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2006 2.5i Limited Manual
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Discussion Starter · #1 ·
After searching through the service history and the service intervals I discovered that it was time to do plugs. Everything went well, the internal connector in one of the boots came off and stayed on the plug which was a bit of a pain so I swapped all the wires as well. I used a borescope similar to this one and took some pictures as I went and wanted to share the pics to see if anyone saw anything notable or for other people to use as reference for plugs with 35,000 miles on them. Piston/plug 3 was one of the standouts, it was harder to get out then the others, had more crud on the threads, and the top of the piston seemed to be more discolored then the others.

Plugs: Noticed some oil around the base of 3 and 4 (none on any of the boots). 4 is missing the platinum tip & the ground terminal is very worn. The tip on 1 is also significantly worn.
485806

485807
485808


Piston Heads:
3 looked like it had more rust/buildup then the others, but 1 also had some weird discoloration going on. At the end of the day this borescope took pretty low quality photos but hopefully there is something to get out of these pics. More then anything I was surprised at how differently they all looked.

1:
485809
485810

2:
485811
485812

3:
485813
485814

4:
485815
 

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2009 Outback XT (5MT)
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Disclaimer:
I am new to the world of Subaru so I may be off basis. If anything I write is flat out wrong by all means correct me.

The increased electrode wear on plug 1 and 4 lead me to believe the air/fuel mixture is somewhat leaner in those cylinders. This may be due to how the manifold is built and is just the nature of the beast.
The increased carbon build up on piston 4 is probably due to a long term weak spark resulting in incomplete burn. Increased carbon on pistons 3 and 4 could also be due to the oil reside you found. Possibly mild ring wear or head gasket issues? Or maybe more of the oil residue that goes through the intake system just ends up in the back cylinders more than the front. Couldn't tell you.
Most of the difference in carbon build up on the pistons will likely normalize within a couple thousand miles, fresh plugs and an "Italian tune up." I assume a piston soak is pointless with these engines having horizontal pistons, but if you want to help it along maybe seafoam the intake through a vacuum line.... Or just dump some in the gas tank.
 

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2005 Outback VDC limited 3.0r
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Did you happen to gap the plugs correctly before installing? With only 35k on those plugs, they look very worn for being platinum. I pulled mine out with 50k and they weren't nearly that worn. Carbon build up is hopefully due to incomplete burn as stated by previous commenter. Could have a fuel leak on the rail for injectors 3/4 causing those cylinders to run lean. Is the vehicle tuned with excessive timing or stock?
 

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2006 2.5i Limited Manual
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74 Posts
Discussion Starter · #5 ·
@SwitchPNW The plugs that were in there were installed by the dealer so I would assume they were properly gapped. I made sure to gap the ones going in, but they had all been pre-gapped properly. I haven't smelled any fuel or noticed any inefficiency, in fact quite the opposite I've been getting excellent mileage despite the worn plugs. The power train/ecu is totally stock.

@shandalo Good idea, I'll look into doing that when I get a chance.
 

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2009 Outback XT (5MT)
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Platinum and iridium plugs are gapped by the manufacturer. The end user should not attempt to gap them with old fashioned gapping tools as you will likely damage the plug. It is good due diligence to check gap before installation to ensure the plugs are correct and haven't been damaged, but that is all.
A fuel leak on 3/4 would result in a rich condition. Rich condition would likely make more carbon build up but would also present with fouled plugs rather than plugs with prematurely worn out electrodes. The injectors on 3/4 could be gummed up thou... Which provides potential for a minor lean condition.
We're you able to read the part number on the old plugs still? Wondering if they were the correct part to begin with.
Also.... Any chance you took a look at they cylinder walls?
 

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Outback 2011 3.6R Premium
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How many miles has this vehicle travelled?

Those plugs have been in the engine for a lot longer than 35,000 miles, more like 100,000 miles.

I am going to suggest that the dealer did not bother to change the plugs when they were last due to be changed (even if they actually charged you for the plugs and the time to change them.

