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Discussion Starter · #1 ·
Over the weekend I replaced the spark plugs in my 2018 Crosstrek. The car is at 96K miles and ran great but the plugs had never been done before.

The job itself was easy enough and things went back together without any issues. During the first test drive after changing the plugs, I noticed the car wasn't idling quite right. The rearview mirror would vibrate and you could hear a low sounding knock. I checked for missed vacuum connections and went through the idle relearn process several times with no improvement.

I re-installed the old plugs on the passenger side and sure enough, the car seemed to idle normally again. The car was fine during several shorter test drives so I decided to use it for a trip with a ~2hr long drive. The car seemed totally normal during the drive. It wasn't until I was close to my destination and ended up idling at a stop light that the rough idle issue and knocking sound came back again. Fortunately the car still performed fine at higher RPMs and highway speeds so I was able to finish my trip. At this point, my suspicion was that I had gotten bad plugs from the Subaru dealer.

I went to the dealer first thing yesterday and they offered me a set of replacement plugs. Last night I replaced all four plugs with the brand new plugs. After doing the idle relearn process again I took the car for a test drive. Unfortunately, the rough idle is now back and seems more extreme than before. At one point during my test drive the car started idling so rough that I thought the it was going to lose power.

Before anyone asks, I used OEM plugs purchased from my local Subaru dealer. I also used a multimeter to confirm that the new plugs I installed were functional and not "dead". The measured gap also seemed to be within spec.

I suppose the issue could be bad ignition coils, but the car seems to run fine with the original spark plugs installed. The ignition coils were also replaced several years ago as part of a recall. I'm hesitant to throw several hundred dollars at replacing the ignition coils without a guarantee that it will fix the issue.

I've gone over everything and know for certain that there aren't any missing vacuum connections either.

Has anyone else experienced this issue? I'm hoping to avoid expensive diagnostics fees at the dealer if possible.
 

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I'll assume you've installed the plugs correctly(proper torque spec, no grease or dirt on ends, etc..).
1) check the connections at the coils and make sure the connectors are at the correct coil. At your current mileage, they are probably due for replacement. I generally do them every 60k on our cars. Having said that you can ohm them out with a voltmeter/multi-meter.
coil
2) check for dtc's with a code reader if you have access to one
3) at 96k, I'd also check your fuel pressures.
 

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Discussion Starter · #4 ·
I just looked up the recalls done on the car and the ignition coils were replaced just over a year ago. So definitely less than 20k miles on the ignition coils.

I'm 99% sure the connectors and coils are correct. I did not need to disconnect them during the plug replacement and the wire length doesn't allow for them to be installed on the incorrect cylinder.

I hooked up a code reader and no codes came up. The check engine light isn't coming on while driving or when the issue is happening either.

Replacing the fuel filter is past due. I'll look into getting it replaced this week.
 

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I just looked up the recalls done on the car and the ignition coils were replaced just over a year ago. So definitely less than 20k miles on the ignition coils.

I'm 99% sure the connectors and coils are correct. I did not need to disconnect them during the plug replacement and the wire length doesn't allow for them to be installed on the incorrect cylinder.

I hooked up a code reader and no codes came up. The check engine light isn't coming on while driving or when the issue is happening either.

Replacing the fuel filter is past due. I'll look into getting it replaced this week.
I'd check the connectors/harness then. They really need to be disconnected and moved away. The harness/connectors are not exactly robust. I'd disconnect them and make sure the tabs are tight and the wires in the back are still in their pinch connections.
 

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2021 Crosstrek Limited
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Hey OP, I know it's a little off topic but still related to your SP replacement, just curios about your access to the rear driver side spark plug well. I haven't seen anyone do a video on our generation, but it seems like the ECM/main wiring harness are much more obstructive compared to the '12-'17 models. The Sub-frame architecture is also a little different, and looks a little tighter. Are there any tricks to getting access?
Thanks in advance!
 

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2021 Crosstrek Limited
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Hey OP, I know it's a little off topic but still related to your SP replacement, just curios about your access to the rear driver side spark plug well. I haven't seen anyone do a video on our generation, but it seems like the ECM/main wiring harness are much more obstructive compared to the '12-'17 models. The Sub-frame architecture is also a little different, and looks a little tighter. Are there any tricks to getting access?
Thanks in advance!
On another thread, it was mentioned that the coils did not have to be disconnected to remove them.
 
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