Spark Plug Centre Electrode Erosion (33K kms)

[dk1604]

I replaced the NGK G-Power plugs that I have been running in my car (2005 Subaru Impreza RS) for ~33k kilometers (~4.5 years) and was surprised to see the centre electrodes on two of the four plugs worn as shown in the following photo:

Because of the DIS ignition and the fact that G-Power plugs have platinum on only one side, I was anticipating asymmetric plug wear. (G-Power plugs are economical enough that I figured I would just change them out at copper intervals.) However, I assumed that most of the wear would have been on the ground strap - not the centre electrode since it has the platinum puck. As such, I was rather surprised to see the centre electrode worn as shown. For comparison, here is the plug from the DIS "pair" cylinder:

It looks perfectly normal with a some wear on the ground strap.

I sent the photos to NGK and they suggested that it could possibly be due to an overly aggressive ignition. I suppose it's possible but the ignition on the car is stock from the factory. No mods have been done to the car.

Does anyone have any theories as to what could cause the centre electrodes on a plug to exhibit accelerated wear? Or, is my assumption that the DIS ignition would wear the conventional ground strap more that the platinum centre electrode incorrect? Is this normal wear for 33k kilometers (20.5K miles)?

Thanks,
David

 

[Johnny248]

That is quite a bit more wear on the G-Power plugs than I am used to seeing. I've seen these plugs in other engines with 100k of use in better condition. Interesting.

 

[WobblyElvis]

If this helps, I know that most many 4 cylinder cars only have 2 coils and pair up two plugs per coil. The coil fires both plugs at the same time but one of the sparks is simply wasted on the exhaust stroke. With this way of doing things, the spark jumps across both plugs in the opposite direction. The spark jumps one plug from center electrode to ground, through the head, back from the ground electrode of the second plug to the center electrode and back to the coil. This might explain the different wear pattern you see.

 

[mattwithcats]

I think that's a waste spark ignition system.

It fires backwards on two of the plugs...

Use a plain copper plug of a double Platinum / Iridium plug.

 

[bvance554]

I wouldn't call that normal for that few miles. I know when I changed the factory plugs on my Subaru there was a noteable difference in two of the four plugs. I won't say how many miles... but two were worn down to a nub while the other two still resembled a spark plug. Still ran great though.

 

[michaelluscher]

That's not right.

And as you know, you'll need Double Platinums for longer life in a DIS ignition.

Back Banks of my Taurii are infamous for this.

 

[splinter]

Time to swap 'em out again. Nice pictures!

 

[dk1604]

Thanks, what surprised me was that the most wear was on the center electrode (with the platinum puck). I thought a DIS ignition would accelerate the wear on the ground strap (which is conventional on a G-Power.)

 

[dk1604]

Thanks for the comments. The plugs were swapped out and replaced with Denso Platinum TT. Since they are also an asymmetric plug (platinum centre electrode with titanium enhanced ground strap) I will swap them out next year and see how they fare. One thing the Denso plugs have in their favour is that their centre electrode is 1.1mm instead of the 0.6mm found on the G-Power so hopefully they will hold up better.

BTW, the car still ran fine on the G-Power plugs. (Of course, it does run better now with fresh plugs.)

 

[y_p_w]

The OEM is an NGK FR5AP-11 "Laser Platinum". The current price on Amazon.com is less than $7 each, although it says only 4 in stock.

http://www.amazon.com/NGK-FR5AP-11-Laser-Platinum-Spark/dp/B001MU28HA

Not sure what you can get in Canada, although I found one reseller through Amazon.ca that has them for less than $10 CDN.

http://www.amazon.ca/NGK-FR5AP-11E-Laser-Platinum-1-Pack/dp/B000GZETK6/

I don't know why you'd use anything other than OEM. It's a PITA to change the plugs, so most go with the long-life OEMs. Even if you insist on checking them, you can just pull them out and put them back in. I use to do that on my Integra GS-R with OEM dual platinums.

 

[dk1604]

Going OE is always a respectable option however:

I still have the original FR5AP-11 "Laser Platinum" plugs that the car came with and despite the description on the NGK website, they do not appear to be double platinum plugs. Close examination of the ground electrode on those shows no sign of a platinum puck or inlay. (The plugs look exactly like the photo shown on Ebay item #160952511119.) At the time, rather than spending $40 for a set of OE plugs, I figured I might as well just get what's easily and cheaply available at Canadian Tire and replace more often. (The plugs are reasonably easy to change on the Impreza.) Some model year Subies came with conventional Champion plugs from the factory so I figured G-Powers would still be step up from conventional despite being "lesser" than OE.

Even today, I'm still not sure I would spend $40 for a set of FR5AP-11 plugs (which appear to be single platinum.) I just noticed the other day that Bosch now has a plug line that has double iridium electrodes and they are available at Canadian Tire for roughly the same $40 price. Autolite XPs or Champion double platinum plugs are also readily available and cost even less.

Actually, the plug I would like to try is the new Denso Iridium TT.

