I just installed a Borla Stinger catback and a mustangtuning O/R X pipe. I put on a set of used MIL eliminators. I get no check engine light. BUT I am getting horrible gas mileage. I just got 100 miles out of 3/4 of a tank!! I think the car might be running too rich. In park, the car will sometimes die at idle. What do you guys think??? What is the explanation???