Seagrass
 

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2005 Outback VDC limited 3.0r
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Platinum and iridium plugs are gapped by the manufacturer. The end user should not attempt to gap them with old fashioned gapping tools as you will likely damage the plug. It is good due diligence to check gap before installation to ensure the plugs are correct and haven't been damaged, but that is all.
A fuel leak on 3/4 would result in a rich condition. Rich condition would likely make more carbon build up but would also present with fouled plugs rather than plugs with prematurely worn out electrodes. The injectors on 3/4 could be gummed up thou... Which provides potential for a minor lean condition.
We're you able to read the part number on the old plugs still? Wondering if they were the correct part to begin with.
Also.... Any chance you took a look at they cylinder walls?
You can still gap them if they are off, just need to be careful. Last set I had, brand new, over .045 spec was .028. Also how would losing fuel on a rail cause it to run rich?... it would run lean, as their would be less fuel for the injectors to use.... rich is excess fuel, lean is not enough fuel.
 

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2005 OBXT Limited, VF37, STI intake, 5MT
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Nothing terribly out of the ordinary, save the odd plug wear. The build up may be oil consumption as the AVLS piston rings use a thinner oil control ring and, with age, tend to gum-up and allow more oil to be burned.

It's nothing to be concerned about as the plug condition looks good, especially for 35k+ miles. Remember, Subaru didn't spec double plat plugs for extra long miles, they require it due to the nature of their wasted spark system. Maybe some less-than-great fuel (the yellowish deposits usually mean increased sulfur in the fuel)?

Have you checked valve lash yet? Should be done about every 105k. Exhaust tend to tighten-up and/or hang, causing abnormal combustion conditions.

Does the ECU report any odd/high fuel trim adjustments? Anything above 10% is a concern, more than 25% is a CEL.
 

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I can also see these plugs not being changed even thou records state they were.

@SwitchPNW
Was thinking of leaking injectors, not a leak at the rail. Leaking injectors would likely cause rich condition. Leak at the rail would be lean, hopefully with a very obvious fuel smell.
 

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Discussion Starter · #11 ·
@shandalo Thanks for the heads up, I did only check that the gaps were correct with a caliper, I didn't adjust them or use the sliding style which could damage them. When I take the plugs out to do a compression test I'll try to move the camera so I can see the cylinder walls.The plugs that were in there were the correct OEM replacements, just like the ones I put in there. No fuel smell at all so if there's a leak it would be at the injectors, but I'd be surprised, I've been getting great mileage recently.

@seagrass The vehicle now has ~150,000 miles so it's definitely possible the plugs weren't actually changed but i'd be surprised.

@RoughDiamond Do you have any resources/links on how to check for valve lash? Prior to 115,000 the car was serviced religiously at the dealership, do you think it's possible it was already done?
 

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This is good. The pics indicate the amount of carbon build up over time that is a contributor to knock. The carbon heats up and retains heat and when fuel hits it, it ignites; detonation/knock occurs. This is when the computer alters timing and fuel to correct it.

The carbon build up on 4 being heavier is due in part to the EGR system. The valve is closer to 4 and therefore when the intake stroke occurs 4 pulls in majority of the exhaust gases which has carbon in it. Shortest path has the least resistance.

The PCV system pulls oil particulate in to the intake and the particulate is burned in combustion. The VE of the engine allows for most of it to burn, but some is always left behind, especially after a shut down where it's able to cool and harnden on the surfaces, usually on the low side of the chamber. The carbon build up looks normal for 150k miles. I'd bet the intake is loaded with carbon.

The spark plugs do look like they were in longer than 35k miles. #4 being a bugger to replace the "tech" that replaced them could have disregarded that cylinder to speed up his time on the car and figured nobody would know. The plug being worn like that also limits the spark. It had to jump farther and this would affect combustion which would also contribute to higher carbon build up in the cylinder. #4 would not have been burning all the fuel, the fuel correction would have shown a reduction in fuel trim.

Google valve adjustment. There's vids on it. Easier than typing it all in.
 

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You can take the throttle body off and look down into it and if there's a lot at the throttle body there's going to be some down at the ports at the head.

There's two different ways you can clean it. One way is feeding seafoam or B12 or some other kind of chemical through the intake manifold while the engine is running and it'll break it loose and burn it up are combustion. The other way is to remove the intake and clean it while it's off the car.

If it is really heavy laden with carbon and oil deposits it'll take more than one treatment of seafoam or b12 with the car running to clean it out. The advantage of doing it with the car running is it will also break up the carbon in the combustion chamber and it'll blow out the exhaust.
 

