[Bobiam]

Is there any reason that I would be sorry for not changing my spark plugs at 60,000 miles? It's a 3.6. The car runs perfectly all right and I've heard of people putting this service off for 20 or 30,000 miles. I drive around 5000 miles a year with this car, so I might be able to put the service off for several years.

 

[HyRax]

Yes, because the service schedule says you should do them, regardless of whether time or distance travelled comes first.

You can of course choose to put it off, but you also have to accept any potential issues that may arise out of that down the track.

 

[seagrass]

Many forum members have reported that they have delayed the spark plug replacement until 80,000 or 100,000 miles.

The early 3.6 engines have double platinum plugs as standard which last a very long time. I removed the NGK double platinum plugs from my 3.6 at around 115,000 miles (when I purchased the vehicle) as I could find no record of the plugs having been replaced and they looked like they could have lasted longer even though they were obviously worn.

Seagrass

 

[provels]

How about pulling the easiest one and checking it out? Infiniti used the equivalent NGKs in my FX35 as OE and the factory change was 100K. They still looked new even then.

 

[donglow2]

I am pretty observant of how the car runs and able to notice small differences. I replaced my 2.5 at 104,000 for the first time and could feel zero difference. They looked fine too, but they are so hard to get out that I haven't heard of anyone putting the old ones back in. You don't know if there is a bad plug or oil in a cylinder until you have them all out and by then all you can think about is not having to do it again for a few years so you put new ones in.

The next generation NKG plug is Ruthenium which apparently is a little more abundant than Iridium and cost less.

 

[johnre]

You are actually supposed to pull them every 30,000 miles for inspection - even when they are called out for changing at 60,000 miles.

But to answer your question, search the forums on "stuck spark plug". There are plenty of stories abounding about this, like this one, and the reason is usually that they aren't pulled per spec and allowed to run too long.

 

[seagrass]

You are actually supposed to pull them every 30,000 miles for inspection - even when they are called out for changing at 60,000 miles.

But to answer your question, search the forums on "stuck spark plug". There are plenty of stories abounding about this, like this one, and the reason is usually that they aren't pulled per spec and allowed to run too long.

Which is why I ALWAYS put anti-seize on spark plug threads when installing them.

Back when spark plugs were changed every 10,000 miles or so you never had a problem with them becoming seized but now with 60,000 to 100,000 mile replacement intervals the chance of spark plugs being seized is very real.

Seagrass

 

[cardoc]

The replacement interval is 60k miles because the ignition system is a waste spark system. The plug fires twice per cycle which means it's equivalent to 120k miles use on a single firing cycle engine.

And it not just the electrode wear you need to look at. If the plugs have burn marks on the porceline where it meets the metal, the spark is leaking.

 

[CdnScooby]

If it's running smooth leave it 20k then replace.. unless you begin having issues during that time. I've changed plugs on many Subarus and have yet to find one with bad plugs at the recommended interval. They are very high quality plugs, yes you may be getting optimal performance by installing new plugs but you also get optimal performance from checking your tire pressure weekly, replaing your air filter every 20k etc. etc. 80k miles seems to be the commonly known limit on these plugs before you begin to run into issues.

 

[subforx]

I would just leave it until you notice something, Sometimes you wonder if 60K replacement is for the dealers to make money on those who are unable to do the job at home.

 

[Bobiam]

THANKS guys..... i'm going to give 'em another 20k of use and have them changed by someone else if I still own the car. My hands aren't too good for such intricate work anymore!!!

 

[ammcinnis]

My hands aren't too good for such intricate work anymore!!!

I feel your pain.

 

[donglow2]

You are actually supposed to pull them every 30,000 miles for inspection - even when they are called out for changing at 60,000 miles.

But to answer your question, search the forums on "stuck spark plug". There are plenty of stories abounding about this, like this one, and the reason is usually that they aren't pulled per spec and allowed to run too long.

The 2019 Subaru Service Manual, which is for the 3.6 and 2.5 Outback and Legacy, says Replace at 60. There is no inspection at 30 in this manual.

