Disclaimer first. I haven't messed with the throttle body or IAC and to my knowledge the transmission has had some adjustments though I think it was just a stock rebuild. My issue is that off the line it runs good through 1st and 2nd gear. At about 25-30mph it was shift in 3rd or 4th and in order for me to accelerate from there or to drop down I have to push the pedal literally to the floor.

The truck is a 1990 F150 lariat 5.0L automatic. Any information or guidance would be great. I just don't want to start ripping it apart until I have a good starting point.

 

How long have you had it? Has it ever shifted correctly for you, or did you buy somebody else's problem?

 

I bought it like 2 weeks ago. The previous owners guess was the fuel pump. I changed that and the fuel filter and no change. If I play the throttle but keep my foot in it pretty good it seems to act just fine.

 

I know enough about automatic transmissions to know that I don't know much about automatic transmissions.

Seems like a transmission problem, not an engine problem. If you set the lever at one of the lower gears and drive around within that gear, it would be a clue about engine performance. You could also shift it manually and see how it behaves. Seems like your automatic is not automatic enough.