Have a 2003 s10 pick up 2wd with a 4.3 came into shop with check engine light not on but code po300 stored.Went to misfire data saw no misfire current but history cyls 1 and 4 misfire mostly cyl 4.Did Tune Up With AC plugs,Cap Rotor AND wires.Test drove and showing misfire only on cyl 4 and only with rpm over 1700.Did comp check have 175 lbs,did in balance all cylinders drop same amount.Did leak down cyl holding ,changed cam and crank sensors.With truck in park It only showing misfire above 1700 rpm.even just to make sure I swapped injector 2 with inj 4 .truck doesn't feel like it's actually misfiring so I'm not sure if e c m needs to be replaced because it's no good,or am I overlooking something.

 

not sure for blazers but full size pickups must have a crank sensor variation relearn procedure done after crank sensor replacement. if not done, false misfire dtc will set at high rpms.

 

Found ecm to be bad grounding cam sensor. Replaced it and all is good thanks for feedback.