Hello sorry if this is to long I'll get to the point.
I'm having an issue with 93f150 4.9liter xlt 4wd
It's bucking wildly at about 55mph and doesn't stop unless you go under 55. As per suggestions from mechanic I've replaced the following parts (mind you a lot were bad and all original) distributor,spark plugs &wires, coil, map sensor,throttle position sensor,air idel control module, ignition control module, pcv valve, ecm leaking parts on its board, non engine parts like upgraded to 11: clutch and pressure plate new flywheel, pilot bearing, rear main seal, slave cylinder, rebuilt m5d2 transmission, rebuilt transfer case, new u-joints, chsnge front and rear axle fluid, fuel filter, heater core, radiator and hubs and rotors with brakes timing checked 5 times . A lot of stuff all in the last 14 months. Fyi after changing ecm bucking stopped but came back after 3 months turns out oriellys gave me ecm for automatic not stick don't know if that is an issue changed it out again yesterday still got bucking at 55mph please help

 

Many times bucking is caused by a miss ie plugs, wires, or a clogged filter etc. You have replaced all of the normal causes. The possibility I see is your mechanic timed the engine when the distributor was installed. The SPOUT must be removed to properly set base timing. Many mechanics are unaware of this Spout. Would have been nice if Diagnostic codes would have been checked. If he did remove and replace the spout when timed then running diagnostic codes should give some clues.

 

Wow! that's a lot of parts that were changed. Hopefully some diagnostics were done. After decades of automotive repair, I've found that the hi-quality, working, factory parts are removed and replaced with low-quality auto parts store parts that barely meet the spec to pass 'quality-control' and fail in a few years. Mean while, a number of drivability problems develop from the low-quality parts.

Has anyone checked fuel pressure? Stumbling and bucking under load can be caused by low-fuel pressure. It can also be caused by low system voltage.

 

Try paying attention to the speedometer and tach when the bucking occurs.

If the speedometer is reading erratically while the bucking is present, it indicates a problem with the vehicle speed sensor or related circuits.

If the tach reads erratically, it indicates a problem with the spark timing/control circuits.

One other thing -

A faulty MAP sensor or a leaking MAP sensor tube can result in bucking or hesitation. Typically ​​​​​​it will be noticeable through several engine speeds/loads, but it’s a quick, easy test to plug the manifold port and test drive.