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Rough Idle & Check Engine Light - 2018 Forester 2.5i Premium

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Mike T. · rep 0 · May 25, 2:17 AM · 1 replies · solved · Question

Vehicle: 2018 Subaru Forester 2.5i Premium

My 2018 Subaru Forester 2.5i Premium, with about 75,000 miles, has started acting up. For the past week, I've noticed a rough idle, especially when the engine is cold. It feels like it's misfiring sometimes.

Yesterday, the check engine light came on. I went to AutoZone and they pulled a few codes: P0301 (Cylinder 1 Misfire Detected) and P0300 (Random/Multiple Cylinder Misfire Detected). The car still drives, but it feels a bit sluggish and the fuel economy seems to have dropped slightly.

Could this be something simple like spark plugs or ignition coils? Or am I looking at something more serious? What's the recommended maintenance for spark plugs on this model, and is it a DIY job for someone with moderate mechanical skills?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:17 AM

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ClearTheCode admin answer:

Those OBD-II codes, P0301 and P0300, strongly indicate an engine misfire, specifically in cylinder 1 and generally across multiple cylinders. Given your mileage, spark plugs and ignition coils are very common culprits for these symptoms in your 2018 Subaru Forester 2.5i Premium.

Common Causes of Misfires (P030X codes):

  1. Worn Spark Plugs: Over time, spark plugs degrade, leading to a weak or inconsistent spark.
  2. Faulty Ignition Coils: Ignition coils provide the high voltage for the spark plugs. A failing coil can cause a misfire.
  3. Fuel System Issues: Clogged fuel injector, low fuel pressure, or contaminated fuel.
  4. Vacuum Leaks: Unmetered air entering the engine can disrupt the air/fuel mixture.
  5. Compression Issues: Less common, but worn piston rings or valve issues can lead to low compression and misfires.

DIY Steps for Spark Plug/Ignition Coil Inspection/Replacement:

  1. Safety First: Ensure the engine is cool. Disconnect the negative battery terminal.
  2. Access: On your Boxer engine, spark plugs are located on the sides of the engine block. You may need to remove the airbox or other components for better access, especially on the passenger side.
  3. Remove Ignition Coils: Each spark plug has an ignition coil directly on top of it. Disconnect the electrical connector and unbolt the coil (usually one bolt). Carefully pull the coil straight up to remove it.
  4. Inspect/Remove Spark Plugs: Use a spark plug socket and extension to remove the old spark plugs. Inspect them for wear, fouling, or oil. This can give clues about engine health.
  5. Install New Plugs: Gap new spark plugs to specification (if not pre-gapped) and apply a small amount of anti-seize to the threads. Hand-tighten them first, then torque to manufacturer's specs (typically 13-15 ft-lbs for Subaru). Use only iridium-tipped plugs as specified by Subaru.
  6. Install Coils: Reinstall the ignition coils, ensuring they seat properly. Reconnect electrical connectors.
  7. Reassemble & Test: Reassemble any removed components, reconnect the battery, and start the engine. The check engine light should clear after a few drive cycles if the issue is resolved.

When to See a Shop:

  • If replacing spark plugs and coils doesn't resolve the misfire.
  • If you're uncomfortable working in tight spaces or don't have the right tools.
  • If diagnostic tests (like fuel pressure or compression tests) are needed.
  • If the misfire is accompanied by unusual noises, smoke, or a strong smell of fuel.
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