2000 jimmy p0300

My daughters 2000 jimmy with a 4.3 has 120k on it has a p0300 code I replaced the cap rotor wires plugs fule filter fule pump ( it quit )replaced any qustanable vac hoses ,it has all ways ben well cared . at low idel it is smooth at about 1/4 throttle it is a little rough . it comes and go's it will run great then miss .tempture don't make a diffrance
It would help to know which cylinders are missing. Also a fuel pressure reading and see if it holds pressure with the key off..

Was it missing like this before changing the ign parts?
Has anyone attempted to change the timing?

Is the check engine light coming on at all?
 

the fule presure is good.


yes it wamissing befor but not all the time you can drive it allday without it missing.
no one has touched the timing

the light comes on for about 5 minutes then goes out
The "check engine" or "Service Engine Soon" light is an indicator for a system that monitors many electronic functions in the system, also called the "malfunction indicator lamp" (MIL). It can come on for any of hundreds of different causes from any of the systems that the computer monitors. Some are very important and can lead to further component or engine damage and some are emissions related not as serious but still a problem and need to be repaired.

Your first step is to have the computer scanned to retrieve the trouble codes that were set when the check engine light came on. Those codes will give you specific information about what the computer saw and will get you pointed in the right direction to find your problem. With a 1996 or newer and some 95 cars and you have an AutoZone or Advanced auto parts in your area, they will read them for free. Once you have these code numbers, we can be a lot more specific about what your problem may be.

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.