Bad gas mileage

Hello, my vehicle listed above is a LWB model. It has an.5 l v6 gdi engine. It was having some fuel problem consuming a lot I think about 2 or 3 kmL. So I took it to a local mechanic and changed the fuel pump and injectors and it became worse. Now my vehicle wont rev past 3,000 rpm's and it takes more then one try to start it after five tries it starts and engine check light is also on. In my town almost nobody knows how to fix gdi engines. I need help.