tech2

if it won't misfire when floored or shortly thereafter...its probably not cats...that is when the backpressure is highest.
p0300 random misfire...."random" means it can't identify which cylinder is misfiring not an intermittent condition. ...but if you go into misfire data...the offending cylinder counter stores it.

tech2

check to see if the tcc is coming out or into full lockup when the dtc sets. typically...the monitor is pretty smart and usually won't set due to a tcc shudder.

Crystalflow

is that the torque converter?

Crystalflow

The counter is always the same on the misfiring cylinders. 4 are at 0 misfires. The other 4 show misfires to different severity. I knocked the cats today and there was no rattling. They seem intact.

I'm leaning towards bad injectors...

I was going to run an injector cleaner through the tank and monitor my fuel trims.

tech2

haven't replaced many injectors on chevy's... they are usually pretty good....if its not misfiring under full load, I doubt its injectors. Wot is when the fuel demand is highest....the fuel trim would be adding to it adaptive limit 30%. Lean misfire would show lower post cat 02 voltage readings. ignition misfire would should high post 02 voltage reading.

gm has an active fuel injection tester that measures injector balance. injector cleaner through the tank won't do much if anything for a plugged injector.

Crystalflow

Thank you for the advice. I won't have a chance to do much troubleshooting until the weekend. Where is the next logical place I should look for issues? I can pull 02 readings and other variables with my scanner to narrow things down if that is helpful?

Crystalflow

I didn't check the fuel trims at full throttle, just misfires. I'm guessing this information might be helpful as well.

tech2

fuel trim is only relevant when the engine is not misfiring...so if its not misfiring at wot you might get a clue

Crystalflow

Ok. So I just swapped injectors from one of the misfiring cylinders to the good one. No change.

Could a leaking intake manifold gasket be the culprit?

To recap: the misfires occur on the same 4 of 8 cylinders at highway speeds, or revving above 2000 rpm in park.
There is no misfires at idle and no difference when spraying carb cleaner around the intake.

mountainmanjoe

Did you eliminate all the causes in the link for P0300 I posted?