Engine Mechanical problem
2005 Chevy Colorado Two Wheel Drive Automatic 136000 miles

i replaced fuel pump due to gas gauge not working...after replacing sending unit the gauge worked but now the engine will not start properly...takes 2 or 3 tries to start. once started it puts off a smell like bad or old gas...but runs fine once it starts...i have driven it like this for about a year and now my gas gauge is starting to malfunction going up and down

any ideas and thoughts greatly appreciated

Jimmy