Very well could be that when the shop replaced the in-tank fuel pump they did not put a hose back on properly. Get underneath the auto.....put two jackets on and gloves....and jiggle some vapor hoses before you install that $40 valve. But again doing any of this won't increase your mileage but it might keep gas fumes out of the cabin....do u smell gas? Back to ur main issue, mileage. How bad is your mileage? My other college boy has an old Taurus that has the 6 cylinder. It gets terrible mileage but runs very smooth. However, it's a solid tank so we live with it. If your reduced mileage is significantly less than before u changed out those nasty plugs then there's an issue that a shop WILL find. The non cheap scanners have a live data feature (while the engine is running) that will pinpoint things like the air fuel ratio, spark curve etc etc. It's easy for a decent mechanic to sort out ur mileage issues. Again, you are going to have to belly up to the bar and pay a few bucks to ur favorite local garage...unless, of course, u have a nice Snap On scanner! Keep us informed!