I need help :(
 

Okay I have a 2004 3.8L mustang, automatic, and really the only modifications I have done that would affect the cars performance is install a cold air intake. There are no bad side affects of the intake that I know of. I've had the passenger side manifolds replaced, the EGR valve replaced, and spots on the actual exhaust welded cause there were holes. The problem now is, it seems like she just doesn't have any power to her anymore. The car was fine about 3 weeks after the new mainifolds but now it's just like she doesn't get the "umph" anymore. She doesn't accelerate fast at all and the RPM's go up to 3,500 when switching gears (from 2-3 gear or 3-4 gear). I'm assuming I need to replace the driver side manifolds also because they're not in the best of shape either. Any ideas of why this is happening? I can't figure it out and it's really frustrating! Sorry for the long post but if anybody has any ideas on how to maybe fix that? Thanks

Oh yeah, and also my gas mileage has gone down incredibly. I end up almost using a quarter of a tank of gas in one day just driving 5 mins down the road and back.

Welcome to the forum.

Pull codes to see if any are stored. Some codes will not light the CEL and you'd never know. That may point you in the right direction.

Also, try starting a new thread in the appropriate Tech section, the one specific to your year Mustang. Good luck