Have a 2003 Mustang GT that is running extremely rich, runs really rough popping all below 1700 RPM. Once I can get it past 1700 RPM it starts running better. It's a struggle to get it there. Mods are BBK TB and long tube headers. I've smoked tested and found no leaks anywhere in the intake or exhaust.

Parts changed out:
MAF
Throttle position sensor
Crank position sensor
Intake Manifold with gaskets for manifold and throttle body
all injectors and fuel rail
fuel pressure sensor
fuel filter
upstream O2 sensors
Header gaskets (removed K member to do it right)
replaced plugs, with correct gap
swapped coil packs from a friend's GT, no change

Now, if I have the battery disconnected for about a day. It will start and run just fine for about 2 mins, then it starts running like crap again.
Driving behind it you can smell fuel coming from it. Seems way too rich.
Any thoughts on what's going on here?

 

I've seen this happen with wrong injectors. There were multiple injectors used on our cars. Are the new ones OEM and same color as the old ones? I believe 03 had pink injectors. How do you know it is running rich?

 

Im having the same problem! Changed out pretty much all the same parts as you we well. 02 GT here. Sluggish and lurching when trying to get up to speed, but does better around 2500 RPM. Have swapped from 23# to 19# injectors along with the things you changed. Hopefully you;ve had some luck!