1993 D350 Clubcab with the 518A auto tranny:
OK, so for the last week or so, I have been "playing" with the TPS , trying to dial it in to the correct setting, and every time I seem to have it set, it will shift into OD correctly for a few days( about 100miles) then the shift point seems to "creep" higher and higher..

this last week, I pulled a 12-14K trailer out to Salina, 110 miles each way thru the Flint Hills( not long grades, but steep up and down for about 70 miles, ussually see the semi's slowing to 40-45 going up them).
On the way there, I could maintain 70mph thru most of it, and the OD was engaging at around 60. I got about 14 mpg on the trip there.
On the way home yesterday, OD would not engage until I had been running 70 for a bit and then would disengage the minute I took pressure off the throttle.
I average 67mph with the pedal floored all the way back....
no power at all.. going up the hills, I was slowing to 50mph with the pedal floored... averaged about 9 mpg going over the same road with the same load as i did on the way out..
I replaced the fuel and air filters before I left, it has a recent oil change, tranny fluid is clean and filled properly, and the rear end is filled to the proper level.

I am going to replace the TPS with a POT today, making a list of parts now, but is there anything else I should look at? what else could be the cause of this??????
I have to tow the trailer on a 500 mile trip this coming weekend and cant afford getting the 9mpg that I got on the way home yesterday...

also gonna install a BHAF today...and relay the headlights

 

in my 92 w250 through Appalaichan Mtns. Found out my Turbo was trashed, bushings etc. when I went to go replace housing.

 

ONce the OD would engage, it ran good, and I can hear the turbo unwind when i shut it down..no oil leaking from teh turbo discharge tube and no indications that the turbo is bad..

 

Good job, you broke it. That's what you get for messing with things.

(That's what are friends for... )

 

my thoughts exactly..it wasnt broke, so why on earth did I start messing with it??????