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Rough Idle & Misfire on 2020 Forester - Spark Plugs?

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Melissa G. · rep 0 · May 22, 3:19 AM · 1 replies · solved · Question

Vehicle: 2020 Subaru Forester Sport

My 2020 Subaru Forester Sport, with about 65,000 miles, has recently developed a rough idle and occasionally feels like it's misfiring under light acceleration. It's not throwing a check engine light yet, but I'm worried it might soon. I've heard that spark plugs can be a common culprit for these symptoms on Subarus, especially with the boxer engine design.

Is 65k miles a typical interval for spark plug replacement on the 2.5L engine in the Forester? What kind of plugs should I be looking for (OEM equivalent)? Also, how difficult is it to DIY this job on a 2020 model? I'm pretty handy, but I know the boxer engine can make plug access a bit of a challenge.

Any tips or experiences from other Forester owners would be great!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:19 AM

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ClearTheCode admin answer:

Your symptoms of a rough idle and misfires on your 2020 Subaru Forester Sport at 65,000 miles strongly suggest that spark plugs are a likely cause. While the official Subaru maintenance schedule for iridium spark plugs is typically around 60,000 to 100,000 miles, individual driving conditions can accelerate wear.

Spark Plug Replacement Interval:

  1. Recommended: For your 2.5L FB25 engine, the general recommendation for iridium spark plugs is 60,000 miles. Your mileage is right in that sweet spot for replacement.

Symptoms and Potential OBD Codes:

  • Rough Idle: Worn plugs can lead to inconsistent combustion.
  • Misfires: You might feel a stumble or hesitation. If a misfire becomes consistent, your Check Engine Light (CEL) will illuminate, and you'll likely get a P030x code (e.g., P0301 for Cylinder 1 Misfire, P0300 for Random Misfire).
  • Reduced Fuel Economy: Inefficient combustion uses more fuel.
  • Lack of Power: Engine struggles to produce full power.

Recommended Spark Plugs:

  • Always use iridium-tipped spark plugs. The OEM plugs are typically NGK or Denso. Stick with these brands for best performance and longevity. Consult your owner's manual or a parts catalog for the exact part number (e.g., NGK SILFR6A11 or Denso FXE20HR11).

DIY Difficulty and Tips: Replacing spark plugs on a Subaru boxer engine can be challenging due to limited access, but it's a common DIY job for those with patience and the right tools.

  1. Tools: You'll need a universal joint or wobble extension for your ratchet, a spark plug socket (usually 14mm or 16mm, thin-walled), and potentially a flex-head ratchet. A torque wrench is essential for proper installation.
  2. Access: The plugs are located horizontally on the sides of the engine. You'll likely need to remove the airbox on one side and potentially the battery or other components on the other for better access.
  3. Procedure: Work on one plug at a time. Carefully remove the coil pack, then use your extensions and universal joint to reach the plug. Be gentle to avoid cross-threading during installation. Torque the new plugs to the manufacturer's specification (typically around 15-18 ft-lbs).

When to See a Shop:

  • Lack of Confidence: If you're uncomfortable with the tight working space or the risk of damaging components.
  • Persistent Issues: If new spark plugs don't resolve the issue, there might be other underlying problems like coil pack failure, fuel injector issues, or vacuum leaks, which a professional can diagnose using advanced tools.
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