4.0 feels like a 4 cyl

[tonyjklh]

well i put this used engine in my truck in February and it has been runing great until now. the past few weeks it feels super week! i have changed the maf, tps, and all the plugs/wires, also changed fuel and air filters. i even changed the ecm. it seems to be running great and smooth, but its just lacking the power.
any help would be great.

 

[chazzone]

Check the timing and make sure it is advancing. I'd also check fuel pressure on the rail.