Originally Posted by Jordan Y.
Maybe the exhaust shop messed up the o2 sensor and it's messing with your fuel trims? That backpressure thing is crap- if you've got a full exhaust w/ test pipe, you shouldn't be having any problems. Running open header, yes, it might throw things out of wack somehow, but you don't need a cat choking up your engine for it to run..
Backpressure is needed for most all NA motors to run properly. Cats and mufflers are what provide that needed backpressure for the motor. Running a 2.5 inch exhaust or bigger on a NA eclipse motor is not a good idea. 2 to 2 1/4 would be the ideal "performance" exhaust size for it. Cats aren't bad, youd be suprised how much they can actually flow.