Intermittent P0304 cylinder 4 misfire ( not plugs or coil)

Hoping you guys can help me ou! Vehicle- 2012 4Runner 4.0 1GR FE
155k miles, daily driver, well maintained. History of rodent damage.

Problem- Vehicle threw a P0304 misfire cylinder 4 code with flashing CEL and was running rough( active misfire)
Put new Denso Iridium plugs in all cylinders and replaced #4 coil with new Denso coil.
Truck ran great for about 25 miles, then it threw the exact same code with the same symptoms. Moved coils around and confirmed it was still cylinder #4 misfire. Cleared the code and it drove ok with the exception of a couple misfires while rollingto a stop but not enough to throw a code and make the CEL pop up.
I took it to another shop and they drove itnearly 70 miles before they got it to throw a code again, P0304.
They claim it’s a valve issue because they checked compression and the compression is good, but saw fluctuations in the vacuums.
They recommended to pull the heads and service the valves at a cost of $8k
Both shops couldn’t find rodent damage and both confirmed good compression.
No backfires or any other symptoms havebeen noted.

My questions are-
1) what steps would you take in further diagnosing or troubleshooting this vehicle?
2) Could a valve issue cause this type of intermittent misfire?
3) Are these motors known to have valve problems?
4)What do you guys recommend ingeneral?
Thank for reading and any helpful advice is greatly appreciated.

 

You seem to have listed off things pretty diagnostically, but man, 8k seems like a big leap to take on a vacuum fluctuations. But maybe that’s an established test for bad valves ??? (I’m no expert). Maybe they can explain why it’s the cause, and not some overlooked chewed wire.

8k is a serious chunk of change I’d only spend if it’s a keeper, and they are absolutely sure it would solve the problem. It would truly suck to do this and still have the problem! Ugh. I’d consider paying someone extra for a second opinion, even paying full freight for an honest guy to do pure diagnostic work. Also, borescopes are cheap nowadays… if it’s a bad valve maybe it’s something that leaves tell-tale signs that can help nail down the cause.

Good luck

ps, are misfire event codes tripped by electrical misfires (no spark/poor spark), or are they sensed via lack of combustion?

 

Exactly. 8K, huge! The vague "rodent damage" doesn't help. If that refers to chewed wires here, there and who knows where then it's a different animal. The 1GR FE is probably fine.

 

Thank you all! This is helpful information.

 

Check the injector wiring for rodent damage. Pretty common issue.

 

I will check the injector wiring. Replacing the injector was suggested by the original mechanic, so when he is going to replace it I will ask him to do a thorough inspection.

What do you mean when you say common? Do you mean mice have been known to chew these (injector) wires specifically?
I live in a rural area on a ranch and have had mice chew the knock sensor wires on this car and another wire that I can’t remember what it was within the last 3 years. Thanks again.

 

For whatever reason the injector wiring specifically is a target maybe because it’s easy to access. I had this issue when it wasn’t even 2 years old and it was almost $9K to fix. I have since removed the engine cover and I spray Grandpa Gus’s peppermint oil.

I think you would see a vacuum fluctuation with a dead cylinder.

This is a pic I took through the intake runners:



These are pics from the dealer:

 

Oh man that sucks. I’m sorry you had to deal with that.
Thank you for sharing and explaining this info to me. This is extremely helpful and I will certainly go this direction for further diagnosis.
I agree you would see vacuum fluctuations in the dead cylinder. I think the shop didn’t want to spend more time trying to find the problem and just threw the valve theory at me so I would just leave.
I like the idea of removing the cover and have been using peppermint oil spray on my Taco because I have a suspicion this was rodent related. The 4R will be getting sprayed thoroughly once I get it back. What symptoms did you have when the inspector wires were chewed?
thanks again

 

Flashing CEL and rough running. I forgot what code was being thrown. A trac light is normal with any CEL is the ECU is designed to shut off any extra driving aids except the essentials to protect from any more issues.

They could perform a leakdown test to see if you have a valve issue as well.

 

You may also want to use green mouse bait in the engine compartment/garage.
Something else to chew on.. instead of your wires.
Will at least confirm vermin presence.

 

Ok thanks. Seems like the exact same symptoms with the exception that mine has been intermittent and first threw a code while driving, not after being parked overnight. The shop claimed they did a leak down but only after I asked them…

 

Intermittent is the key word, I would suspect something electrical. Check the injector and coil lines, I mean check them with a tester not just visually. Also check connector corrosion or damage. If all that is good you can use an endoscope to take a look at the intake valves. You can also use a valve cleaner, the spray used for GDI engines. But if the intake valve is dirty I would suspect a bad injector. Don´t forget to check several cylinders for the intake valves.

Good luck!

 

Update-#4 and #6 injector wires were “repaired” previously. They obviously didn’t do a good job. New pigtails ordered and hopefully this closes the case, fingers crossed. Thank you all for the information!

 

Wow! That looks like I fixed it.

 

Wow...Is that from the $9k repair bill done previously?

 

Wow, hopefully that’s it. Doesnt inspire confidence in both of the shops that looked at this (or didnt) and were fine with throwing out wild ass guesses. 8k quote without suggesting maybe doing a once over on those wires. Shaking my head on that. Lol

 

No this is from a year or 2 ago. Completely different shop.

 

Hopefully that solves the issue. Keep us posted.