98 Blazer Misfire
 

Hi everyone, to start I have a 98 Chevy Blazer with 195k miles but only 50k miles on the engine and transmission. I've been having a misfire problem for the last 9 months or so, or at least 9 months where it's been bad enough that it's easily noticeable. It will misfire during acceleration around 2k rpm and is usually worst in first gear, also will misfire on highway (mainly over 65). If I accelerate very fast the problem seems to get better but sometimes will still buck a little when about to shift under high acceleration. Usually wont shift into overdrive when missing and lately I've been getting the check engine light to flash on the highway its missing so bad. Problem seems to be getting steadily worse. It's only missing in cylinders 3,4,5 when it is missing, what the misfires per each cylinder usually look like below.


Cylinder 1: 20
Cylinder 2: 20
Cylinder 3: 5000
Cylinder 4: 60000
Cylinder 5: 10000
Cylinder 6: 20


Those are approximate as I don't remember exactly the last time I scanned it. That pattern is what always shows up, always a ton in 4 and less in 5 and even less in 3 and almost none in the rest.


From what I've noticed is that it runs worse the colder the air is outside, not so much the engine temp, though when it's fully warmed up it will drive slightly better. Whats odd is when its hot out (95 deg) it will run perfect and you wouldn't even know it has a problem, but when its cooler (65 deg and lower) it drives pretty bad.


Only codes it sets are P300 (Random misfire), P304 (Misfire in 4) and P0135 (bad O2). I've already replaced that O2 it's referring to so I'm assuming that’s coming up since the misfires though. Alright now I'll list everything I've done,


New Cap and rotor, wires and plugs (with ACDelco)
O2 sensor (bank 1 #1 I think it was)
Camshaft Position sensor
Injectors
Fuel filter
Checked for vacuum leak
Ignition Coil
Compression test (Compression was solid)
EGR valve
Also checked throttle position sensor and intake air sensor with a multimeter and they seemed like they operated correctly. Also looked at the MAF and it looked VERY clean but never tested it.


Also when I checked the compression I must of pulled/pushed the wire off/on the ignition coil as it started to have a spark jump across the ignition coil. Whats odd though is I plugged it into my uncle's scanner and was watching the misfires as it was running (this was all before I realized that the spark was there) they were all showing up in 3 and 4 and sometimes 5. So there is something that's causing a very hard time for the spark to ignite in those specific cylinders.


If anyone has actually read this all and made it to the bottom then thank you!!! I've been pulling my hair out trying to figure this out and it makes me more mad I haven’t fixed it yet then it running bad. If you know anything about these 4.3l engines and a similar situation to mine please let me know, it would be GREATLY appreciated.


Thanks to all!


Hopefully I didn't miss anything, have any questions just ask.