Warm engine rumble after spark plugs change, VQ35DE

[Dude2005]

Hi guys, it's me again with another weird issue. TL;DR at the end.

 

Story time first, it's a 2005 DE btw. For the past 2-3 months I had an intermittent random cylinder misfire (P0300). I did the obvious thing at first and ignored the problem. Don't get me wrong I wanted to sort it out, unplugging one coil at a time and looking for change, but by the time I was parking at home the problem would go away. It had this weird characteristic that at first it was fully random and sometimes it wouldn't even throw a code. Recently it was only manifesting itself when it was warm outside up until the engine was really warmed up, at which point it would start running properly on all 6 cylinders. One time I went home and it was still misfiring, so I've did the unplug one coil and listen for change procedure. I've confirmed that the cylinders on the left (1,3,5)  work just fine. The right side was more tricky as it's buried under air intake.

 

I've finally decided it's prime time to change spark plugs. Car has 115k miles on the clock, so about time to change plugs anyway. This week I got to it, bought a set of NGK PLFR5A-11, got my tools out and did the job. And here comes the problem. I've found, what I think was the offending cylinder. When I removed coil pack from cylinder number 6 (furthest on the right when looking at the car), it made an ungodly gurgling sound I never wanted to hear and there was a fair bit of oil. The coil pack was dripping with it and I went through 3 dozens of paper towel sheets to slowly but surely remove most of the oil from the plug well. I cleaned the coil pack and all. At that point I already changed the other 2 plugs in that bank on the right so I also changed this one. Then I went to change the plugs on the left and everything went smoothly. Some were easy to remove some required a lot of force, but no surprises. Nothing chipped, or anything like that. Everything went as planned and expected aside from the surprise oil where it was not supposed to be.

 

When removing engine cover to get better access and following guides I've noticed that the surface of the engine block right under the mounting point of that plastic cover was absolutely pristine, indicating that it most likely was never undone since the car left the factory floor. With that in mind I don't know if these spark plugs have ever been changed. The service book has a P3 entry at the right time though.

 

Now here comes the problem. I put everything back together triple checking to not forget any plug or bolt that I've undone. The car started smoothly as if nothing changed. It was cold and running perfectly. I didn't notice it running better, but it didn't run worse either. I went for a short ride to a store and about a mile down the road as it warmed up a little it started making a low rumbling noise when accelerating. From idle to about 2000 RPM, the noise can be heard both when stationary and just revving the engine and when accelerating in gear. The car shakes and the noise is very audible. It also comes when idle with A/C on. It causes some odd vibrations that can be felt on the steering wheel too and pressing brake pedal and revving also causes the vibrations to be felt by foot.

 

To clarify this is not a gurgling sound, it's not misfire, the engine doesn't sound like V8, doesn't really shake, I checked the plugs to coil packs, all are in correct order, including the infamous 4 and 6 cylinder that some people swap by mistake.

 

It only happens when it's warm; cold runs as it used to.

 

I did put the plugs by hand at first. Hand tight, until I felt they hit the washer, to make sure I don't cross thread them, then used very short wrench to squash the washer and as I felt it was done I didn't turn any more. I though that maybe they weren't tight enough, so I did a second pass of tightening by feel on warm engine, each plug went another 1/6 of a turn on average, but that didn't help. Today I got a square drive adapter for my torque wrench and torqued all plugs to 30Nm. I know it's the max value and should be closer to 25Nm, but that wrench has a range from 30-210 Nm. Again as I did torque them to spec each one went another 1/4 of a turn before wrench clicked at 30Nm, so I thought maybe they were indeed too loose.

 

Then I went for a ride and again at first cold engine smooth as butter, but once warmed up started making that same rumbling sound. Starting it when it's warmed up also causes it to sound very "harsh", that kind of low key scraping maybe?

 

I don't think anything got into the cylinders, I did one plug at a time, there was no time for anything to fall in.

 

The best way I can describe the nature of the sound and that is my leading theory right now, is that spark plug(s) are not tight enough and when I press gas it causes pressure in the cylinder to go up and some of it escapes around the plug. This could explain why the sound is the worse when accelerating and revving the engine and why it goes away at higher RPMs. It doesn't persist when I hold RPMs only when they go up. The other theory I have is that some of the oil from that cylinder 6 made it's way into the cylinder, burned and turned into a soot residue and as the engine warms up it causes premature ignition, hence the sound.

 

My last resort would be to put old plugs back in and see if the problem is maybe with new plugs.

 

TL;DR

Changed spark plugs after having a random misfire for a while. Cylinder number 6 had a lot of oil in spark plug well. Now the car makes rumbling sound in RPM range 650-2000 when either revving or accelerating but only when warm. Sound fine when cold. Problem only manifests itself when revs go up, holding it at set RPM in that range does not result in the rumble.

 

Picture of old plugs in attachment. I think the one that was in cylinder 6 with oil in well is the first one (from the left), but I'm not sure. They all look about the same. Worn, but otherwise fine and shouldn't cause the misfire. I blame that on oil getting between plug and coil pack.

 

I know that oil in plug well means I need to replace valve cover as that particular gasket/o-ring is integrated in the cover. In the mean time I'm looking for wisdom. Did I miss something obvious when puting everything together. Should I put something on spark plug threads to help it seal or is it indeed oil residue in the cylinder that causes premature ignition. Should I take the car for long ride and let it burn in or should I change something and not drive it until it's fixed. Have anyone experience this before?

[Olly350z]

Are you absolutely sure you've not mixed up the firing order on the last two spark plugs by connecting the coilpacks the wrong way round? It's super easy to do and many, myself included have made the same mistake ? 

[Dude2005]

I'm absolutely sure I got them in the right order. The wire that comes from the middle of the harness on this side goes to the cylinder 6 and the one from the end of the harness closest to the firewall goes to cylinder 4. As described here (I hope link from another forum aren't a bannable offence)

 

I also put back old plugs and the problem is still there, so that rules out any kind of fault with new plugs too.