Over a week ago my CEL came on and I took it to the local garage. The mechanic said it was a bad gas cap so I replaced it with a new Ford gas cap. About 100 miles later the CEL came back on. When you fill the gas tank up the CEL will go out until you get down to about 3/4 of a tank and then it comes on, fill the tank back up and it will go out. Does anyone know what could be causing this? It also does a quick hesitation like it wants to stall when starting out from a full stop. And it sounds a little louder than it used to, almost like an exhaust leak. Other than that, it runs fine, all fluids are full, and the temperature is normal. Just wondering what I may be looking at before I go and take it to a Ford garage.