98 missing under load could it be o2
 

ok I have a 98 its missing under load I get these codes
po171
po300
po134
could this be o2 sensor
for a long time I ran with a crack in y pipe on ds elbow I have now welded it up

I had issues about a month ago then it went away and now its back

truck has had new plugs cap and rotor fuel filter
and the fuel pump replaced with used one

i cant seem to find out whats the real cause of the poor running it does smell bad rich like and sorta smooths out at higher speed

I don't have lots of cash or time to mess hope to narrow issues down

P0171 is B1S1 lean, (upstream sensor, driver side) which could also cause the P0300 to set. The P0134 means the PCM has not detected activity from B1S1, which means the PCM will richen the fuel mixture. This is probably why the exhaust smells rich;) Oxygen sensors are not accurate if there is an exhaust leak upstream from it. If the exhaust leak has been repaired, and the DTC still sets, chances are pretty good the oxygen sensor is faulty, or there is a fault in the circuit to it. https://blazerforum.com/forum/diagno...plained-35154/