Hello! I have a 2000 Chevy Blazer 4.3 RWD that is continually having a cylinder#5 misfire. I started having an intermittent misfire under load when going up hills. I started with a tune up replacing my spark plugs, spark plug wires, cap and rotor button. This did not take care of the issue, which was beginning to happen more often and not be so intermittent anymore. So far I have also replaced the spider injection, mass air flow sensor, the cap and rotor again with A/C Delco this time, the distributor, and the coil pack. I have also tried swapping the plug to #1 to see if the misfire followed it and it did not. I have also done a fuel pressure test which was fine. I have not done a compression test as there is no issue at an idle. I'm at a loss now of what to do and could really use some input on if anyone has had a similar issue and if so what took care of it for you? Thank you.

 

Mist the truck in the dark and see if you have arching

Swap ignition wires and see if it follows

Check your cmp retard value. Look for carbon tracing under the old distr cap

Is this a P0305 code or are you looking at a misfire counter?

Check the condition of the plug

Do the compression test


George

 

I was having a p0306, I did most of the same as you did (Ac delco iridium pre-gapped Spark plugs, ac d spark wires, rotor and cap,took a chance with an amazon distributor lined up at Top Dead Center of Cylinder one and all, and mass airflow sensor) Still had problems. Was going to do new spider injector, ignition coil and cat converter (I still have p0420 caused by the original p0300) just to be sure, until my battery died which initially didn't show signs of going out. I replaced the battery after a few weeks and low and behold absolutely no more misfires. I think my weak battery was causing a misfire in cylinder 6 somehow. It is nice to be able to go 80 again.

 

In addition to the things George has mentioned (I'd try these first, especially misting the truck in the dark). I had a cracked cap that I didn't even know was cracked and didn't result in misfires, but it resulted in a nice shock when I went to change out the oil sending unit and when to put the distributor back in and lined up correctly. One other secondary ignition thing I've run into before is it's really easy to mis-wire the cap (this doesn't sound like your issue, but if it's only gotten worse it might be worth verifying top to bottom on the cap you have the wires going from 5 at the top, 1 in the middle, 3 at the bottom). You'd be surprised at how 'well' these trucks run with the cap mis-wired.

If you have a scan tool that can read codes and graph o2 voltages / fuel trims that might be helpful (you'll need this to check the CMP Retard value as well). If there isn't anything obvious with the ignition system you might have to start looking at other things like this.

I mention the o2 sensor for this because a few years back, I had a dead cylinder 5 misfire caused by a clogged CSFI poppet valve on cylinder #5. I could pretty much tell it was a no fuel type misfire because the exhaust didn't reek of gas, but also it was very obvious on the o2 graph for bank 1, because cylinder 5 is in the middle of the firing order for this engine (1-6-5-4-3-2) so this resulted in the o2 graph dropping lean in the middle of the firing cycle. It doesn't sound like this is your issue either, but I figure I'd mention it.

 

Do you mean graphing O2 sensor voltage or fuel trims? Did you do a screen capture by chance?

George

 

I unfortunately don't have a screenshot, this was from around thanksgiving of 2022 or so.

Both actually kind of pointed to this, in my case graphing the o2 voltages did show this. IIRC what I did was compare bank 1 and bank 2's waveform, and you could very clearly see bank 1's voltage was switching lean (moving towards 0v) where I'd expect to see it switch rich (it almost looked like there was a missing cross count). Then was Bank 2 remained relatively "normal" looking.

Now that I think about it, I'm not exactly sure where I got the idea that you can tell looking at the o2's graph alone you can determine that cylinder 5 is the culprit. IIRC, I pulled the plug wire off cylinder 5 and the o2 graph for bank one looked exactly the same, so I believe I assumed that cylinder wasn't getting fuel, although that alone is probably a bad assumption. The fuel trims did really show this though, I don't remember exactly what the graph was doing, but I remember seeing the fuel trims for both banks and being pretty suspicious of the spider right after all that. (Edit: I also had a P0305 code, so I knew already cylinder 5 was misfiring well before I started looking at graphs).