Hello, I am new to these forums, so please keep that in mind. I have searched through these threads for something similar to my condition, but I have come up with no luck, so I started a new thread. If by any chance I happened to have missed it, please direct me to that thread so I can take a look because this has me at my wit's end. If I have posted this in the wrong forum topic, I am sorry for that as well. I will give a quick run down for those that don't feel like reading a small story and then go into detail to completely explain my problem and steps I have taken to try to correct it.

Long story short, I am misfiring in cylinders 3, 6, and on occasion 1. I have changed out my cap and rotor, plugs and plug wires. I have also done the MFI conversion for the fuel injectors to get rid of the poppet nozzles. My compression is good all around (150PSI+). I have cleaned my MAF and I still cannot get my check engine light to stop flashing at highway speeds. I get P0300. Hope this gives someone any possible fixes for me. And now the details.

Okay, so I about a week ago I got a 2002 Chevrolet Blazer Extreme 2WD. It has almost 83,000 miles on the 4.3L Vortec V6. Everything seemed fine until I drove it on the freeway for the first time. Once I hit about 70MPH, my check engine light started flashing. I know that means that my catalytic converter is going to clog up and get ruined if I keep driving it while it was flashing. So I slowed down and was about to get off. Once I got down to 65, the flashing went away and the check engine light did not stay lit. I sped back up and the same problem happened. I basically knew that I was misfiring.

I assumed that a tune up has never been done, so I figured I would do it. When I got home, I changed out the spark plugs and wires. Changed the oil as well. I go back on the freeway and I get on it while I'm going on the on-ramp. As soon as I hit 70 and above, the Malfunction Indicator Light begins to flash again. I slow down and this time it stays on. I go to my shop because we have a scanner there. The code was P0300- Cylinder misfire detected. I go into the datastream on the scanner and it shows that cylinders 3 and 6 are misfiring. It does it at idle as well, but not enough to set the check engine light off or cause it to flash. I drive it home in town and the light goes away.

The distributor is what popped into my head. So I changed out my cap and rotor. Problem still not fixed. Same thing happens. From there, I started thinking fuel problem. I read that the poppet nozzles on this are notorious for going bad and saw that many people to the MFI fuel injector conversion. I spend the $300 and sprang for the conversion. After I get them in, I check it with the scanner and it is still misfiring and now cylinder number 1 does it every once in a while when I rev it. It might have been there before but I might have not noticed it.

Having put in a new fuel deliver system, new cap and rotor, new plugs and plug wires, I now think that maybe there is something wrong with the internal components of the engine. And easy way is to check my compression, so I did that. It was 150+ PSI all around. So my valves are good so it can't be something as serious as that, which is a sigh of relief.

I read that the MAF sensor can sometimes cause this problem, so I get some MAF cleaner and clean it out, but I had no luck with that. I am seriously at my wit's end with this truck. I have tried everything that I know and that comes to mind. I need your guy's help.

I used to be able to set the check engine light off and cause it to flash when I would rev it in park or neutral, but after I replaced all these parts, it only does it when I am driving at high speeds. So I guess I fixed the problem a little bit at least. The engine does not seem to be greatly affected. It does not bog down, it starts right up, I can cruise just fine in town. Once I get on the freeway, it goes downhill once I hit 70MPH. It seems to begin to vibrate a bit and then the light comes on. My gas mileage isn't doing that great, I don't know if that because of the constant misfires even while I drive it in town or if the Blazer does not get that good of mileage to begin with.

So yeah, that's basically about it. Well at least all I can remember off the top of my head. Did I miss something in my diagnosis that could lead to me fixing my truck? It's a nice truck and I would love to keep it, but I don't want to keep pouring money into it if I cannot rectify my problem. Please, any insights are more than welcome. I have run out of ideas. If you have read this far, I am very greatful. Thanks to all of you who have taken a few minutes of your time to read through my thread. I hope to get some responses soon so I can finally not have to worry about my new vehicle.

 

sounds like my current headache. my '98 has developed the dreaded P0300 code, but causes a retarded shift out of first gear if i drive like an old white man. if i accelerate quickly (but not floored) it doesn't do it, and isn't as noticable when warmed up. i've done complete tuen up as well when it first started (use only AC Delco parts on these engine), even pulled the distributor to see if the gear was worn/chewed up, still no luck. i've been told by a few it's the lower intake gaskets, but i still think it's an electrical issue (sensor).

here's a link to my post (similar to your problem). see if there is anything there to help.

https://blazerforum.com/forum/2nd-generation-s-series-1995-2005-tech-41/runs-like-s%2A%2At-st-st-stutters-34744/

 

The lower intake is a good theory that I never even thought of. It can cause a vacuum leak which in turn would cause a misfire because it's running lean I suppose. I changed out my upper intake plenum gasket when I did my CSFI to MFI conversion. I didn't do the lower though. I will talk to my auto teacher and see if he has a smoker to see if we can find a leak there. Thanks. I will definitely put this on the top of my list, especially since it is the first theory haha. Really, thanks though. As to your problem: I have heard that the ignition switch is tied into the TCM and if that is going bad, it can cause that sticking before it shifts and also causes the misfire. Look into this post https://blazerforum.com/forum/2nd-generation-s-series-1995-2005-tech-41/symptoms-p0300-33612/ I am not sure how accurate this is, but maybe this will help you a little.

 

I would have the ICM and coil tested. It sounds like load and RPM are a factor which could be contributing. If/when you have these two components tested, have it done repeatedly.

A few other good threads include:
P030x Explained (x=0,1,2,3,4,5,6)
The dreaded P0300 CAUTION LONG POST

 

The flashing check engine light happens to me too.. It only does it when im at highway speed and when im going up a hill in over-drive.. And turns off within 10 seconds after it shifts out of over-drive.. Still don't know what it is....

 

If my coil were bad, wouldn't I have a random misfire instead of multiple of always 3 and 6?

 

There may be something else (carbon in the cylinder, etc) that is causing it to be consistently those cylinders, but an ICM and coil test would rule out those to components from being the cause. This problem just screams ignition/spark problems.

 

I agree. The coil would be my first suspect.

 

the first thing i did was take it to my trany guy, whom i've known for more than 15 years, and had him test drive it. then i drove it to show him how it would act up. we scaned it (found nothing relatng to the TCM), he even pulled off the valve body to visually inspect it when he did a full service on the underside of the truck. so there's nothing wrong with the trans.
and driving like i do, it would show up very quickly, 'cause the damn thing only acts up when you ***** foot it around (drives my wife nuts when ever she needs to use it).

 

How would I test my icm and ignition coil?