Single Cylinder Misfire Mystery

2008 Tacoma V6 4.0 Pre-Runner
Milage around 140k

Code- P0306 Cylinder 6 misfire

Parts Changed- PCV valve, All Coils, all spark plugs, and all new fuel injectors.

Still have Cylinder 6 misfire after changing parts. I did a compression test on all cylinders and they all showed 178-185 psi. I have read that MAF sensor, O2 sensors, fuel filter and vacuums leaks cause misfires but they would cause more than one cylinder to misfire. (Not sure if that is true)

I cleaned MAF and I “tested” unplugging it while engine running and it turned off engine. (Supposedly that means its working) and I also do not hear any loud hissing noises in regards to vacuum leak. I also cleaned cam shaft sensors.

Also when I first bought the truck it had rough idle, resolved with seafoam. I had to replace head gasket shortly after. (No signs of warping) After replacement it ran fine. I haven't driven it much, but right when I tried CEL pops up with misfire code.

Could it be the electrical wiring to that cylinders coil or injector?

I was going to run seafoam but would it be pointless if compression test was good and i have new injectors. Or is that a dumb question.

 

Before the head gasket replacement, was the issue also a misfire on the same cylinder?

The question is going to come up, so I might as well be the one to ask it. Were your replacement parts all OEM? if the problem was unchanged before and after the parts replacement, bad parts are probably not a factor, but nevertheless, good to know going in.

Also, which spark plug type did you put in? There is a lot of anecdotal evidence here on TW that the copper plugs work better in the V6 than the platinum type. Seems unlikely that would manifest as an issue specifically on a single cylinder only, tho.

I wonder if a vacuum leak at the intake manifold inlet for that cylinder would localize the problem to that one cylinder? I think probably yes. I can only recall one post in the last couple of years where I saw someone indicate they fixed a problem by replacing the intake gaskets, tho.

 

I think I'd be checking out the wiring to #6 injector or #6 ignition coil. Could be that a wire got pinched or broken during the head gasket replacement.

 

Prior to head gasket truck had hard rough idle but never threw a code, I just noticed my coolant kept getting low and oil was gettin milky. So it never threw a misfire code. All my parts are OEM, and my spark plugs are NGK Iridium XI.

 

With the same code being thrown before and after the parts replacement, I think I have to go with @jjsinaz and the wiring issue for spark or fuel. That or a localized lean condition.

I saw a South Main Auto video where he used a spark tester that was essentially a spark plug with a grounding electrode. It was an easy way to confirm whether there was a spark or not. He also once used a current clamp along with his pico scope to detect that there was less current being drawn on an injector at the time of a particular misfire than on other cylinders. I doubt you have that piece of equipment, but it was an interesting diagnostic method.

 

I don't think running seafoam through it again is going to help, but if you do I'd try something better like Techron or ATS 505. I think Seafoam does have some benefits but there are better products out there.

https://www.atschemicals.com/505-pour-ins/

 

@WagonSnatcher did you ever figure this out?