Did I get bad plugs?

SC Maintenance

SC Maintenance

Joined
Jun 8, 2022
Messages
11,216
Location
Lowcountry South Carolina
Back story - this is from my Xterra - 410K miles now approx. At 390K a couple years ago I changed the plugs - NTK from Rock Auto. This car now resides at my daughters which is a couple hours away.

2 months ago she gets a misfire on cylinder 6. I drive down and swap the plug and coil with RA parts. The plug looked pretty knarly - figured though it was just a bad coil because they have 400K on them. Think nothing of it. Problem gone.

Last night she gets misfire on cylinder 3 - other bank. I drive down - swap plug and coil on that one (I ordered a whole set after last issue - again RA). Plug looks nasty again. So I pulled the one behind it - number 5 - just to look. Looks nasty - swapped it and coil also. Its raining and dark - I really didn't realize how bad these plugs were then. Misfire gone. Long term fuel trims are normal on a test drive. Lots of power. So I return home.

The plugs below are cylinder 3 on the left (the one with misfire) and 5 on the right. Also they have some oil on the bottom of the threads - which I am not sure where that comes from? The car does not use oil. For comparison the 3rd pic are the plugs that recently came out of my Frontier at 100K - same VQ40 Nissan engine. These engines are not hard on plugs -normally.

So thoughts. Did I get a bad batch of plugs? 20K miles - they look terrible. No codes for misfire on the 2nd one. Do I have something else going on?

Xterra-badplug-2025-2_3261.webp

Xterra-badplug-2025-2_3261.webp

Xterra Bad Plugs 2025-2-cyl 3 (left) and 5 - misfire on 3

Xterra-badplug-2025-3_3261.webp

Xterra-badplug-2025-3_3261.webp

Xterra Bad Plugs 2025-cyl 3 (left) and 5 - misfire on 3

Frontier-Plugs-200K.webp

Frontier-Plugs-200K.webp

Frontier Plugs 200K
 
Looks like too rich fuel trim. But you said fuel trims looked ok. To me oil burning would have some shine to it. Bad PCV??? Clogged air filter??? Does it have a MAF??? Can you see where the fuel rings are at down in the plug??? Good Luck!!!
 
oldhp said:
But you said fuel trims looked ok.
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After the repair. I did not check them before. It may have been running rich for a while. They don't smell like gas though?
oldhp said:
Bad PCV??? Clogged air filter??? Does it have a MAF???
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PCV is maybe 15K (OEM). Air filter pretty new also. Yes it has a MAF - but at idle it seems to be about the right gram / s. Also - fuel trims are good so?
oldhp said:
Can you see where the fuel rings are at down in the plug
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I don't know what this means?
oldhp said:
Good Luck!!!
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Thank you. Seems to be running well now - so I am told. Vehicle will come back home in a few weeks, so maybe I have time to check more things out.
 
1744163870559.webp
1744164180829.webp

Spark plugs provide clues to various engine conditions, noted our source from E3 Spark Plugs. Shown, from left, are cases where a plug has been idled on for an extended period of time; oil is present in the cylinder; the fuel mixture is a little fat; the plug is too cold for the engine; and the engine is lean.
 
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The oil on the plug removed from cylinder #5 appears to be from a leak from the valve cover tube seals. That can be causing some of the misfires as well as running rich. The tube seals in the valve cover of the VQ40 are molded into the plastic valve cover and cannot be replaced, thereby requiring replacement of the entire valve cover. It's not a terrible job, but the upper intake manifold has to come off. Search YouTube for a few videos that demonstrate the process if you determine that is part of your problem.
 
Nukeman7 said:
The oil on the plug removed from cylinder #5 appears to be from a leak from the valve cover tube seals. That can be causing some of the misfires as well as running rich. The tube seals in the valve cover of the VQ40 are molded into the plastic valve cover and cannot be replaced, thereby requiring replacement of the entire valve cover. It's not a terrible job, but the upper intake manifold has to come off. Search YouTube for a few videos that demonstrate the process if you determine that is part of your problem.
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If the spark plug tube seals were leaking I would expect to find oil at the bottom of the coil boot or on the hex part of the plug, but that part is dry. The valve covers were replaced fairly recently also - the seal is integral to the cover and are not serviceable. I don't think that is the source since the only discoloration is at the very bottom of the threads?
 
I've had NGK fakes from Amazon, but not RA. I've switched to Densos, and I usually buy from local stores.
The fakes I got were really worn; for 20K those plugs look pretty bad. How's the gap? Could be fakes.

I have to believe the coils are OK if it didn't throw a trouble code... Good luck!

400K, you got your $$ worth! Wow!
 
JeffKeryk said:
I've had NGK fakes from Amazon, but not RA. I've switched to Densos, and I usually buy from local stores.
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The NGK number is the OEM number, and everyone that strays from them on these engines seem to have trouble. That would be the 5th set including the original set, in that engine. First possible spark plug problem. My truck has same engine. Its on the 3rd set, never a problem either.

