P0301, Tried everything, Suggestions?

Hello, new/old member here, had a 2018 a few years back, sold it for a hybrid lease, ended up back here in a used 2nd gen.

Anyway, info on the vehicle
2015 2.7L RWD SR5 with 168k on it

Some info about myself, been working on cars with my father since I was old enough to hold a tool, he was a master tech at Toyota for 35+ years, we are both stumped by this issue.

About 2 weeks ago I got a code P0301, figured start with the basics check the plugs, looked original, got new NGK iridiums and installed. Pending code came back after a few cycles.

Swapped coils, again, came back after a few cycles, same code for cylinder 1 misfire.

No signs of coolant in the oil, no coolant usage or oil usage, still averaging 24.6mpg, runs fine after a rough start that lasts 2-3 seconds. No puff of white smoke, no strange smell, no sign of head gasket leak or failure.

Swapped injectors 1 and 4, also checked fuel pressure at this point. 41-43psi while running, drops to 30-32psi when shut off and stays there for roughly 10 minutes, drops to 22psi after 1 hour. (Is this right?)

Did block test, fluid remained blue

Did compression test:
Cyl 1: 170
Cyl 2: 165
Cyl 3: 165
Cyl 4: 175

Did leak down test, no hissing from anywhere etc, all cylinders tested around 5-10% this was done with a cheaper setup from Amazon so I’m not sure how accurate that thing was, may have to revisit this test.

Checked plugs again, plug 1 had carbon build up and a dark spot on the electrode insulator, crush washer not crushed, weird because we had tightened it down, also threads dirty further up than every other plug. Plug was dry, just sooty buildup. Swapped plugs 1 and 4, made sure to tighten again. This is the only questionable thing we had found, the threads on both the compression and leak down tester were shorter and they both sealed but the plug itself looks like it got 3/4 of the way in and then tightened up, maybe threads are messed up and the plug isn’t seating all the way? Not sure on this one, going to have to check again.

There is no pattern to this pending code, it happens day or night, cold or warm start, parked uphill or downhill. It can happen twice in a day or once in a few days. There’s no consistency to it at all. Always cylinder 1 misfire, doesn’t always go past pending because it’s so random. No other codes, the only hint I get is the rough 2-3 second start and then plug in the scanner and see it. No power loss, no other stumbling or idling funny.

My father and I are stumped, I’m going to pull plug 1 tomorrow and check it again, I will also be checking the cylinder and valves out with a borescope but with the problem being so inconsistent I don’t know if or when I’ll see anything.

Any suggestions are greatly appreciated! Maybe there’s a test we missed or something we haven’t looked at.

 

I’m going to guess spark plug tube gaskets are toast and letting oil wick up the outside of plug and into the coil.

 

Being it only happens at startup I think I'd want to pressurize the cooling system and look for coolant dripping in the cylinder with a borescope. It only takes a few drops to cause a misfire.

https://www.youtube.com/watch?v=Rurhi-IGaiU

 

no oil on or in any of the coils or tubes that we could see.

 

This is why I want to run the scope into it tomorrow, it’s just odd because even sitting a few hours, few days etc sometimes it happens sometimes it doesn’t. I’d think if the cooling system got up to temp and I shut it off, the next morning it would do it every time. I’ll give it a go, never know, honestly if it is a head gasket, yeah that sucks but it’s such a small leak at least it’s caught early before anything really detrimental to the engine happens. I don’t look forward to a headgasket change and rebuild but at least I can do it myself for far less.

 

I mean hopefully it's not but that is the most common cause of a misfire on startup that goes away after a few seconds, certainly something I would want to check/rule out before doing anything else.

 

That’s what I keep reading, I’m hoping it’s something we overlooked on it, but it’s seriously approaching the head gasket issue at this point. I’ll give it a look tomorrow, crossing my fingers I don’t have to get that far into it.

 

perhaps this might help - more potential candidates

 

I will go down that list and check everything if the scope goes well, thank you

 

Well, I finally figured it out, and looks like I’m ordering a head gasket kit etc, camera revealed a very very small leak but it’s enough to be problematic, this should be fun.

 

Dang, well at least mystery solved.
I'm not sure why but it seems headgasket issues are becoming more and more common on both the 2.7 and the 4.0 as these are getting older now, not sure if it's a coolant maintenance issue or if it's just heat cycling is eventually causing the gasket to fail.

They often don't leak bad enough to cause any overheating or to test positive on a combustion gas test but leak just enough into the cylinder to cause a cold start misfire.

 

I was going to say bad gas or water or some sort of condensation crap getting into the fuel system hence causing a bad air/fuel mixture. Yeah, i know...seems like the story only fits in my head. Glad you got it all figured out.

 

The head gasket problem is also a VERY common problem with the Gen3 Prius (2010-2015) and would also manifest (initially) as misfire at startup (along with shaking and rough running until the coolant in the cylinder is burned off).

PERHAPS it was a quality problem the head gasket source that Toyota used at the time?

 

very possible, makes me hesitant on ordering the gasket kit from Toyota directly, but it does look like it has been updated with new part numbers over time.

 

Took a week and a day only working on it 2-3 hours each day, but it is all back together. I did end up going with OEM parts, just got the entire gasket and seal kit. No leaks, double checked everything and it’s running great! Thank you to everyone for the help/suggestions!