 

[skipw]

The 2019 Subaru Service Manual, which is for the 3.6 and 2.5 Outback and Legacy, says Replace at 60. There is no inspection at 30 in this manual.

Nor is it in the Maintenance Schedule for the 2015 Outback.

 

[donglow2]

I would just leave it until you notice something, Sometimes you wonder if 60K replacement is for the dealers to make money on those who are unable to do the job at home.

When the going dealer cost to replace, I am told, is now about $600 who wants to replace them earlier than needed? If my calculations are correct, evaluating this completely on fuel cost, at $5 a gallon with an average mpg of 30, you would need to improve mpg by 4.5% or 1.35 additional mpg before you would pay for the cost of a $600 spark plug replacement by changing them at 60,000 rather than 80,000. This is without taking into account if you replaced at 60, at 80 you would have plugs that are 1/3 used up. Without figuring any changes in costs then you add another 4.5% or 1.35 mpg to get 9% and 2.7 additional mpg. From fuel savings-only point of view, it looks hard to justify new plugs if you are going to have a dealer do it and you are not experiencing any symptoms of worn plugs.

If you DIY, without considering your time, tools, and frustration cost, then you could break even with a substantially less increase in mpg. The tool cost isn't really that much, because if you have a 3/8 rachet and 3/8 socket set with metric sockets for the battery box and a few other small items that need to be removed, plus a 14mm open-end wrench, which most people do, then all you need is a special plug wrench like this https://www.amazon.com/gp/product/B08FRH2PYL/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 with a 1" extension. I used a 3/8 to 1/4 adapter with a 1/4 back to 3/8 adapter which turned out to be about 1". I got the NGK Ruthenium plugs from Rockauto for $8.55 a piece. Follow the thread here
StickySpark plug DIY? plus look at a few youtube videos and with a little first aid cream, you are all set for a few frustrating hours, but rewarding in knowing you accomplished something a lot of people won't try.

 

[DugsSin]

Watch out for the HUGE selection of FAKE NGK plugs on that rain forest site. Don't ask me how I found out about it.

 

[blkprlz]

You are actually supposed to pull them every 30,000 miles for inspection - even when they are called out for changing at 60,000 miles.

I didn’t see the 30K inspection, & this chart shows 100K plug change:

 

[ammcinnis]

... this chart shows 100K plug change:

That's 100,000 km, not 100,000 miles. (100,000 km = ~60,000 miles.) Details matter.

 

[johnre]

The 2019 Subaru Service Manual, which is for the 3.6 and 2.5 Outback and Legacy, says Replace at 60. There is no inspection at 30 in this manual.

Nor is it in the Maintenance Schedule for the 2015 Outback.

I didn’t see the 30K inspection, & this chart shows 100K plug change:

Right you are - I always thought that it was in the manuals.

What I do recall seeing somewhere is wording to effect that if your manual specifies 30k miles, as my 2008 does, and you go longer on the replacement cycle because you use a longer life plug than what they spec, you shouldn't exceed that replacement interval for at least pulling and inspecting them.

 

[BISCUT]

17 OB 3.6. IIRC my OEM plug was NGK Iridium. I replaced with NGK Iridium after 105k. Individual choice, how you maintain your oil changes and filter changes along with driving style and conditions matter. I have very little oil burning even over 8k OCI and use Mobil 1 5-30 and recently moved to Pennzoil Plat 5-30 due to super cheap purchase price of about 50 quarts. I also do UOA's so I have a degree of confidence in running my plugs to 105k. Below is a thread I posted with images of my plugs after removal at 105k. Only you can answer for your own 3.6.

2017 Outback Limited 3.6 Plugs

I was able to get my plugs changed recently. They were removed at 103k. Pics below show them to be in good condition. I replaced them with the same OEM NGK Iridium plugs. Interested to see these plug s at 200k. Will they show the same good condition or be toasted a little more? I'll know...

www.subaruoutback.org