I searched the forum for a situation like mine, but this might be an obvious fix that I dont know about. I replaced my alternator about a week ago on my 98 f150 4.6 (motor has about 10k miles on it so it SHOULD run fine), before I replaced the alternator it ran fine, plenty of power that a 4.6 should have. After I put everything back together and got it back on the road it feels like its only running on about 6 cylinders, I can go wide open on a wet road and not get any wheel spin, just to give you an example of what kind of loss im getting here. Suggestions?

 

does it misfire or is it just accelerating like its down 2 cylinders? what kind of plugs are in the engine? wires? coil packs? was dielectric grease used? did you check fuel pressure? how olds the fuel filter? maybe a bad cat?

 

Let's see you just replaced a component on top of the motor.
Every thing seemed fine just before the replacement.
Matts Ford says check the fuel system and I agree. Except you said that every thing was good before you replaced the Alternator.
There are at least three significant vacuum lines in the vicinity of the alternator. Have you checked back around the alternator for a cracked vacuum line?. Loose vacuum lines can cause at least two of the symptoms that you mentioned. This could have been caused by pushing and bending of that covered cable while removing the alternator. Those vacuum lines are essentially hard "spaghetti" plastic, and can crack causing vac loss.

I'll add checking of all vacuum tubes and lines. Check all vacuum lines from the engine block to the left fender wall for a disconnect or crack. Those lines affect the IP-HVAC, Emissions EVAP, and EGR, and additionally the Fuel Pressure Regulator Unit receives its vacuum source from the LH manifold source.
This could be the source of poor fuel supply during acceleration that you noticed. I hope that helps some of us have had to learn the hard way.