I have the Roush quad tip exhaust, and the inside tips have (had) a build up of black smoke while the outer tips remain clean. I pulled the tips off and cleaned them, reinstalled them, and started it up. White smoke came from the inside tips, but not the outer; however, there was exhaust coming from all of them. I put my hand down there to check. I installed a UPR catch can within the first month of getting the car, and my oil level is perfect. Does anyone have the same issue or know why this is happening? Thanks everyone!