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

Roger Rogers · October 10, 2021

On 10/7/2021 at 9:37 PM, CrawlSlow said:

Long shot but try the following, clean MAF, clean Throttle body and perform idle relearn with scan tool.

I've done this. It's interesting. The idle gets better. Didn't go away entirely, like when I cleaned all the intakes and exposed parts of the valve. But it's not much different. I think it's just better for now and then I feel like the computer starts trying to learn how to solve the misfire issue and things get worse.

I'm a noob mechanic, but the fact that the misfire could go away entirely, even if for 20 minutes of idling, and always when at highway speeds, would seem like the cam shaft isn't a problem, right?

The misfire code is 100% consistently 0305 (cylinder 5). Is there any possibility this is actually a more random misfire, but being attributed to only one cylinder, or the cylinder is wrong? Cylinder 5 isn't AFM, so my understanding is that it is much less likely to have a collapsed lifter. But then, the problem doesn't seem to indicate that anyway... so.

Next things I'm going to try unless someone tells me DON'T DO THAT!:

- Run Marvel Mystery Oil in the oil for 500 miles

- Pop open the valve cover on cylinder 5 bank and see if anything is sticking, etc

asilverblazer · October 11, 2021

I've had bad new parts... Swapping plug wires, plugs and coil packs around aren't terribly hard and might confirm that the new parts aren't bad.

Any difference in misfire count hot vs cold operation?

What about running premium fuel?

Joseph J Pahler · October 12, 2021

I’m thinking injector

On 9/24/2021 at 11:27 AM, Roger Rogers said:

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

BTW, really appreciate your feedback!

I’m thinking bad injector