low mpg

hi need help yesterday i let my xterra 2001 in service because the service engine soon light and bad power and low mpg they say me that the sparks are bad they change for new ones and the throttle body was dirty and now i have it yea it recover some power but mpg not and when i stop the engine...

hi i need help mi x mpg is les than 4 or 5 mpg on highway and also has very low power when is on drive and complet stop the rpm are 500 what i have to check or how can i best my mpg and hp. is a 2001 v6 se 4x4 46k.