrade.

 

I think you need to start tracking your MPG by recording every tankfull for a couple of months, then you will have a concrete number.

If you plan in keeping the van, and since you have no history on it, it wouldn't hurt to change the plugs. I would change all fluids in order to have a baseline to go on. At 10 years and 67K, all fluids could be original.

 

Most of the fluids have already been changed/flushed. I did that.

 

With my 02 150 4.6 I'm getting 15 city, but if I try to run with traffic off the light and drive it like a mad man it will get 11-12 mpg. I haven't check it on the highway yet!