Hello everyone! Well like the title says my 2008 LNF has a misfire. Specifically on cylinder number 3. So I've changed plugs, moved coils and all that and it stays on cylinder 3. Checked compression and I'm also good there (thank god). Chevy dealer did the top end service to clean the valves and ran an injector test. Injectors 3 and 4 came up having a bit of difference than the others but still in spec. Now heres where this gets interesting. When I bought the car and brought it home I had no lights for days then all of a sudden it came on but it never once flashed like a misfire does. So I dove into diagnostic work and fixed some small issues like a bad map sensor and the lower charge pipe was leaking so I replaced it with a new stock one. So fast forward after the dealer had it and the light stayed out and it was no longer counting misfires. Drove it about a week and out of the blue light cones back and its counting on cylinder 3 again. So I checked some stuff and saw the downstream sensor wasnt reading right so I replaced it, cleared the light and it's been good for about a week again and then today it popped back on and was counting misfires on 3 again. I have a feeling my throttle body is starting to go due to other reasons but I'm really leaning towards cylinder 3 injector going out. Any other thoughts? Thanks for any help. Also car is all stock and is GMS1.

 

Just out of curiosity what gap are you set to and what plugs are you using?

 

I'm running factory AC Delcos and they're gapped at .032-.035

 

That’s a bit of a head scratcher. What kind of driving are you doing when you get the misfire? Try adjusting the gap .030 maybe? Hopefully someone here can be of better help than me

 

So heres the other thing. I never feel the misfire. Driving or idling. And I'm usually going through town when it happens so top speed of maybe 35 or 40. Not driving stupid or anything. Yea I've been scratching my head for the last month now. This site definitely helped with the earlier diagnosis steps but now I'm at a point where idk where to go next other than injectors or maybe a pcm.

 

Yes but how old are they? Can still be a bad plug. Did u try swapping it?

 

Plugs are maybe 2 weeks old and I have swapped them. Nothing seems to change.

 

Try replacing the pcv valve inside the intake manifold. Its cheap and relatively easy to diy.

 

Found my issue. PCM took a crap. Did what everyone here suggested and no change. Drivability ot the computer with a new one from ZZP and is been running like a top. Thanks for your help!