I got them April 2021 according to RA, even though I didn't install until a year later. I wonder if I got pandemic quality?

Hopefully it runs another 6 weeks - which is likely 1200 miles, then its coming home for a while.
JeffKeryk said:
400K, you got your $$ worth! Wow!
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Just getting broken in! Yes, it has saved me a lot of money. My daughter got an internship and needed a car at school almost immediately. I gave her that one figuring I would find something newer for her in a short time. That was Dec 23. Objects at rest, still looking. I did title and insure it under her name so everything is perfectly legal.
 
zzyzzx said:

Did I get bad plugs?​

No. You have a different issue.
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So walk me through it.

The fuel trims are fine.

I swapped coil / plug on the last one - problem gone and has not come back - 2 months ago.

I swapped coil / plug on this one - runs fine now. Fuel trims returned to near zero. No more misfire.

The plug I pulled randomly on the second one above and it showed the same plug sysmptoms, with no code / pending code.

If its not the plugs, I am sort of at a loss for where even to start looking. If it was another sensor or something it should be showing rich or lean??
 
SC Maintenance said:
Every mechanics favorite word - intermittent :ROFLMAO:

It certainly could be an injector. But even if it was intermittent - with two plugs (and those were the only two I pulled) both being carboned, that would mean I had 2 bad injectors at minimum - which would be sort of odd no?
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At 410k miles why not? Could it be a poor spray pattern? Maybe they're just dirty? Spit-balling here.
 
BMWTurboDzl said:
At 410k miles why not? Could it be a poor spray pattern? Maybe they're just dirty? Spit-balling here.
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Yes, I agree its a good idea. New injectors should be on the list at some point.

I also wonder about the earlier suggestion about the PCV valve also - even though its new. But you would think if it were stuck open it would show it running lean in the fuel trims? Definately not running lean.
 
SC Maintenance said:
If the spark plug tube seals were leaking I would expect to find oil at the bottom of the coil boot or on the hex part of the plug, but that part is dry. The valve covers were replaced fairly recently also - the seal is integral to the cover and are not serviceable. I don't think that is the source since the only discoloration is at the very bottom of the threads?
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I was focusing on the oil/black contamination that is apparent on both sides of the crush washer on plug #5. There has to be a reason why plug #3 and all plugs from the Frontier are clean and shiny on both sides of the crush washer, but plug #5 is not.

1744210303144.webp


I know you said that it is not consuming oil, but look at the significant carbon build-up on the ceramic insulators of the center electrode of plugs #3 & #5 as compared to the clean white insulators on the Frontier plugs with 5 times the mileage. I'm not convinced that all of the carbon residue is just due to a rich fuel mixture at 410K miles.
 
Nukeman7 said:
I was focusing on the oil/black contamination that is apparent on both sides of the crush washer on plug #5. There has to be a reason why plug #3 and all plugs from the Frontier are clean and shiny on both sides of the crush washer, but plug #5 is not.

View attachment 272644

I know you said that it is not consuming oil, but look at the significant carbon build-up on the ceramic insulators of the center electrode of plugs #3 & #5 as compared to the clean white insulators on the Frontier plugs with 5 times the mileage. I'm not convinced that all of the carbon residue is just due to a rich fuel mixture at 410K miles.
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The plugs are definately very odd. I was looking at them again last night. There is oil on the threads of both - but only on one side - like one half or 180 degrees - not top and bottom. There is no oil reside at all on the very bottom of either above the threads. If the valve cover seals were leaking I would think it would pool in the bottom? The only affecting 180 degrees of the threads is very perplexing.

Also odd that both are the same. Valve covers were replaced (with OEM) shortly after the plugs - pictures underneath are in my sig. Not to say OEM valve covers can't leak.

I stupidly forgot to take my borescope. I might head that way this weekend to collect more data.
 
SC Maintenance said:
The plugs are definately very odd. I was looking at them again last night. There is oil on the threads of both - but only on one side - like one half or 180 degrees - not top and bottom. There is no oil reside at all on the very bottom of either above the threads. If the valve cover seals were leaking I would think it would pool in the bottom? The only affecting 180 degrees of the threads is very perplexing.
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Very intriguing indeed!
SC Maintenance said:
Also odd that both are the same. Valve covers were replaced (with OEM) shortly after the plugs - pictures underneath are in my sig. Not to say OEM valve covers can't leak.

I stupidly forgot to take my borescope. I might head that way this weekend to collect more data.
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I agree that with OEM valve covers that were recently installed, the likelihood of failure this soon is unlikely unless a tube seals was accidentally nicked during the installation process. Borescope inspection is a good next move after you get the Xterra back to your home.

Please update this thread if/when you get to the bottom of the problem with the Xterra plugs. Good luck!
 
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