i got a 1990 chevy 5.7L 350 I got a service engine soon code #13 which is for the o2 sensor so i replaced it. But the engine still goes up and down in idle and when i try to put it in gear it dies does anyone have a clue what it could be

 

egr valve needs cleaning/replaced. maybe idle air control valve.

 

where is the idle air control valve

 

should be on the side of throttle body, can't remember which side. it should have a square, 4 wire connector if memory serves right.

 

i switched the air flow and throttle positioning sensor from my 87 350 but it still runs rough and now it wont stay started anyone know what the hell could be going on