2000 Deville 98k 13.8 mpg ?????????

i have a 2000 deville with 98k. the engine was replaced with a 2003 model previous to my purchase. the car won't average above 14 mpg city or hwy. the codes have been checked by dealer with none found. the only thing they suggested was a full induction cleaning witch included 4 oil changes the same day. It ran a little better, ( it ran great before) but the mileage has not improved. Any ideas?:banghead:

whens the last time you've tuned it up?? also could be your timing....should get more on hwy....

timming is computer controled it;s a DIS system. Although it has a new air filter, fuel filter and AC delco plugs. There is no PCV valve per Cadillac