Originally Posted by Pyroproblem
Sounds like a bad o2 sensor to me... A plugged fuel filter will cause it to run lean all the time and not only when its warm. Since you said it runs good as its warming up(open loop) and when it gets hot after 5 minutes it uses the o2 sensor feedback...
Are you using a different o2 sensor than the one with your previously engine?
you can take an o2 out of the car completely and it will still run properly... that is why i dont think it is the o2...