Hey all, new guy to the forum here. I've got a 96 blazer (4.3, auto, 4wd) that has been having a misfire for a while now. It's just an extra vehicle, so it doesn't get driven much but the miss is becoming worse quickly. I guess it is time to fix it. The misfire only used to happen when the truck had sat a few days between starts, but now it is happening almost all the time. It is the worst the first start of the day and gets better as I drive it. I performed a tune up in the spring (unrelated to the misfire, it was just time). That included plugs, cap, rotor, wires, pcv, and air filter. This didn't have much of an effect. Yesterday it was plugged into a scan tool and I found that cylinders 1 and 4 were misfiring quite regularly at first, but after running about 20 minutes I had much fewer. I also checked the misfire history, and numbers 1 and 4 were well above all of the other cylinders. It seems like I've got 2 injectors that are going bad or dirty. Does this sound about right?

These injectors can be replaced individually right? not the whole spider assembly like my 94 s-10 had. I know the injector manufacturers say I should replace all 6, but on an old extra vehicle that gets driven twice a week is that worth it?

Thanks for any help or opinions.... sorry for the long post.

 

Bump.... any input?

 

While the injectors could be replaced individually, if you have to replace more than 2, it really is more advantageous to do the MFI swap.

As far as your misfire problem goes, check out the link below:
P030x - Random/Multiple Cylinder Misfire Detected

 

Wow, that link in the link that you gave me was a loooooong read. What I learned was.... pretty much anything can cause this problem. I'm going to do a few inspections on the ignition system and if it checks out I guess the injectors may be a safe bet. I thought about the MFI conversion, but I'm not sure. It's another 100 bucks, and this truck isn't used much. Thanks for the input, any other ideas are appreciated.

 

Well, I replaced the injectors last night and ran pretty good... but not the way it should. Still a misfire from 1000-2500 rpms, but it wasn't too bad. This morning I checked #1 spark plug (that was the one with the worst misfires on the scan tool) and it looked alright. I then checked out the cap and rotor, and they also looked fine. I then drove the blazer and the misfire was worse than the night before. The injectors have helped quite a bit, but truck still isn't running right. The miss is usually under light throttle between 1000-2500 rpms and much worse in 1st gear. It hardly misses at all 2nd gear on. Under full throttle the miss disappears in any gear. I forgot to mention earlier that the SES light also does not work.

Anybody have any ideas on a system to check next? I've dealt with other misfires on 4.3's and know they are a pain. Any help would be greatly appreciated.

 

I hate to add more confusion, but you may also have a vacuum leak, or a LIM gasket leaking. Grab a vacuum gauge, remove your pcv valve and any breathers going into the valve covers and plug the holes on the covers, then hook a vacuum gauge to the dipstick tube. When you start it, it shouldn't show any vacuum. If it does you have a leak. To see if its LIM gasket, do a compession check, if it reads quite a bit higher on some cylinders (like 155-160) it's a LIM gasket leak.

 

Well, here is the update: I checked the truck out on the scan tool yesterday and found that cylinder 4 isn't misfiring anymore, but number 1 still is. I'm assuming that the number 4 injector was bad, but that something else is wrong with #1. When the truck was on the scan tool I let it run for about 25 minutes, and the misfire all but disappeared. That makes me wonder if I might have a LIM leak. Maybe once the metal surfaces warm up and expand it seals the leak? The misfire is also worse in colder temperatures, until the truck warms up. Not just warms up on the gauge, but is driven for about a half an hour. Would a bad LIM gasket just cause 1 cylinder to misfire or would it be all of them? Any ideas on what would cause just the #1 cylinder to misfire cold but then be alright after a drive?


The other weird thing is that my CMP retard was -17 as read by the scan tool. I've never messed with the distributor, but the truck supposedly has had an engine before. Somebody else could have messed with it. This could be unrelated, because it's probably been this way since I've owned the truck. Could the sensor be reading wrong? The CMP retard also doesn't change from -17 no matter what the engine does. Is this normal?


Thanks for the help.

 

Update!

I checked the misfiring with a scan tool again, and this time every cylinder was misfiring about equally. I decided it was time to change the distributor. That solved the issue, the truck finally runs good again. The gear was warn pretty good, I'm assuming that is what was causing my retarded timing, the new distributor ended up a little advanced of the old mark.

I'm not sure if my truck has the wrong balancer on it or what, but I couldn't get the timing zeroed using the timing marks on the balancer. I did finally get it on the correct tooth (when I just went by the mark I had made on the plenum where the old distributor pointed.) It was a pain in the neck, but I'm happy that it's fixed.

 

Glad to hear you got it all repaired!

 

I just a had a problem some what like that. When I went out to start the truck, It ran like crap missing and I thought at one time it was going to die. So I floor it HARD. Smooth right out..but my "Service engine light" came on. So I took it up to the shop and explain to them what had happen and it was running fine now except for light still on. So they pull the code history and 2 and 4 cyl didn't fire at one time. But when I floor it, they went away. They suggested to clean the Mass Airflow and have them run a injector cleaner thru it using their equipment. Did that and it runs fine. But I have my eye on that Mass Airflow...I don't trust it now requardless of being cleaned up...