 

[tommygunn]

Does that car call for a 2-prong spark plug? Many wasted spark engines come with spark plugs that have two ground electrodes.

 

[circuitsmith]

With Japanese cars at least, OEM is the only way to go, unless the engine is modified.

 

[threeputtpar]

You should go one heat range colder as those plugs show that they are too hot. The color change on the ground strap should be in the middle of the strap.

Too hot means it's closer to the threads, too cold means the color change is closer to the tip.

 

[PandaBear]

Is it just me or does the plug looks like it has a green tint? any head gasket issue?

 

[y_p_w]

Originally Posted By: dk1604
Even today, I'm still not sure I would spend $40 for a set of FR5AP-11 plugs (which appear to be single platinum.) I just noticed the other day that Bosch now has a plug line that has double iridium electrodes and they are available at Canadian Tire for roughly the same $40 price. Autolite XPs or Champion double platinum plugs are also readily available and cost even less.

Having bought a set of NGK Laser Platinums, I was under the impression that the line specifically meant it was a double platinum. However, I noticed that there's a lot of talk that they're not with the OEM plugs for a base Impreza. I bought those for my wife's Civic, and they've been fine even though I couldn't get the crush washers to crush and my attempts to use a torque wrench were foiled because it always seemed way too loose. In the end I just tightened them "good and tight" and they haven't shot into the hood yet.

I have experience with high-quality double platinums going back to the factory ND plugs on my 1995 Acura Integra GS-R. My only issue was that nobody seemed to have the correct gapping (1.3mm) except for dealers. Even when a shop installed plugs for me, all they could get were the 1.1 mm gap and they regapped them (which wasn't recommended since it can damage the platinum). I remember spending time on the line with Denso Sales US, and a rep assured me that he could ship the correct plug to me on the honor system with an exchange, and I could return my 1.1 mm plugs to the closest authorized dealer. I waited and waited, and apparently I never got them. However, I did get a cool Denso Spark Plugs trucker cap.

Still - double platinums are fine and everything, but for the most part the center electrode is what's subject to the most wear. When I see your photo, the center electrode is of course messed up, but the ground electrode looks fine. Of course a quality double platinum might even last indefinitely event though you might want to check. Back with my Integra, I heard that platinum plugs were pretty much a requirement for that engine because standard plugs would erode - especially with the big required gap. The 60K rated life was a bonus, but they were ridiculously easy to reach.

 

[dk1604]

Thanks for that tip. (Learned something new today!) I wasn't aware that you could fine tune the heat range by seeing where the colour change was. Do you look for the color change on the back of the ground strap?

Here is the back of the ground straps for the plugs from cylinder #1 and #2 (#1 is the plug with the worn center electrode, #2 is his DIS partner):

 

[dk1604]

The green tint is due to the camera's white balance. Looking at the plugs with a magnifier I would say that the "green residue" is actually a light gray (for the most part.)

That said, I wouldn't be surprised if there is some coolant seeping into the cylinders. This generation of motor is notorious for the head gaskets leaking coolant. The coolant usually just leaks outside and not into the oil or cylinders. Subaru's solution is to add "Subaru Cooling System Conditioner" whenever one does a coolant service. It is well know that Subaru Cooling System Conditioner is really just Holts Radweld:

http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2305516

I have a little bit of coolant seepage but it doesn't seem major (yet). This last year, I had to add about a 1/2 cup of water to the system to bring it back up to the full mark. It's been a few years though and I'm due for a cooling system service so maybe the next bottle of "Cooling System Conditioner" will button things up

 

[dk1604]

Originally Posted By: y_p_w
Having bought a set of NGK Laser Platinums, I was under the impression that the line specifically meant it was a double platinum. However, I noticed that there's a lot of talk that they're not with the OEM plugs for a base Impreza. I bought those for my wife's Civic, and they've been fine even though I couldn't get the crush washers to crush and my attempts to use a torque wrench were foiled because it always seemed way too loose. In the end I just tightened them "good and tight" and they haven't shot into the hood yet.


I know where you are coming from about the Laser Platinums. When I removed my original plugs I was fully expecting to see platinum pucks on the ground electrodes. When I didn't see them I was wondering if the pucks had fallen off. (Still wonder that actually...) For torquing new plugs on, I don't wrench enough to have calibrated wrists and sometimes it's tough to maneuver the torque wrench into position so I have just been using the turn angle method. So far it has worked alright and I don't need to worry about anti-seize throwing off the torque settings.

Now, in appreciation for everyone's input, for your entertainment here are photos of the original plugs that the car came with from the factory: NGK FR5AP-11 after roughly 55K kilometers (also about 4 years). You can actually see a similar wear pattern to what the G-Power plugs have. It's just not as obvious since the center electrode has a thicker "pedestal".

First the center electrodes:

No platinum pucks

:

And finally the back of the ground straps:

 

[SHOZ]

My car came with Deso Iridiums FK20HQR8. It has a Iridium pad on both the electrode and ground strap.