I posted about a week ago, but I didn't get any responses. Let me try again and hopefully, somebody who knows more about cars than I do can help me.
I have a 07 Hyundai Elantra SE with auto transmission. Since I bought it, the car has consistently gotten about 27-28 mpg for mostly city mileage. However, recently, with the same driving, it's dropped to about 25 or below. This coincided with a yellow indicator lamp that, according to the dealer, produced two codes: P0304 (intermittent misfire in cylinder #4 spark plug) and P0442 (small evaporative leak). The dealer replaced the spark plug and tightened the gas tank cap, but, the mileage hasn't improved and I've driven sufficient miles to see a difference. Anybody know whether the first code wasn't produced addressed by the replacement of the spark plug and that's why the mileage hasn't improved? I haven't noticed any performance difference in terms of smoothness of acceleration or power.