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2010 Forester
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5 Posts
Discussion Starter · #1 ·
I will try to be brief:
2010 Forester 2.5L non turbo has a stumble at takeoff from idle.
Took the opportunity to put in crankshaft sensor, camshaft sensor,
both O2 sensors, cleaned MAF, new air filter. Reasonable prices but
NO improvements.
Developed an intermittent misfire on cylinder #1 ( 3 times ). I was not certain if
this was a cause or effect of the stumbling.
I had hesitated in checking the spark plugs, since I had replaced the plugs
with NGK Iridium plugs and only had 35K on them. See attached photo.
Since the #1 plug is easy to get to, I put a new plug in cylinder #1
and voila, problem solved, idle is smooth and no hesitation now.
My question(s): any ideas why the Iridium plug deteriorated so soon?
Do the discolorations on the plug hint any any other problem?
Would you expect the other 3 plugs need to be replaced? I know that is a
trivial question, but I don't drive the car but about 10 miles a day, and
at 155K miles, do I want to open up trouble by changing the other 3 plugs.
I am thinking of inspecting cylinder #3 which isn't too difficult to get to.
I hesitate because the plug on cylinder #1 was a bit tight coming out and
the others aren't as easy to get to.
Comments are welcome.
TX
Household hardware Gas Nozzle Auto part Fastener
Household hardware Gas Nozzle Auto part Fastener
 

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2021 Forester Limited
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1,357 Posts
I used NGK IFR5E11 in my 2010 (Impreza), and got 100,000 miles out of them. No misfires, but a couple had the fine wire electrode gone similar to what yours looks like. Did you notice if the gaps were all consistent when you installed them?

I also replaced the wires with the NGK blue spiral wound type with the set of plugs before the iridiums. IIRC they were platinum/iridium plugs. I got around 89,000 miles from those, but they weren't quite as well off. But again, no misfires or other issues. Perhaps the upgraded wires helped the coil make the huge spark gap.

I think the IX is the platinum/iridium plug. The Laser Iridium plugs I refenced above have IR on the porcelain.
 

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2010 Forester
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Discussion Starter · #4 ·
I used NGK IFR5E11 in my 2010 (Impreza), and got 100,000 miles out of them. No misfires, but a couple had the fine wire electrode gone similar to what yours looks like. Did you notice if the gaps were all consistent when you installed them?

I also replaced the wires with the NGK blue spiral wound type with the set of plugs before the iridiums. IIRC they were platinum/iridium plugs. I got around 89,000 miles from those, but they weren't quite as well off. But again, no misfires or other issues. Perhaps the upgraded wires helped the coil make the huge spark gap.

I think the IX is the platinum/iridium plug. The Laser Iridium plugs I refenced above have IR on the porcelain.
Thanks for the response. I also put NGK blue wires on when I changed the plugs. I'll probably bite the bullet and start over with wires and plugs.
 

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2010 Forester
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Discussion Starter · #5 ·
Assuming the correct plug for the application, results like that @35K miles indicate a problem. I'd be looking at fuel mixture. Yes, change all four and start looking at it.
Thanks for the reply. I'll probably bite the bullet and start over with wires and plugs. ( I did put the NGK blue wires on when I last changed the plugs.)
 

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2021 Forester Limited
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One thing I noticed when going with the new short block, was the injector to manifold o-rings weren't the tightest seal.

Unfortunately I couldn't find any that were exactly like what was there, so I ended up reusing them. I didn't have any issues, but I think luck had more to do with that.

If there is a vacuum leak at the injector o-ring(s), it could create air fuel ratio issues. The leaking cylinder would be lean, and the rest rich, because there's only one o2 sensor for all 4 cylinders.
 

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2015 Forester 2.5i Premium 6-speed manual
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185 Posts
Thanks for the reply. I'll probably bite the bullet and start over with wires and plugs. ( I did put the NGK blue wires on when I last changed the plugs.)
Spark plug wires do not normally "wear out" between spark plug changes. Unless roughly handled or otherwise mechanically damaged they should be fine. Checking for resistance and continuity, aside from obvious physical damage or wear and tear should be all that is needed to confirm if the spark plug wires were or are the source of your problems.

Modern fuel injected computer controlled gasoline engines since the early 1980s have been designed to run the leanest fuel mixtures possible under all operating conditions. Therefore, it is seldom a good idea to deviate from the exact spark plug specified by the vehicle manufacture. Correct timing and the absence of vacuum leaks anywhere between intake valve and the throttle/ MAF/ MAP etc. is critical. To placate the American market's desire for low MPG vehicles running on "cheap gasoline", many manufactures have now allowed Regular 87 pump octane gasoline with the disclaimer in the owner's manual that using the highest octane available is best for engine performance which means for best engine health. I have been running only premium and brand specific gasoline and diesel in my cars since the 1980s with few exceptions. Chevron on the West Coast where I am from and Shell on the East Coast where I now live.

The secondary side of an ignition system is a "demand" system in that it will try to jump the spark plug gap no matter the actual distance (correct or not) and any resistance in the circuit which includes the spark plug itself.

To my eye in the photo (which is a bit under lit) the plug shown it too dirty with soot, both on the electrodes and the spark plug threads. It would be nice to know what the top of the pistons look like through the spark plug holes. Without any other evidence, it would appear that you are running rich or burning oil to some degree. Also, what are the black marks on the insulator? They look like "carbon tracks" due to shorting of the secondary circuit to ground across the insulator to the spark plug base with the spark plug boot in place. Note the faint "brown / tan" discoloration on the insulator inline with the black marks. Although the spark plug looks sooty the insulator at the base is discolored like it is running a bit "hot". This and the suspicious black and brown / tan marks the length of the insulator make me further suspect the spark plug is not suitable or correct, but I lack enough information to make it definitive.

