Hi, I am new to this forum and I could use some guidance on fixing my truck. I recently purchased a 1989 f150 lariat 5.0 4x4 short bed automatic with 115k original miles. Runs smooth except when I am on the highway trying to keep a steady speed in the 65-75 range. it feels like it is starving for gas when i apply the accelerator. when i let off the pedal and cruise around 55mph it is smooth. I have replaced the following. plugs,wires,coil,cap and rotor,ignition module fuel pressure regulator and fuel filter. I had a garage check the cats and they said they are fine. previous owner said he replaced the egr and fuel pump(single tank). Also the CEL is not on.
I could use some help. Thanks!

 

do you feel this in your seat or steering wheel? are the tires balanced? and have you removed the driveshaft to inspect the u-joints for any play or binding?

 

steering and seat are fine. new tires balanced. have not checked drive shaft or u-joints yet. there is a small amount of squeel when I take off slow. so that could probably be the ujoints? when i am trying to keep a steady speed at 65-70 it feels like the wind is pushing me back. when i let go of the gas pedal it stops. could it be the drive shaft?

 

get the driveshaft out and inspect. then let us know

 

Drive shaft and u joints are good. Thinking about changing the map sensor and tps.