CYL 4 Misfire issue

Ok so for starts im a comm small engine tech and familiar with in depth efi engine diag but dont have access to diag software for this vehicle. having said that, i also know how stupid it is to "shotgun approach" a diag but since that parts were cheap i did exactly that. My wife has a 2010 cobalt 2.2l. it is throwing a cyl 4 misfire code and its completely iso'd to that cyl (at least for now). I first swapped the ign coil with another cyl and it still threw a 4 code. Then i went ahead and did the service parts i had yet to do that might affect run so i did fuel filter, plugs and even ran some injector cleaner thru system. Still not helping i went a little deeper and actually replaced the #4 injector.. surprise surprise still didnt help. then as a last ditch effort i thought poss so issue in connector to coil so replaced it. I have noticed on posts for other years people talk about ground issues but im not aware if they have sep ones for each cyl bc if not i would guess it would cause random misses and not always the 4. This issue has been going for about 4 months so my thinking remained at something that would be iso'd to the 4 but since im not familiar w this vehicle that way i could be wrong there too. Any help from someone experienced with this car would be greatly appreciated. Thanks

It sounds like you've done the basic checks, ignition, plugs, fueling. Next up I think is going to be valves and seats. You can also try tossing in a tank of premium to see if the misfire goes away.

YelloEye said:

It sounds like you've done the basic checks, ignition, plugs, fueling. Next up I think is going to be valves and seats. You can also try tossing in a tank of premium to see if the misfire goes away.

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yea already ran some premium when i first ran the bottle injector cleaner but it prob wouldnt be a bad idea to just run straight prem for a month and see what it does... it can come and go so im holding off on assuming its a value (mostly bc i dont want it to be) ... i was wondering if anyone know if could poss be CPU related or sensor of some sort.. again any help from people really familiar with this car is appreciated..thanks

The KR sensors are really touchy on these motors, but that shouldn't be the issue unless you're seeing massive knock.

no knock and the issue will smooth out at times but when it's acting up its always poor idle followed by hard shift from 1st to 2nd with mid-high rpm (bc of the miss) but even when its doing that it'll still shift smooth thru the rest and then ride smooth.. no other real noticeable issue other than slight mpg diff

Any chance the ignition loom has damage?

YelloEye said:

Any chance the ignition loom has damage?

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now that im not 100% on and i will check asap... when i replaced the coil conn.. i went back with the new wires past the orig bend bc didnt like the look of it but once had that old section off (at least the first 6 in.) it ohm'd fine.. i also had that ground off thats not far from there but didnt look bad... is there any chance this could be CPU related at all? also are there any other trouble spots where the loom bends that you can recommend? thanks

The usual spots I see are less than 6" from connector ends. When are the misfire events happening? (load, cold start, etc.)

YelloEye said:

The usual spots I see are less than 6" from connector ends. When are the misfire events happening? (load, cold start, etc.)

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YelloEye said:

The usual spots I see are less than 6" from connector ends. When are the misfire events happening? (load, cold start, etc.)

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yeah that was the spot i noticed on the wires about 6'' back that actually led to me r&r the connector bc it came with about 10'' of wire... as far as when its usually always at low idle situations.. occasionally right after start up the engine light will flash then stay on.. other times it just comes on at some point during a drive cycle w/o flashing.. when it does start missing i can tell bc if at idle it will get rough and then when i accelerate it'll jerk on the auto shift from 1st to 2nd.. then its fine cycling thru the rest and (to this point) does show symptoms at driving speeds... the nature of those symptoms kinda makes me think buildup on value poss causing to hang up but since im not really familiar w this car i hate to guess esp on that... i did put some royal purple in and switched to high oct fuel (only 1 tank in so far) and will cont to run that fuel for few weeks and see if helps.. let me know what u think..thanks

Sounds like a compression issue. I would run a compression check on that cylinder and compare the readings with the other cylinders. If it is more than 10% more than the other cylinder then you have a compression issue causing the misfire.