Hello all,

I can't find anywhere in the FSM or Chilton manual on troubleshooting injectors. Any advice on that? The engine ran fine until I replaced the valve cover gaskets (externally leaking) and ever since I put it back together I've had a misfire on cylinder 4. I've swapped around the coil packs countless times, misfire stays on 4. The coil pack connectors for 4 & 2 are ambiguous but I get multiple cylinder misfires if I swap them the other way. I've put in new spark plugs and after only a few miles all are black except cylinder 4, which is clean as new, and 2 which only has a tiny black spot.
I pulled the #4 spark plug and had my wife start the car and I can visually see it's sparking.

I checked compression and that's fine on all cylinders.

It leads me to suspect injectors. I can't find any procedures to go through in the FSM for injectors at all nor the chilton's 350Z manual. The only thing I can think of is swapping them around but of course the injectors are under the plenum so I'd have to disassemble the entire plenum then reassemble to do this. I don't understand why an injector would do this all of a sudden after a VCG change.

I did the VCG job 2 years ago just before our 2nd baby was born, so I found myself with zero free time, which is why it's taken me so long to resolve this. I'm tired of not being able to drive the Z for 2 years and being unable to solve this never ending problem.

It's a 2003 DE, engine is stock except an Injen CAI.

 

I'd look at this from the point of "what changed since it ran well? At least 2 possibilities, maybe more.

It sounds like you've inspected the valve gasket repair and are satisfied with it, so we're back to the injectors and harness. The vinyl over the wiring is pretty delicate, possible it got nicked during valve covers? Pull the fuel pump fuse, have a helper run the starter in a dark area, watch for sparking. You should be able to hook a simple volt meter onto the harness and a ground and safely check for signal too.
Since you've rotated both the connections and injectors for me it really points to a harness or connection problem. You can search for a shop that cleans and inpects injectors for proper function and cycling, I think trying it at home would be immensely tedious. Gl!

 

Your plugs shouldn’t be black after a few miles, but yes, there seems to be a problem in cyl 4. Step one is to go back through everything you did, basically do the job again paying close attention to what may be related. It could be something very simple, maybe you knicked a wire while you were in there doing the vcg, which really would be my guess.
Do you hear any vacuum leaks?
You can “test” injectors using a stethoscope or very long screwdriver by holding it on the body of the injector while running the engine, it should make a constant audible click, compare it to other injectors to be sure. If you hear nothing it could be the command from the pcm, or a bad injector.

 

Diiiiiiddddddddddd you swap cylinder #4 and cylinder #6 wire harness?

Cylinder #4 and #6 are opposite in logic …

if you’re not sure, grab a picture of the wire harnesses and we will help.

 

Well I finally had time to take the upper and lower plenum off and I swapped injector 4 with 6, unplugged and replugged their connectors a bunch of times, put it all back together and turned it on. For the first time since I did the valve cover gaskets in 2019 I don’t have a constant flashing specific cylinder misfire! It got to operating temp idling with no misfires, a whole bunch of random codes were there, throttle and pedal position sensor and air intake temp codes, I cleared them and they never came back. It did have periods of misfiring but then would stop. However it was always random multiple cylinder misfire. I cracked a vac line so that could be why and the gas is from 2020 and 2019 so that could be why. I’m thrilled all cylinders are combusting.

At this point I did swap coil pack connectors 2 & 4 but it constantly flashed the cel at me and hadn’t up to that point the way I had it. So that solves that.

I still don’t know what I did that resolved it for the most part. It’s a mystery. Just as mysterious as how it came about in the first place.

now to replace the vac hose I cracked, clean the maf with maf cleaner.

 

Update: this ended up being the problem. I couldn't find any info on which connector goes to which coil pack on the drivers (US) side anywhere on the whole internet until this guy's video in July 2022.