My 1988 f250 2wd EFI has recently started running poorly. It has the 351 windsor and c6 auto transmission with 158k miles. The truck seams to not shift into third consistently going 55-65 mph. The motor sounds like it is stuck in second and the power seams to be lacking. If I floor it at 65 mph it will excelerate slowly and then finally shift and quite down. It looks as if I am getting 6 miles per gallon. I use to get closer to 12. The tranny shifts hard most of the time around town and feels strong. The only time is when I am on the freeway and experience the above problem or if I am going up a long hill and then around 65 mph the truck will shift erratically between 2nd and third especially if i have a load and then I have to keep the peddle to the floor to maintain speed. The truck has had a new exhaust and cat installed in the last couple years as well as a full tune up last year. Could this be as simple as a Throttle Position Sensor (TPS) or any other sensors or could this be a tranny problem?

Thanks for your feedback

 

Have you checked the vacuum line and the modulator valve on the top of the Tranny? I just had a C6 rebuilt about a month ago but i tried everything else first that modulator seems to be a pretty common reason for poor shifting or elongated shifting i'd check it there only about 13 buck at you parts store.