I recently put on a 4 inch turbo back (3 1/2 down pipe) off road system (no cat) with no muffler on my 05 Excursion 4x4. I haven't drove it far (approx. 20 miles) since putting it on but something has already set the check engine light. It seems to lack on power in the mid range driving it normally and is getting worse mileage than with the stock exhaust. Before everyone goes and tells me the mileage loss is because you're driving it harder to hear you new exhaust that is NOT the case. I'm just wondering if anyone else has expierenced this or not and if so what needs to be done to solve the problem without putting the stock system back on??? Or could it just be by chance that something else has gone wrong that is non exhaust related to be causing this???