Hi all, I have searched for days, but in case I missed a thread that covers this issue, please feel free to redirect me.

I have a 99 Blazer 4wd 4dr with the finicky 4.3 vortec. When it is cold, runs fine, however when it gets up to normal temps, acceleration is jerky, misfiring from stop to cruising speed, where it levels out and behaves normally. If I floor it, it seems to go through the gears a lot better, if not completely normal. Had it read at AutoZone, the dreaded code 0300 of course.

What I've done:
1. fuel pressure test- 64psi with key on engine off, when the pump finishes the cycle it drops to 58, with no considerable drop thereafter in the 1st 10 minutes, maybe a couple lbs. 56-58 psi while idling, no miss or shuddering. Checked behind the filter, I get the same readings.
2.New AC Delco iridium plugs, Bosch premium wires, swapped coil and ignition control module. New air filter, pcv valve. Cap and rotor look reasonably well, not new, but not worn excessively.
3. Before I knew any better, I put an aftermarket fuel pump on about 13 months ago, but it has acted fine and i'm not positive but I believe the pressure is in the ballpark, as well as the fact that it responds fine when I stomp it. (I know, I know AC Delco or bust from now on, lol)

Could this be a vacuum issue? Timing off? Cam retard? Push the darn thing in the lake?

 

P0300 is almost always caused by a fault in the secondary ignition system, (Plugs, wires, cap, rotor and ignition coil). Low fuel pressure, (yours is fine) and incorrect camshaft retard adjustment can also set it. Can't tell much by just looking at the cap, unless it's an obvious problem. If it's been a while since it was replaced, or it's an aftermarket cap, replace it with AC Delco and put a dab of silicone dielectric grease on each terminal, inside and out. If that doesn't cure the problem, next step is check/adjust camshaft retard. Must be done with a capable scan tool, the el cheapo $500 jobbies that the auto parts stores have won't access the data.

 

The captain is right about the cap and rotor; I just had the same problem with a "good" looking aftermarket cap and rotor. As soon as I got the AC delco parts in my missing and PO300 codes were gone.

 

Thanks, strangely enough, I stepped away from it for a few days, and now it seems to be running great. I can feel what I think is a slight occasional miss now, but nothing even close to how it was. I plan on swapping the cap and rotor anyway just to get it all new in the tune up dept. The one thats on it is Delco, but as I said, it's not new, so better safe than sorry. The CEL is off, though so that's great. I don't really get why it ran rough again on the 1st test drive, then 3 days later after letting it (and my brain) sit, runs like a top. Maybe the truck was compensating for the problem, then was slow to go back to the right settings after it was repaired? Oh well, better to not look a gift horse in the mouth, right? Thanks again for the help, good to see I'm not the only one who loves these fickle things enough to put up with all the crap that can go wrong with em.