Interesting Spark Plug Photos (30651km - Subaru Legacy 2.5i)

[dk1604]

Pulled the Denso Iridium TT spark plugs from my 2008 Subary Legacy 2.5i (naturally aspirated) while trying to diagnose what felt like an ignition problem after some major engine work. The issue ended up not being the plugs but I was "intrigued" by the wear pattern on the platinum ground tips of the spark plugs and decided to share them for your viewing pleasure.

One of the iridium center tips isn't looking too happy either.

I've had similar aggressive wear on an Impreza (also naturally aspirated) that I used to drive prior to getting the Legacy. Seems that Subaru's waste spark ignition system on their EJ25 engines is pretty tough on plugs.

 

[Silk]

2 will be jumping centre to earth, the others earth to centre.

 

[WobblyElvis]

On the top two plugs there are small aluminum looking balls on the ground electrodes. I've never seen this before but I think that is a sign of serious detonation. Perhaps someone else will have a better idea. Fantastic photos!

 

[Hammehead]

Those balls are the metallic additive ash deposits. It´s burning some oil on those cylinders.

 

[spasm3]

In the last pic, the center electrode looks like it is about to snap off!

 

[jorton]

How is the gap? Great pics.

 

[Inspecktor]

Deposits look like lead build up we would find on aircraft spark plugs, although that was usually around the base of the porcelain insulator.

 

[Trav]

A little more information may be helpful.

What was the problem that turned out not to be the spark plugs?

What octane are you running?

Which if any fuel or oil additives do you use?

Any other useful info, such as ignition timing, ignition and fuel system service history,etc ?

 

[Whipcitywrencher]

Originally Posted By: spasm3
In the last pic, the center electrode looks like it is about to snap off!
.

It sure does.
I wonder if it did break off if it would do any damage,
or just go out the exhaust valve?
Very nice close up pics, thanks for sharing.

 

[WobblyElvis]

Detecting detonation by reading spark plugs is explained here
https://books.google.ca/books?id=8R8xBgK...lls&f=false

 

[slacktide_bitog]

That's a lot of wear for iridium with only 20k miles

Although the issue may have ended up not being the plugs, the issue certainly could've caused excessive wear like that!

What was the issue anyway? And I hope you replaced those plugs! If you did, did you install new TT's?

 

[dk1604]

Lots of interesting feedback. Thanks everyone.

To answer some of the questions raised:

The major engine work done was replacement of the head gaskets. Like many Subaru EJ25 engines, this one developed an external oil leak right on schedule at 100,000 miles. The engine wasn't consuming abnormal amounts of oil (usually about 1/2 qt for each 6000km) but the bottom of the engine would be coated in oil and she would mark her parking spots with a few drops after each trip.

I never thought that the metallic spheroids on the ground electrodes could be deposits from the oil. I thought it was actually the platinum pucks breaking down but upon closer inspection of the photos it definitely could be deposits. The oil used during the tenure of these plugs was Maxlife or a blend of Maxlife and Synthetic Maxlife. (The shop that did the head gasket replacement used Motul 5w-30 but the plugs only saw that oil 630km before getting pulled.)

The issue I was investigating when I pulled the plugs for inspection was what felt like an ignition miss whenever I would let the car creep forward at idle while crawling in slow traffic. Initially I thought perhaps one of the plugs was damaged so I replaced them with the car's old original OE plugs which I keep in the trunk as emergency spares (NGK FR5AP-11). This didn't resolve the issue; if anything it made it worse. After that I measured the ignition leads with a multimeter and found an open wire. A-ha, found the smoking gun...or so I thought. Changed the ignition leads and the problem persisted! Finally did an idle relearn procedure and that did the trick.

The gasoline used is normally Shell or Chevron 87 or 89 (on hot days.) Once in a while I run a tank of premium to get the additional cleaning additives. I do not use any additional fuel additives. I'm reasonably sure the engine is stock and unmodded though I can't be 100% sure since I'm likely the car's third owner.

WobblyElvis, thanks for that super informative link. I sure hope that it's not aluminum from the top of my pistons melting. The shop that did the head gasket replacement would have had a good view of the pistons when the heads were removed however and they didn't tell me about anything unusual there. Fingers crossed!

Slacktide_bitog, I actually just re-installed the Denso plugs as they idle smoother than the stock FR5AP-11 and I wanted to try every permutation of plugs and wires. (This is Bitog afterall!) I'll run them for a few weeks while I order some new plugs. Given the rapid erosion I've seen on this car as well as my previous Impreza, I'll probably go with something else with a thicker center electrode. Perhaps Autolite XP, Champion PLatinum or Iridium, or maybe go back to Bosch Double Iridium plugs which I was running prior to the Densos. (I don't run the stock FR5AP-11 since they are only single platinum plugs and I think a DIS/Waste Spark systems should run at least dual platinum.) The choice of plugs will depend largely on availability though and I do need to get something quick since, as many of you have observed, the center electrode doesn't look like it will hang on for much longer.


For the curious, here's a link to my old post showing how NGK G-Power plugs fared in an Impreza I used to drive (similar kilometers):

https://www.bobistheoilguy.com/forums/ub...re_Electrode_Er

You can see undercutting of the platinum center electrode there also.

 

[Trav]

What number are those Denso plugs? Does the engine ping under load? I know, only questions but I am trying to figure out whats going on inside the chamber that would cause this in such a short time.

