i have a 2012 edge limited 2.0 ecoboost 67k miles. until 3 to 6 months ago the mpg was 27 to 28 avg, now can barely avg 23 to 25. i have put a new air cleaner & 4 spark plugs & this has not helped. my local dealership sayes that's all that can be done since no engine warnings are on. ive noticed that the tail pipes are blacking up really quick after washing, wondering if the fuel mixture could be a issue ?