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Rough Idle on 2020 Forester 2.5L - Time for Spark Plugs?

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Sarah K. · rep 0 · May 22, 3:19 AM · 1 replies · solved · Question

Vehicle: 2020 Subaru Forester 2.5i Premium

Hi everyone,

My 2020 Subaru Forester 2.5L (FB25 engine) with about 72,000 miles has recently developed a noticeable rough idle, especially when the engine is cold. It smooths out a bit once warmed up, but it's still not as smooth as it used to be. I've also noticed a slight dip in my average fuel economy.

I haven't seen any check engine lights yet, but I'm wondering if this could be a sign that it's time for spark plug replacement. The owner's manual suggests a longer interval, but with the rough idle, I'm concerned. Is this a tough DIY job on the boxer engine? Any specific plug recommendations for the FB25?

✓ Accepted solution

ClearTheCode Admin

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

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ClearTheCode admin answer:

  1. Spark Plug Interval: For your 2020 Forester with the FB25 engine, Subaru typically recommends spark plug replacement every 60,000 miles. At 72,000 miles, you are past the recommended interval, so worn spark plugs are a very likely cause of your rough idle and decreased fuel economy.

  2. Symptoms & OBD Codes: Rough idle and reduced MPG are classic symptoms of worn spark plugs or ignition system issues. While you don't have a Check Engine Light (CEL) yet, if the misfires become more pronounced, you might eventually see a CEL with codes like P0301, P0302, P0303, P0304 (indicating a misfire in a specific cylinder) or P0300 (random/multiple cylinder misfire).

  3. DIY Difficulty on Boxer Engine: Replacing spark plugs on a Subaru boxer engine can be challenging for a DIYer due to the horizontally opposed cylinder design. The plugs are located deep within the engine bay, close to the frame rails, making access difficult, especially on the passenger side. You'll need:

    • A good set of wobble extensions and universal joints.
    • A spark plug socket with a rubber insert to hold the plug.
    • A torque wrench.

  4. Recommended Plugs & Torque:

    • Plugs: For the FB25 engine, use NGK Laser Iridium spark plugs (part number SILFR6A). These are OEM equivalent and designed for long life.
    • Torque: Ensure you torque the new spark plugs to the manufacturer's specification, typically around 14-15 ft-lbs (19-20 Nm). Overtightening can damage the cylinder head, while undertightening can lead to misfires.

  5. When to See a Shop: If you're not comfortable with the tight working space or don't have the necessary tools, it's best to take your Forester to a Subaru specialist. They have experience with the unique challenges of the boxer engine and can perform the service efficiently and correctly, potentially diagnosing other issues if the plugs aren't the sole cause.

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