I would install the exact specified spark plug Subaru calls for and start over. Unless there is no other choice ( very rare) and especially if one spark plug shows problems, spark plugs should always be replaced in complete sets and the old ones examined and compared as a group and matched to their corresponding cylinder. At 155k miles, now would be an excellent time to do a cylinder compression test. If the results are uneven and out of Subaru specifications, a cylinder leak down test should be the follow up..
 

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2012 Forester 4 speed auto
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1,289 Posts
It takes higher voltage to jump a wide gap. Check that the gaps are within specifications and adjust. Then have a shop check the firing line with an oscilloscope to see if the spark duration is correct. Good luck!
 

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2021 Forester Limited
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There is no way to adjust the spark duration. Except changing the coil in hopes of improving it.

Dwell time, where the coils are charged is controlled by the PCM.

Too much dwell time, and the coil pack will overheat and possibly split open.

I would suspect if it was on the rich side, there would be less damage on the spark plug. It would be sooted up, but not burnt off.


Owners Manuals specifically state to use 87 octane unless it is turbo charged. Then higher octane is recommended or required depending on the manufacturer.

The only instance where I have seen recommendations for higher octane are on turbo or for the naturally aspirated performance trim levels. Not Subaru specific in that case.

The owners manual for my 2010 Impreza explicitly said to use 87 octane, that anything higher would not benefit the engine or performance.

That was with 10:1 compression.

It ran just fine on 87 octane for the 6 years

/230,000 miles I owned it.

It's funny you are adamant about the the spark plugs but not the fuel.

Higher octane merely burns slower meaning it takes longer for combination to complete. Longer burning would also keep those spark plugs hotter longer
And the cylinder pressure will take longer to build, so the pistons are farther down the bore, effectively making less power in an engine/engine management designed to run on 87.

It's not going to magically advance the spark timing enough to make use of the longer needed burn time.
 

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2010 Forester
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Discussion Starter · #10 ·
Spark plug wires do not normally "wear out" between spark plug changes. Unless roughly handled or otherwise mechanically damaged they should be fine. Checking for resistance and continuity, aside from obvious physical damage or wear and tear should be all that is needed to confirm if the spark plug wires were or are the source of your problems.

Modern fuel injected computer controlled gasoline engines since the early 1980s have been designed to run the leanest fuel mixtures possible under all operating conditions. Therefore, it is seldom a good idea to deviate from the exact spark plug specified by the vehicle manufacture. Correct timing and the absence of vacuum leaks anywhere between intake valve and the throttle/ MAF/ MAP etc. is critical. To placate the American market's desire for low MPG vehicles running on "cheap gasoline", many manufactures have now allowed Regular 87 pump octane gasoline with the disclaimer in the owner's manual that using the highest octane available is best for engine performance which means for best engine health. I have been running only premium and brand specific gasoline and diesel in my cars since the 1980s with few exceptions. Chevron on the West Coast where I am from and Shell on the East Coast where I now live.

The secondary side of an ignition system is a "demand" system in that it will try to jump the spark plug gap no matter the actual distance (correct or not) and any resistance in the circuit which includes the spark plug itself.

To my eye in the photo (which is a bit under lit) the plug shown it too dirty with soot, both on the electrodes and the spark plug threads. It would be nice to know what the top of the pistons look like through the spark plug holes. Without any other evidence, it would appear that you are running rich or burning oil to some degree. Also, what are the black marks on the insulator? They look like "carbon tracks" due to shorting of the secondary circuit to ground across the insulator to the spark plug base with the spark plug boot in place. Note the faint "brown / tan" discoloration on the insulator inline with the black marks. Although the spark plug looks sooty the insulator at the base is discolored like it is running a bit "hot". This and the suspicious black and brown / tan marks the length of the insulator make me further suspect the spark plug is not suitable or correct, but I lack enough information to make it definitive.

I would install the exact specified spark plug Subaru calls for and start over. Unless there is no other choice ( very rare) and especially if one spark plug shows problems, spark plugs should always be replaced in complete sets and the old ones examined and compared as a group and matched to their corresponding cylinder. At 155k miles, now would be an excellent time to do a cylinder compression test. If the results are uneven and out of Subaru specifications, a cylinder leak down test should be the follow up..
Well you could be correct. I used the NGK BKR5EIX-11 plug last time ( from Rock Auto - stated as an "alternative"-MY BAD). It appears that NGK FR5AP-11 is the correct plug, so I will use them this time. Since the burnt mark on the plug indicates a spark through the boot, and I have no way of investigating the inside of the boot, I'll replace the wires - not really that expensive. Thanks for the responses.
 

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2011 Subaru Forester
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I don't think using the NGK Irridium IX for your application would be a problem. It should be an upgrade on the laser platinum plugs. I will be curious to know if the new laser platinum plugs make any difference. Copper NGK's should also be good but just won't last as long.
 

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2021 Forester Limited
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Like I said above, I have used both the IX and the Laser Iridium plugs with zero issues. They lasted well beyond the recommended spark plug change interval.

As far as a performance gain, if there was, it wasn't a noticeable one.

I averaged around 30 mpg, with a fairly well bound up center differential, so there was probably a lot more bias to the rear than normal, taking away some fuel economy.

It could be a defective plug, only removing and inspecting the other 3 will tell more of the story.

No sense speculating on some complicated issues that aren't easily solved until there is more of the picture...
 

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Is it possible that the spark plugs were not sufficiently torqued down when installed?
.
 
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