I'm currently out of ideas for my problem. I own a '97 f-150 and it has very sluggish acceleration between 0-2000 rpm. In that range it would barely inch forward no matter how much I push on gas pedal. Currently I have replaced the spark plugs, ignition coil, wire set, throttle position sensor, serpentine belt, catalytic converters, exhaust manifolds and gaskets, o2 sensors, fuel filter and mass airflow sensor. I also just took it to the mechanics and they checked everything including all the work I did and even the fuel pressure and checked for vacuum leaks. They turned up with nothing. This issue started after I redid the entire ac line. I'm stumped and any ideas would be helpful. Also there is no check engine light and no codes being thrown
An IC engine needs air, fuel, spark, and exhaust to run. Assuming you have an automatic, needs a functional torque converter to accelerate.
Easy stuff first.
Engine runs smooth with smooth idle?
Warmed engine, parking brake on, service (foot) brake depressed, floor accelerator and read rpms. This is your "stall speed" and should be about 2000 rpm. Higher is ok. If in the 1500 range, bad torque converter. You need about 2000 rpm for the engine to reach a speed that develops enough launch torque. Too low rpm and the torque is not there.
Assuming that's ok.
Confirm you have good air entry, no dirty air filter, or blockages.
You seem to have confirmed good fuel flow. A dirty fuel filter or poor fuel pump will "run out" of fuel during hard acceleration...that is you will sputter after fuel throttle shift into 2nd, etc. I have seen where stuff was thrown in the gas tank and slowly disintegrated, periodically blocking the inlet screen and fuel filter.
Confirm that exhaust flow is good. Have someone rev engine while you feel exhaust flow. Should be strong flow (and not burn your eyes from rich unburned fuel). I have seen dual walled exhaust pipe that internally separated and blocked flow.
Now the hard stuff.
Stretched timing chain or worn out camshaft. What is the idle intake manifold vacuum? How does the vacuum behave on sudden throttle opening? Vacuum gauges are cheap or use the bluetooth OBD II dongle and the Torqe app. Search the internet for "engine vacuum test" to learn about how problems affect the vacuum.