well i calculated my fuel mileage and im getting about 12-13 mpg, i recently had some exhaust studs drilled out and taped to get the exhaust issue fixed b4 i was getting about the same, bumped it up to about 16-18mpg in town driveing, well i started having engine problems and bogging down while driveing, i took it into nissan and had then diagnose it and all they said was to replace my fuel pump 20psi and droping, and my sparkplugs that are not even 1 year old, i had in autolight, i replaced with ngk platnum plugs. i watched my fuel mileage about 2-3 weeks after work on exhaust then didnt watch it untill about 2 weeks ago, and noticed i was geting 12ish MPG, my new coworker has same year but merc villager and gets about 22 mpg intown, i was like WT heck can i trade you? lol, but i did some testing and i dont know if my ecm is reading my o2 sensor, i completely disconnected it and no check engine light, so now i dont know if i have bad wire somewhere or if there is a quick fix. i dont have a check engine light on eather.

Any thoughts will help. thanks.