newer EGR valve If I unplug the map sensor the service engine light is on. but if i unplug the o2 sensor nothing changes , engine ran great before last 250km. hwy trip. how can you tell if the o2 sensor is broken.? I have a 1991 gmc sierra 5lt manual trans . engine seems to be running rich has stinky exhaust .barely Idles ?probably clogged. this is giving a wrong air to fuel ratio making it run rich.I have a 1991 gmc sierra 5lt manual trans . engine seems to be running rich has stinky exhaust .barely Idles ?you can tell the o2 is bad just by lookin at it if it looks bad it porbably bad and you will have a check engine light on
check fuel and air delivery
also check all vacuum hose
your problem lies some where in one or all of those categories
