Misfire (mostly on Idle) troubleshoot

Y'all, finally posting after reading over the past 5 years.
Here's my current troubleshoot - similar to other misfire threads but there seems to be a few different causes people have found - so let's see what mine ends up being.

2008 DC 5.7L w/ 308k - I've beaten on this thing hard w/no major issues.

• Developed small rough idle about at month ago - notably when the weather got cold here in Colorado.
• It grew and eventually threw P0301 Cylinder 1 Misfire
• Cleared codes, Swapped coils 1 and 3, replaced spark plug #1 - plug was covered in oil...
  • "OK need to replace spark plug tube gaskets" -haven't yet
• Idle Misfire remains sporadic, no code yet. It occasionally seems to bog down over bouncy roads or the engine cuts out for a split second, so I'm hesitant to keep driving it.

• Coincidentally or not: It's never not started in its life, but last saturday it wouldn't engage the starter and instead threw a bunch of Camshaft and Crank codes. Came back the next day and after checking fuses and relays, and cleaning Coil 1 and 3 connectors with Electronics Cleaner to no avail -- it started after I banged on the starter. Possibly side effect of the misfire when I shut it down...? or coincidence, weird.

So I'm thinking I should open up driver side valve cover and replace all the gaskets and plugs - since I know that's an issue anyways.
While I'm in there I should probably clean injectors? and inspect the valve springs given it's very high mileage. (there's been no additional ticking, but that seems to be one of the most common engine killers on these eh?).

If that doesn't work replace bank 1 coils with four Denso's from rockauto @ $55/ea just to be safe.
I should also inspect wiring in the engine bay, a squirrel had made his own acorn palace when I had it parked up for a month this summer. (didn't notice any bad wiring though).

I'm ordering gaskets today and will update! What are your bets on?

 

Congrats on 300k miles. That’s awesome. Check out this thread I started on ih8mud regarding oil on spark plugs in 3UR engines - apparently the source of the leak is almost always where the spark plug tubes are pressed into the head and not the seals.

https://forum.ih8mud.com/threads/oil-on-spark-plug-threads.1327761/

 

AZ, now that's a development I hadn't thought of. Will read up before opening her up!

 

Replace the starter, pull that valve cover, and inspect those valve springs. My money is on a busted valve spring and the valve is stuck closed. I found those UR series engines can throw a misfire at idle or cold start caused by a partially broken valve spring. Surprisingly, they run okay on three valves per cylinder.

 

You might check your fuel pressure as well. 300k if that is the original fuel pump, it may be getting tired, or the filter could be plugged. just a thought.