biglittlepickup

well, hello. I am a newbie here and a virgin chevy owner. Moved out to california and bought a 1988 c1500 ls v8. I quickly learned that i should have looked for a smog certificate when i bought it. I went to smog it and it failed, quite terribly. All levels were high. It failed for a faulty O2 sensor, which i replaced, a leaking gas cap, also replaced, and an EVAP leak. How in the world do i find where it is leaking from and what else do i have to do to lower my HC, CO, and NOX levels? any help at all would be appreciated. thanks