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2014 Sierra 5.3, 200K miles, code 0305 (misfire cylinder 5)

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Hey y'all. Just joining this forum. Seems like a great community! 


My post title pretty much states the issue. It started in the past few months.


Symptoms: Mild rough idle, gets better with increased RPM - or maybe is just drowned out. Sometimes shows code O305.

Already done: change plugs, wires, coil pack. 

Considering: pressure test, fuel injector replace, maybe isolated manifold leak? Of course, also dreaded collapsed lifter


Have read quite a few posts on this issue already, many point to AFM. However, wondering, does AFM affect cylinder 5? I didn't think so, unless something changed in 2014? Even if not, would you suspect collapsed lifter anyway? Any other thoughts?


The symptoms are mild at present, but I'd like to address sooner than later if that can help avoid (further?) engine damage.


Would appreciate any thoughts at all. This engine has high miles, but they are highway and I'd love to keep it going.



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Only cylinder 1 and 7 are AFM on that side of the engine.   You could do a compression test to make sure that cylinder is even with the rest on that bank. Past that I'd be thinking injector

Right, compression is not a problem here. It's fuel or spark right now and spark seems to be pretty well covered.

Well, I spoke too soon. The misfire is back. So strange, because it entirely disappeared for about 100 miles. I could idle with no misfires, but it's now back to misfiring at idle.   To reca

Only cylinder 1 and 7 are AFM on that side of the engine.


You could do a compression test to make sure that cylinder is even with the rest on that bank. Past that I'd be thinking injector issue.

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Ran compression test on cylinders 3 and 5 and I'm getting exactly the same compression from each (~140 psi). It might be slightly higher, as I couldn't screw in the tester more than hand tight. While not high, that seems decent, right?

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I'm just re-reading your post and realize I only ran on two cylinders and you suggest on the entire bank. I thought I would run on the two non AFM cylinders, but I suppose I should compare across the bank. I'll redo and report back.

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Ok, just pulled all plugs on that bank and retested. I also watched a video on compression testing and learned that I need to turn the engine over about 4 times. I was just doing once before. This resulted in higher values:


Cylinder 1: 205

Cylinder 3: 195

Cylinder 5: 198

Cylinder 7: 190


Cylinder 7 is hard to reach so I couldn't tighten the tester as well, whereas cylinder 1 is easiest and I could tighten it properly. I think that could be a reason for the slightly lower values toward the back. That said, the values seem pretty good and consistent, right?


To recap what's been done so far: plugs, wires, coil pack on cylinder 5. Going to order a fuel injector next. 


Edited by Ethan Erkel
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  • 2 weeks later...

Well, changed all the spark plugs and... I'm still getting 0305 misfire in cylinder 5.




I did find a P219A code. I think this happened once. Not sure if it's recent and I've now cleared the codes in trying to make sure I was in fact still getting a 0305 misfire, which, after waiting a while, showed up again.


What next? Any ideas would be much appreciated.


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Ok, I did a hack vacuum leak test using a cigar. 😉 No smoke came out anywhere. It seems tight.



A few more details: This problem is most noticeable at idle. Acceleration is good and if I am running on the highway a lot (not stopping at lights, etc., meaning not idling, the light eventually will go away. This tells me that when above idle the misfire is happening much less (feel free to correct my thinking).


So I'm at a loss.


Could this be a sticky lifter/valve? Would this happen only at idle? 




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7 minutes ago, CrawlSlow said:

Is it tune by a shop? Or tune with one of those useless junk handheld? (I know it's junk because I have one). Go back to stock and your problem will be solved. 


Sorry, I don't think I understood what you were asking. No special tune. This is all factory tune. Is that what you mean?

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7 minutes ago, CrawlSlow said:

Another thing to check. Not sure if you have gas cap or not. When you open your gas cap, is there back pressure coming out. You can hear pressure being released. 


Just tried. No sound of back pressure. Granted, the truck is almost empty at present. Should there be?


BTW, really appreciate your feedback!

Edited by Roger Rogers
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