exhaust manifold gasket. on 2004 Ford Mustang

does a bad exhaust manifold gasket cause poor gas mileage?

1 answer
Yes, because the fresh air hits the Oxygen Sensors and makes the car run REALLY RICH, which cuts about 40% off your mileage. You may also have a Check Engine Light on as well. This article explains about the Check Engine Light

http://repairpal.com/check-engine-light