I have a Chevy Blazer 99 with a 4.3L Vortec, I had to replace the injectors because one of them was bad. I did it myself. Ever since I Changed them, I hear a sound from the dashboard like a hiss or water rushing. The car was running normal for a few weeks then I started to have misfires , stutter and bad MPG, I took it to my friend and he replace all spark plugs, cables, cap, rotor. Sparks were in a really bad condition with carbon build up. The car was running fine for a few days(still hiss sound from dashboard) . Although no misfire , I do get stutter when idle and some foul smell. Today I put scan to it and I get these codes P0146 02 Sensor no activity detected (Bank 1 Sensor 3) P0174 System too lean P0300 Random/ Multiple Cylinder Misfire. Could it be after putting all together after changing the injector spider I did it wrong? Bad plenum gasket? What could it be?

 

..did you check to see if the O2 sensor was even there, hiss sound and a dead sensor signal...