2018, 6.2L, Rough Idle now a P0305 code

[The General]

My 2018 GMC Denali 1500 started idling rough 10 months ago or so and I read here on the forums that the seafoam spray top engine cleaner & lube could help or fix my issue, which it did.  I wouldn't say it fixed the rough idle 100% but it was definitely better!  Fast forward to earlier this week we returned home from a 80 day vacation and I noticed the truck idling rougher than ever but intermittently, yesterday wile sitting in line @ Sam's to buy gas it got REALLY bad almost to the point I thought it was going to die!  After filling up it seemed better but then it threw a check engine light so I swung into Auto Zone and they read the code which was P0305.  Any help is appreciated, think the two are related?   Where do I start?  

 

I bought the truck new, it has 54k miles on it, I have always run 87 octane in it.  

[SILVER SLED]

P0305 is a misfire on cylinder #5.  The easiest thing to do is check the #5 cylinder plug wire and make sure it’s secured on the plug and coil.  

[diyer2]

P0305 cylinder 5 misfire. Lots of possibilities, lifter, push rod, worn cam lobe, coil, spark plug, plug wire etc. 

Good Luck.

[CamGTP]

Start with the easy stuff like already mentioned. Look at the plug wire, make sure it's on the coil and the spark plug all the way. Next step would be to move the wire if it's still on cylinder 5, then check the spark plug itself or just replace it for the $8-10 they run. Past that it could be the coil pack and then if it still there it's internal or an injector failure.

[GMCustomerService]

On 12/3/2022 at 12:49 PM, The General said:

My 2018 GMC Denali 1500 started idling rough 10 months ago or so and I read here on the forums that the seafoam spray top engine cleaner & lube could help or fix my issue, which it did.  I wouldn't say it fixed the rough idle 100% but it was definitely better!  Fast forward to earlier this week we returned home from a 80 day vacation and I noticed the truck idling rougher than ever but intermittently, yesterday wile sitting in line @ Sam's to buy gas it got REALLY bad almost to the point I thought it was going to die!  After filling up it seemed better but then it threw a check engine light so I swung into Auto Zone and they read the code which was P0305.  Any help is appreciated, think the two are related?   Where do I start?  

 

I bought the truck new, it has 54k miles on it, I have always run 87 octane in it.  

This is certainly not what we like to hear, The General.  Please know, customer satisfaction is extremely important to us. Our team would be more than happy to take a closer look into the concerns with your 2018 GMC Denali 1500. To do so, please send an email to [email protected] with ATTN: The General/GM Trucks in the subject line.

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[The General]

So after some simple diagnostics by me and not able to find the problem I took my truck to the dealer for rough idle / engine light on.  After being there for a few days turns out I have a collapsed lifter and since it has 54k miles on it GM is paying to put lifters in one bank of the engine.  I am curious as to reliability others have had after these repairs were completed, also since I am so close to my 60k powertrain warranty expiring I am concerned about problems on the other bank.  

[maxfli11]

I'm a little late to the party on this, but my sons 2014 sierra 1500, 5.3 is giving that code, and the Stabilitrac light comes on with it. Now get this. We took the air plenum off so we could inspect the throttle body, and when we cranked it up, with the plenum off, the code is gone and the miss is either gone or not nearly as noticeable. Can't smell raw gas anymore. Got mass air flow codes and several others but not those. Stabilitrac is gone. Put the plenum back on and it's back to P0305 and stabilitrac. There are a lot of threads on this and most have resulted in replacing the throttle body, some times the TPS. What in the world is going on to make just #5 misfire?