P0301 - Stumped as to cause 5VZ-FE

Check Engine Light keeps returning with P0301, the motor runs perfectly by all accounts. I have checked everything I can think of nothing clears the light for long. Here's what I've done and all were normal:

Replaced plugs (needed new ones anyway).
Swapped coils and plug wires to different cylinder banks
Swapped Fuel Injector to another bank
Measured Fuel Pressure
Performed Compression Check (205psi all 6)
Pressurized coolant and inspected cylinder 1 with a boroscope - no leaks, and coolant level is normal never had to add.
Boroscope inspection of cylinder looks normal.
Measured harnesses from ECM to ignitor, ECM to injector, ignitor to coil - no shorts
Measured ground to chassis, ground to alternator, ground to intake, ground to block, found that all resistances were 0.1ohm except block, which was 10.5ohm. Put secondary ground, CEL came back.
Checked for intake manifold vacuum leaks - none found.
Inspected vacuum hoses and none looked cracked.
Pulled the camshaft and crankshaft sensors, both nominal resistance, crankshaft sensor mount was corroded, so cleaned that up - CEL came back.

All engine parameters pulled at time of CEL, and normal - normal fuel trim, normal oxygen, temps, etc.

Pretty much at a loss of what to look at next. Again, the motor if you just listen to it purrs without any noticeable misfire. Only thing you can notice is there is a small exhaust leak somewhere in the pipe.

 

maybe try cleaning the mass airflow sensor. it worked for me but I was getting a cel for lean conditions in the o2 banks according to the code.
it worth a try and it doesn't cost much except for the cleaning spray. good luck

 

Since you checked every thing but the ECM they do fail !!

I had two get wet before I found the cause on my third one now problem seems fixed

Mine both kept tripping the PO505

No problems for about 20,000 miles

 

Cleaned it, now will drive it around and see. There was a little black gunk on one side of it.

 

Yeah I was hoping to not buy such an expensive part without a diagnostic, but kinda at that point now. The dealer said maybe you have a valve that hangs up every ten minutes. Don't think so personally.

 

I see no mention of any testing/inspection of the igniter module. It's an integral link in the ignition chain of command. Checking for good connections is easy, actual testing is tough for the shad tree. If connections/wires are good, most try a junk yard swap. They are not cheap but far less than an ECU. Then there is the risky world of after market.

Scratch that, I see a cursory inspection of the wiring, module itself could still be bad.

 

Thanks, I had wondered about the igniter itself too, but most posts on web seem to indicate they just die and cause noticeable driveability issues. Will add that to the list to eliminate though. Wondered about connecting a DVA to the cylinder 1 and checking peak voltage vs. other coils. Maybe cylinder 1 has a weaker spark, but not dead?

 

Heck, any testing you can do with what you have couldn't hurt. Pretty much all igniter wiring issues should set a code. The 3 "IGT" circuits can be watched with an o-scope or possibly with some of the higher end diagnostic tools. If you can monitor them in some way and see some inconsistency, might provide a clue.
Then on the other hand one could expect a misfire from the igniter to show on both cylinders served by the coil, or not.
All this is why most go for the "swap test". Have any local friends that might that might enable a quick "loaner"?

 

After cleaning MAF, the CEL returned. Same code P0301. Another thing checked off the list!

 

Unfortunately I no longer have an oscilloscope (sat in basement unused for a decade so sold it on Craigslist). Don't know anyone else with a 6cyl tacoma, that I can think of. Maybe I'll check local junk yards. Thanks for suggestions!

 

Since you've ruled out just about everything, I would suspect either the cam or crank position sensor, in spite of a good resistance check. One of those sensors could still be faulty.

 

so when you swapped the coil pack the code still says #1 misfire?

 

Yes, the P0301 (misfire cylinder #1) code returns after all the changes above.

 

Had the same issue with my wife's 04 4Runner. Po306. Did everything you did to check. Ended up being the head gasket. Wasn't burning a noticeable amount of coolant, and wasn't low, but that was the issue for sure.

 

Funny you say that, I was just about to type the same thing.
I was working on a Honda Civic, a regular client so I know this Civic really well. One day he brings it to me with this same MIL code - cylinder misfire. Easy peasy, he wants all 4 new, so we do all 4 new coil packs. Comes back a few weeks later with MIL on again. I scan it and find the same code on the same cylinder. Weird. So this code pops on every now and then for a couple months and I visually check everything and for kicks and giggles, I swap a coil pack with a different one each time. Always the same cylinder. Then one time the civic comes back and I notice the coolant expansion tank a little low. No biggie, fill it up and send him on his way. Then he tells me that his a/c hasn't been working well. I check the refrigerant and it's fine. I notice the expansion tank a tad low again. Long story short, well still long, lol, my bad, the misfire has been caused by an ever so small coolant leak from the head gasket. He didn't want to pony up for a head gasket repair so he brings me some bottle of head gasket fixer shit. Against my recommendation I used this bottle of stuff to try and fix his head gasket. It was a pretty involved procedure (for a magic in a bottle deal). Drain all coolant, remove thermostat, put in straight distilled water with this magic stuff and let car idle for an hour and then let it sit for 24 hours, or something like that. I then remove half of the systems water and add the other of straight antifreeze. Here's the kicker, and I shit you not, it actually worked. He's still driving it today and it's been a year or so. Blows my mind and I still won't recommend it. But holy shiz, that magic bottle actually worked. I wanna say it was blue devil or something like that. Yeah, I think it's Blue Devil. I definitely do not recommend it to anyone, but I still can't believe it. Anyways, sorry to clutter up your thread, but it might be pertinent

 

I'm still suspecting a valve. Had this same issue on my 06 Colorado and it ran fine but for the CEL

 

I had pretty much ruled head gasket out based on 205psi compression and no leak when pressurizing coolant tank but seems plausible given nothing else seems to be it. Maybe I should do a leak down test and look for bubbles in radiator cap. Could do much higher pressure than was able to on coolant side.

 

Have you actually felt the misfire, not that it changes anything just curious. I would think if it was coolant leaking into cylinder you would feel it on start up after overnight.

 

Interesting, I'm running into the same CEL code. Same truck too. Tried most of what you've done and got the same results. I've read a number of articles about the fuel injector becoming clogged, which causes the misfire. But I see you've tested that theory. Never thought about the head gasket with a small leak though. Mine seems to flip the CEL when accelerating - like entering the freeway or going up a hill.

@cory02taco how did you know yours was the head gasket?

 

a watery (but only just) spark plug. Like super clean like you just put it in. Then swap the plug, coil, and injector, get the same code, and pull out another super clean, only very slightly wet plug.
Then took to an excellent tech to verify.