I recently picked up an '02 Blazer and have been becoming increasingly confused and frustrated with it. I am having a misfire issue that I can't seem to chase down. Here is everything I have done so far and my current symptoms:
-Replaced the fuel filter when I first acquired the rig.
-Did a tune-up with all new parts: AC Delco Cap and Rotor, AC Delco ignition coil, NGK Wires, AC Delco Spark Plugs, K&N Air Filter.
-Replaced the fuel injector 'spider' (when I first bought it, it had a more significant misfire and hard starting that I traced to a leaking 'spider' component by performing a fuel pressure leak down test). Ended up going with the MFI conversion assembly (P/N: FJ503) because I had to get it done in a weekend and didn't want to try to chase down individual leaky component(s).
-After replacing the injectors, the engine ran MUCH better but it still has a bit of a misfire that I can't seem to figure out. Seems to run fine at speed but has a slightly rough idle.
-Yesterday it threw a CEL with a P0301 code (misfire, cylinder 1) so I checked the plug, wire, etc. and reset the codes. It then shortly came up with a P0306 code (misfire, cylinder 6).
-I had originally installed copper plugs but today swapped to the OEM iridium in an effort the fix the misfire to no avail.

A couple things that may or may not be related:
-When I hook up the fuel pressure gauge it will initially got to 45-50 psi when I turn the key and then drop to about 30-35 psi and maintain about there when running. After shutting the engine off, it will retain about the same pressure. This seems like normal operation but I am not sure; usually I deal with pre-fuel-injection engines.
-Sometime when I put gas in it, it will be difficult to get the pump to stay on. It will keep shutting off like it is full but it is not.
-When it is running, I run the 'EVAP system leak test' on my OBD scanner and it will fail. Once I shut it off, it will pass; again, not sure if this is normal or not.

Any ideas/insight for what I can look at next? I am thoroughly mechanically inclined but I mostly deal with pre-1980 vehicles so I am admittedly not very knowledgeable when it comes to EFI, emissions components, and all the computerized stuff. I have been able to fumble my way through most of the checks I've found so far with some guidance but now I am at a bit of a loss and getting tired of messing with the thing. My OBD scanner is pretty basic but if there is info I can pull from it that will help with the 'forum diagnosis', let me know.

Thanks in advance for any help or insight.

 

I am just throwing it out there, but maybe try cleaning the Throttle Body and Mass Air Flow Sensor... Might not be the right fix but it definately wouldn't hurt either

 

A few updates (but unfortunately no solution yet):
Cleaned the TB and MAF to no avail. TB definitely needed to be cleaned but the misfire is still there. In the process, I also found a broken vacuum hose and replaced it; that made it run better but I can still feel a misfire.

Just double checked the fuel pressure last night and it actually builds to ~55 psi with the first key turn and will go up to ~60-65 after a second or third key turn. Doesn't seem to hold above 55 for the prescribed 10 minutes but it holds for a long while. When idling, pressure drops to about 50 psi. Found a good deal on a new fuel pump so I'm going to replace it for S&G but that doesn't really seem like the issue.

One page I saw mentioned Long Term Fuel Trim. Mine shows 1 @ ~14% and 2 @ ~7%. Not sure if that helps but does seem like a big difference. Fuel mileage is about 16-17, which seems low but I'm not sure. My girlfriend was behind me yesterday and told me it smelled like it's running rich. My CEL came back on with a cyl. 1 misfire but has since gone away.

Still at a loss here. Might just leave it for the next owner to figure out...

 

I acquired a 2000 Silverado a few years ago, same 4.3 engine and trans. I had the same symptoms as you described, and consequently did everything you have done so far, to no avail. I ended up pulling the distributor again and checking the gears since I didn't really look at them when I pulled it replacing the spider. They were very worn ( truck had 220K miles). Went to Advance, picked up a new one for around $65, installed it and have been running absolutely great ever since. I hit the key and it's purring with half a crank. So you might check your distributor gears for wear while you still have some hair left.

 

Check it at night when running. looking for jumping sparks.

 

Seems like a good suggestion; hooked up my scanner today and it shows the spark advance all over the place between 15.5 and 19.5, not sure if that's normal but doesn't seem like it. Guess I know my project for next weekend.

 

An update and a few new developments:
-Replaced the distributor to no avail.
-Also swapped the oxygen sensors for S&G, no change.
-CEL seems to come and go, as does the roughness of the idle.
-The idle seems to be smoother and the CEL goes away after I fill the tank.
-When I do fill the tank, it has a tendency to shut off early (at times immediately) and will be hard to start right after I fill up.
-CEL comes back on somewhere between 3/4 and 1/2 tank.

Still at a loss on this, hoping someone has other ideas?

 

I have a 89 4.3 tbi it was running rough at start and would shut off of sat idleing for a little .. I replaced the ICM and that fixed it but I’ve seen them so the same thing with a bad vac line or egr ...lots of things just have to narrow it down

 

I'm curious if you have solved this? Thanks!

 

I did not, I ended up getting it to a point where I can live with it. I suspect it is related to the EVAP system (doesn't like to start well right after I fill it with fuel and the misfire/CEL seem to come and go based on fuel tank level) but I gave up trying to figure it out.