What would cause my 1996 Mercury Villager to have reduced mileage to the gallon?

i would say have the intake throdle body and egr valve cleaned. and if it hasnt had a tune up in more then 3 years thats another option too. with out looking at your car myself id say it prob needs the o2 sensors changed too. theres alot of things people neglect that should be changed and would cause fuel mileage problems lastly before you do any thing take it and have the ecu reset then drive it a week if its continues try some of my or other peoples sugestions. good luck.