If anyone could help me with my issue that would be lovely. I decided to change the exhaust manifold on my 99 mustang from stock to shorty headers and x pipe. l also in the process did an egr valve delete and installed a cold intake. The first time and i started the car it ran fine but sounded like it was off so i checked the wire placement and re-did it and started it again and didn't want to start. than went back to original wiring and started right up but still sounded off. But than when i went to test drive it afterwards it started to backfire which sounds like pops but only when accelerating and decelerating. But at idle it seems to have a hanging idle and idles around 6-7 k. Also when putting the car into gear(automatic) it seems to bog down when under-load and the rpms drop to 5k. I have also seen that my fuel economy drop because i have to fill it up every two to three days. I think the car needs to flashed. Can anyone help? Only car.