Ok,

So I got the motor in the car, and it bump starts clean, and Idles. It is surging a little bit between 300-900 RPMs, and that may be the FITV/IACV. However the real problem is that under load I cannot use more than 1/4 throttle without inducing backfire, and hesitation. Could the IACV/FITV cause that as well?

Let me know if you have had a similar problem/Resolution.

I have NO CELs.
If I stay under 1/4 throttle I can drive the car normally.
If the car is not in gear, I can use more than 1/4 throttle, but not WOT.

Thanks,

 

I've seen similar problems when the O2 sensor are reveresed, but it normally cleans up after about 4,500 RPM.

Remove the intake and start the car. Look in the throttlebody, there should be 2 holes. 1 should be the air bypass port for the IAC adn the other for the FITV. Use your finger and plug 1 at a time. If it stops surging, then follow the path to the problem. also, look for a vacuum leak.