It’s obvious that you have had the codes read, either with your own code reader or by someone like what Hammer has suggested, otherwise you wouldn’t even have/know the P0300.
Are there any other code(s) is my question?
Also going back to Sidom’s question; it would help knowing whether this random/multiple misfire was happening too all cylinders, a couple or just one.
Knowing which particular cylinder(s) or whether it was all cylinders randomly misfiring, doesn’t tell us what the problem is but it could narrow down the search.
As in, if it were all cylinders; we would look at things that are common to all. If it were only one or two we would be looking at things that will only affect those particular cylinders.
With a proper scanner (not a code reader) you would be able to watch the misfire counters for the individual cylinders; thus determining the above.
With what I can make out from the other posts; you replacing the mentioned ignition components and fuel pump; didn’t make a difference?
As for the ¼ throttle roughness; well that might be normal; it is a 4.3 and they are not the best balanced engine; that Chevy has ever brought out. I have literally seen cigarette butts get shaken out of the ashtray during a no load (in park) rev of the engine.
If you’re going to give this repair another shot in the dark attempt; I would have the central sequential port injection (CSI) professionally cleaned.
CSI systems use a single fuel control valve that supplies fuel too mechanical poppet nozzles (injector nozzles). The nozzles are opened mechanically with the use of fuel pressure acting against a spring. They were/are well known for intermittently sticking; either allowing to much fuel (leaking) or not enough (no) fuel; too one or more cylinders.
Like I said professionally cleaned; don’t waist your money with the pour in the tank carp; that stuff has more propellant in it, then cleaners. I call most of that over the counter, gas station cleaners, feel good injector cleaner. All they do is make you feel good that you’ve done something; which you have, spent money on nothing. LOL
That being said the pour in the tank cleaners are more of a maintenance item; when there isn’t a running issue due to dirty injectors.
Most up to date shops will have some form of cleaning system; that they’ll shut down the vehicle’s fuel system and run the engine via a machine that has a very concentrated cleaner.
Dan.
Canadian "EH"
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