[mwebb]

GM has misfire counters in Enhanced data, there may be misfire counts present that are not high enough to code the system
so , no Code .

you will need a scan tool with Enhanced GM software , a generic scan tool can not view misfire counters .

and - there' s more
intake vacuum at hot idle should be around 20"
and
the needle must be steady smooth with zero flutter , if there is any flutter , your new used engine has a valve sealing problem

and
if there is a local shop that has Emisfire OR AceMisfireDetective
either of those systems can pick a misfire out of the exhaust pressure pulses synced to spark or IGT on any cylinder.

they work by looking for a vacuum pulse in the pressure pulses , if a cylinder did not fire or did not fire as strong as the rest of the cylinders , when the exhaust valve opens at about 150 degrees after TDC , there will be a Lower pressure or vacuum pulse added to the exhaust pulse train .

not too expensive to connect
i charge 1 hour of labor



the upper yellow trace is exhaust pressure pulses
you can see a regular dip down , a vacuum pulse , and i have overlayed a running compression waveform (720 degrees) below in yellow also with Exhaust valve open EVO time marked by the green cursors.
sync to the pink / red trace .


the software normally picks this out for the user , i just made this screen cap so people can see what the software uses to arrive at the misfiring cylinder
Both programs Emisfire and AMD work very well