I have a 1998 Mustang GT that has recently developed an idling problem. After accelerating and coming to a stop the car idles very rough and hesitates. I've noticed that the right exhaust tip has accumulated a black soot while the left tip has remained normal. The car only has a few modifications including a Magnaflow catback exhaust, K&N air fliter, and Mac 70mm throttle body. Does anyone know what's wrong? Maybe you could assist me with troubleshooting. The check engine light is on but the computer reports no trouble codes. I'm stumped.