UPDATE -

I tested the exhaust back pressure, and was getting ~1.5 in. of H2O, which is way below the maximum allowable pressure (13 in H20). I am starting to think that I have a bad engine computer. Any thoughts?

In general, high back pressure will hurt performance, (clogged cat, smashed exh pipe, etc.)

You mention max allowable, but not @ what rpm - @ idle, 2500?

The back pressure will change dramatically @ different rpms (more rpm, more pressure)

Do you have stock exhaust?

13in. water column is just that - a column of water 13" high in a tube - you could blow that easily.

I doubt that that would cause a misfire

luck,greg

The spec was < 13in of H2O @ 2000rpm, and like I said, it was well below that. Also there is no apparent misfire. The vehicle runs fine, as far as I can tell, but the engine computer doesn't think so.

I do have the stock exhaust, and none of th pipes are crushed. The pressure test I conducted was to check for a clogged cat, which I was able to determine is not the case.

tkb488, you may have already read this

http://easyautodiagnostics.com/gm_co...oil_test_1.php

since you have coil-on-plug, I'm not leaning towards bad coil(*) with a p0300 (random misfire)

I'm more inclined to question the efi or as you said, possible Ecm, although I'm don't think the Ecm would be "random"

I'll read some more,

luck,greg

I read some more about the 4.8 & p0300 and got several hits regarding bad fuel press reg, did you check your pressure @ load as well as idle? (i.e. "brake torque it")

luck,greg