P0301 after engine oil overfill

Hello folks -

This was some time ago, so I'll try to describe the sequence of events as best I can from memory; and then talk about where I'm at today.

Vehicle, mileage, and driving conditions:
-2014 PIP
-243K miles, 90%+ highway miles (60mph+) of which maybe 5-10% of that is mountain highway
-Plugs last swapped at 145-150Kish (NGK Laser Iridium)
-EGR system & intake cleaning at 175K
-The usual cold engine knocking returned at some point after EGR cleaning
-Oil burn that has been going on since around 75-80K miles
-Frequent topping of oil, but not the greatest drain/fill, filter replacement intervals (figure 12K miles between oil and filter changes)
-I don't think that I have ever changed the engine coolant or even topped it off; but I could be wrong on this.

Back in February, the morning after an oil change, I backed out of the driveway and I immediately heard a noise of which I've never heard before. Immediately pulled back in the driveway and placed the car out of service. The next time I fired the car up for diagnosis I had a P0301 misfire CEL. I also noticed that the oil was overfilled.

Over the next couple of days, I played around with swapping coils and inspecting plugs and also siphoned out a decent amount of oil. I also think that I put the plugs back in the original cylinders; but I could also be wrong on this. Unfortunately, I don't recall the sequence of events or exactly how much oil I drained; but I also had some other random misfires during this process (I don't recall if before or after oil draining; but I think before since I had to buy a pump before I did that). I also placed a piece of tape around 3/4 inch below the LOW level on the coolant reservoir to mark it's level at some point as well. The coolant eventually rose up to the LOW line. I cleared all the codes and misfires eventually stopped; but I've probably only had a couple of cranks until this point, and maybe 1 mile of driving.

Getting a bit lengthy here; but I finally got around to doing service over the past couple of weeks.

Cleaned the EGR system and intake. Drained and refilled the coolant with Toyota Super Long Life. Replaced the PCV valve. Installed an OCC. Things seemed pretty smooth doing test drives, about 50 mile total with the cowl, wipers, and everything still removed. No codes, no pending codes. Engine sounded good, but not perfect.

Today, I decided to do a more thorough inspection on the plugs. I bought a compression tester; but decided not to open it. Instead, I opted to do a paper test and had a small amount of soot come from cylinder 3; but no liquid or signs of coolant in the wells (other than sludge on plug #3. See pictures. Plug #3 also wasn't torqued down when I removed it, so I'm guessing that explains the sludge on the insulator). All plugs had white on the ground electrodes, I'm guessing from overheating, and also clean looking oil on the insulator grooves. Plugs 1, 2, & 4 all had the electrode insulators damaged and parts chipped away (I've read that this is due to "detonation"; but I don't know). These plugs just have an estimated 95K miles on them and I did buy them from AutoZone brick & mortar.

Anyways, today I also put in new SC16HR11 plugs in from the dealer; and reinstalled all the cowl and assembly pieces. The engine is running seemingly brand new now after roughly 40 miles of test driving today. Not sure if this is due to all of the insulating pieces being added back in or not; but I can hardly tell when the car goes to/from EV and ICE, the transition is nearly seamless. No codes, and the coolant level seems stable after a few days and maybe 100 miles of hard driving. Cold startups have been good since the EGR cleaning last week.

Now, I've read nearly every thread here on misfires and head gasket failure, and I know the working assumption should be a HG failure. I'm theorizing that my plugs were bad and, additionally, something happened due to oil overfill.

Shoot me your thoughts! Would you trust driving this car for long distances? Do I seem to have any signs of HG failure? How many miles without a misfire would you be comfortable with to see if I'm out of the clear of a HG failure?

This site has been a great help. Thanks to all that have posted questions and comments over the years!

All_plugs by Ceeb posted Nov 1, 2023 at 6:57 PM

Plugs_1-2 by Ceeb posted Nov 1, 2023 at 6:57 PM

Plugs_3-4 by Ceeb posted Nov 1, 2023 at 6:57 PM