 

[dk1604]

Originally Posted By: Trav
What number are those Denso plugs? Does the engine ping under load? I know, only questions but I am trying to figure out whats going on inside the chamber that would cause this in such a short time.


No problem with the questions. I too would like to know why the plugs are wearing quicker than they should.

The plugs are IK20TT and is one of the plugs spec'd by Denso in their application guide for this car (2008 Subaru Legacy, 2.5i, Naturally aspirated).

The engine doesn't ping much if at all with these plugs whereas with the stock NGK FR5AP-11 the car would consistently ping mildly under low-RPM/high-load (lugging) situations (eg. cruising up a slight hill). My previous Impreza (also naturally aspirated) exhibited this same behaviour and I've heard other Subarus of similar vintage do the same thing when they're driving next to me so I've always assumed it was Subaru trying to maximize fuel economy.

A bit off topic but related: the IK20TT is actually a temperature range cooler than the stock NGK FR5AP-11. Normally NGK heat range 5 gets mapped to Denso 16. Now, if you look up in Denso's application guide my previous car (2005 Subaru Impreza, Naturally aspirated) which also came stock with FR5AP-11 plugs, they will cross the IK16TT. I never ran iridium plugs in my Impreza but did run the standard platinum Denso TT's in it with a heat range of 16. It pinged as much with those plugs as the stock plugs. I've often wondered whether running cooler plugs was the reason that the Legacy pinged less with the IK20TT plugs.

It's shame I can't keep running the IK20TT as they're really smooth and hardly ping at all. I see that Denso has an "Iridium Long Life" line. Maybe that would be more appropriate for this car.

 

[Trav]

I would run the Bosch OE double iridium 1 heat range colder, bump the octane up 1 level. I also wonder if you may not have a knock sensor going a bit deaf, not failed as that would throw a CEL.
Some engines have "deaf" areas and manufacturers fitted 2 sensors. The fact you heard pinging at some point is indicating all may be not well with that part of system or that just the way it was made.

You should see the timing being pulled under higher loads with your scan tool when you hear it ping or tap around the knock sensor lightly with a hammer it should also pull the timing back.
It can also be checked with a VOM engine off, google for the correct values IIRC its oscillating around 0.500 VAC.

Its hard to screw up plug wire location on the coils of a Subaru but make sure they are correct.

 

[dk1604]

Thanks for your insights Trav. The Bosch OE double iridium plugs are readily available at our Canadian Tire stores so that's definitely a good option for me. I've was running those plugs (9600) prior to switching to the Denso plugs and had no complaints. I can't remember how they were wearing but I still have them in a drawer at home and will examine them more closely when I get back there this weekend. As you suggested, if I go back to the Bosch plugs, I will go one range cooler (9603).

Great idea on testing the knock sensor. I never thought of inducing knock with a hammer. I do have a scan tool for Subies so I'll definitely give it a try. That said, I kind of suspect that's just the way they're designed. My previous car, a 2005 Inpreza, was purchased new and she did the same thing. Seems like when they went to a throttle by wire system in 2005 they also tried to eek out every bit of fuel economy.

Back in 2014 I sent similar photos to NGK about how my Impreza was wearing through G-Power plugs quickly (similar undercutting of the precious metal on the center electrode), they responded that it could be due to an aggressive ignition system. Perhaps Subaru went a bit overboard on the size of the ignition coils.

I suppose it's also possible that the gasoline in the Vancouver area is lower spec that elsewhere but I haven't noticed any different behavior when tanking up during US road trips. There does seem to be some disagreement in the Subaru forums on whether to use 87 or 89 octane. Some say they notice a difference, others say they don't. Perhaps those that don't live at higher elevations where they can get away with lower octane fuel...dunno. Personally, I think I get a bit more power with higher octane fuels but it's slight enough that I can't say whether it's placebo or not.

 

[Trav]

Next time you have 89 in the tank at half a tank fill it with 93 that should get you somewhere near 91 octane and see how she runs.
No way would I run 87 in that engine.
I hope they do away with 87 and 89 sooner than later, it really is absolute swill. Low octane fuel may be the root cause, it may not be able to pull the timing back enough to stop the pinging.

 

[dk1604]

Thanks. I do fill up on Premium once in a while to "treat" the car (and myself).

Only problem is that premium gas here costs about cdn$0.20 more than regular. (Current gas prices are around cdn$1.45/1.55/1.65 per litre for regular/mid/premium.) So on a typical 50 litre fill-up I would be paying an extra cdn$20. I can understand why some people are insistent on using regular. Oh well, just more evidence that I should have bought a turbocharged GT model. Much fewer head gasket issues, and if you have to run premium anyways, you might as well have the extra horsepower.

 

[Trav]

Tell me about it, everything I own takes premium even the winter rat but it is what it is, they do run better and get better fuel economy.

 

[nthach]

I changed plugs in a 2003 Forester that also used the DIS waste-spark system - the factory called for regular NGKs, I went with Denso Platinum TTs. The person who owned it didn't want double platinum plugs and Subaru didn't call for anything special.

They worked fine, and should hold up better than the stock NGKs. I think waste-spark systems too should use double platinum/iridium plugs. I know Toyota was adamant about using only the "proper platinum-tipped spark plugs for this model" on the 1996-2001 Camrys using waste-spark DIS before they moved to full COP even to the point of requiring a double-ground plug on the 1997-2001 Camry 4-cylinder. But the trucks with the 5VZ-FE called for a plain jane copper plug but used the same waste-spark system.