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Valve lash can be checked in the car, no problem. It's a little tricky as the rear cylinders can get cramped on the exhaust side.

The FSM (factory service manual) has the procedure. I'd grab it from sl-i.net from their "Literature and Manuals" subforum. Overall, it's a simple procedure with rotating the engine around, measuring with feeler gauges, and turning a screw above the valve to adjust (if necessary). It's also a good excuse to change the valve cover gaskets if they are old/leaking.
 

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'17 OB 3.6 , '11 OB 2.5 , '11 Legacy 2.5
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This is good. The pics indicate the amount of carbon build up over time that is a contributor to knock. The carbon heats up and retains heat and when fuel hits it, it ignites; detonation/knock occurs. This is when the computer alters timing and fuel to correct it.

The carbon build up on 4 being heavier is due in part to the EGR system. The valve is closer to 4 and therefore when the intake stroke occurs 4 pulls in majority of the exhaust gases which has carbon in it. Shortest path has the least resistance.

The PCV system pulls oil particulate in to the intake and the particulate is burned in combustion. The VE of the engine allows for most of it to burn, but some is always left behind, especially after a shut down where it's able to cool and harnden on the surfaces, usually on the low side of the chamber. The carbon build up looks normal for 150k miles. I'd bet the intake is loaded with carbon.

The spark plugs do look like they were in longer than 35k miles. #4 being a bugger to replace the "tech" that replaced them could have disregarded that cylinder to speed up his time on the car and figured nobody would know. The plug being worn like that also limits the spark. It had to jump farther and this would affect combustion which would also contribute to higher carbon build up in the cylinder. #4 would not have been burning all the fuel, the fuel correction would have shown a reduction in fuel trim.

Google valve adjustment. There's vids on it. Easier than typing it all in.
I'm going to have the plugs on my '17 3.6 changed at the Stealership ; it's worth it to me right now to NOT have to mess with it. What is a tactful way to ask for the old plugs back, so it doesn't seem like I don't trust them? Part of me wants to actually inspect them, and another part of me doesn't trust them :)
 

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#4 lost its electrode. Plug failed. The cruddy gunk is a result of that. The others look ancient.
Someone mentioned precious metal tipped plugs should not be gapped. I think it's due to the extreme hardness of precious metals which do not bend. Cracking will happen. If enough stress was put on the tiny electrode "trying" to gap it, a stress crack may have happened.
 
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2009 3.0R Outback
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#4 lost its electrode. Plug failed. The cruddy gunk is a result of that. The others look ancient.
Someone mentioned precious metal tipped plugs should not be gapped. I think it's due to the extreme hardness of precious metals which do not bend. Cracking will happen. If enough stress was put on the tiny electrode "trying" to gap it, a stress crack may have happened.
I think the myth of not gapping fine wires plugs comes from people using the ramp style gapping tools and damaging the tips. It's perfectly fine to use a lever style tool to increase the gap. To close the gap you just carefully press on the top of the ground electrode.

And never assume that plugs come pre-gapped properly. I recently bought some NGK Ruthenium HX plugs from RA and even though they're listed as having a gap of 0.044" on both RA and NGK's site, they were actually gapped at 0.028".

This is an inexpensive tool I recently bought with blade gauges, wire gauges, and lever style gapping tool...
 

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I think the myth of not gapping fine wires plugs comes from people using the ramp style gapping tools and damaging the tips. It's perfectly fine to use a lever style tool to increase the gap. To close the gap you just carefully press on the top of the ground electrode.

And never assume that plugs come pre-gapped properly. I recently bought some NGK Ruthenium HX plugs from RA and even though they're listed as having a gap of 0.044" on both RA and NGK's site, they were actually gapped at 0.028".

This is an inexpensive tool I recently bought with blade gauges, wire gauges, and lever style gapping tool...
Never seen that lever style gapping tool before. Far superior to the classic stuff. Agreed it is completely acceptable to gap any plug with this given it is adjusting the ground strap only without messing with the electrode. Also agree one should never assume the plugs you bought are gapped properly out of the box. Always check.
I also like gearwrench tools. Miles better than Chinese crap, but still affordable. Gonna go buy a new gapping tool now!
 
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