Another, expensive, problem with these engine's is carbon buildup in the combustion chamber. It is a standard failing of all internal combustion engines. Carbon from the fuel condenses out of the exhaust and deposits itself on whatever it can. If enough of it accumulates on the piston or the valves, it will change their shape, reducing power. This reduction in power will cause you to press on the gas pedal more to get the power you want, resulting in your lower fuel economy. If the carbon has been on there long enough, it will melt and turn into a glass crystal (obsidian) that can not be removed with fuel additive carbon cleaners. The only fix I know is to disassemble the engine down to having the heads off and scrape/wire brush/chisel(ouch) the piston tops and the combustion chambers of the head clean. Be careful, don't scratch or crack anything.
Had something similar happen to a 1998 Tarus. Caused the head gaskets to blow, and clog the Cat converter with carbon. Once the gaskets were replaced and a new converter was installed, i doubled my fuel economy. 14mpg to 29.