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2018 Forester 2.5i Spark Plug Replacement - DIY Advice Needed

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Mike T. · rep 0 · May 24, 1:25 PM · 1 replies · solved · Question

Vehicle: 2018 Subaru Forester 2.5i Premium

Hey everyone,

My 2018 Subaru Forester 2.5i Premium with about 70,000 miles on the odometer is starting to feel a bit sluggish lately. I've noticed a slight rough idle sometimes, especially when the engine is cold, and maybe a tiny hesitation on acceleration. The maintenance schedule suggests spark plugs are due around 60,000 or 75,000 miles, so I'm thinking it's time to tackle this.

I'm fairly handy and have done oil changes, air filters, and even basic brake jobs before. However, I know the Subaru Boxer engine can be a bit tricky due to its layout.

Is replacing the spark plugs on a 2018 Forester 2.5i a reasonable DIY job, or is it too complex for someone without extensive experience with Subarus? What specific tools will I need? Are there any common pitfalls or things to watch out for? I've heard the rear plugs can be a real pain to access. Also, what brand and type of spark plugs are generally recommended for this model?

Any tips, tricks, or step-by-step guides would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 24, 1:25 PM

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Hi John,

It sounds like you're on the right track with diagnosing your 2018 Subaru Forester's symptoms. Rough idle and hesitation around 70,000 miles are classic signs that spark plugs might be due for replacement, aligning perfectly with Subaru's recommended maintenance schedule. Replacing spark plugs on a 2.5i Boxer engine is a common DIY task, but it does require patience and the right tools due to the engine's horizontal layout.

Here's what you need to know:

Tools & Supplies You'll Need:

  1. Spark Plug Socket: A 14mm thin-wall, magnetic spark plug socket is crucial. A swivel-head or universal joint attachment is highly recommended for the rear plugs.
  2. Extensions: Various lengths (3-inch, 6-inch, 10-inch) to reach different angles.
  3. Ratchet: A standard 3/8-inch drive ratchet.
  4. Torque Wrench: Essential for proper tightening to prevent damage (typically around 13-15 ft-lbs for these plugs).
  5. Dielectric Grease: For the inside of the ignition coil boots to prevent arcing and ease future removal.
  6. Anti-Seize Compound: A tiny dab on the spark plug threads can help prevent seizing, though some plug manufacturers advise against it on plated plugs.
  7. Flashlight/Headlamp: For better visibility in tight spaces.
  8. Gloves: To protect your hands.

Recommended Spark Plugs:

  • Always use OEM-equivalent plugs. For your 2.5i engine, NGK Laser Iridium (SILZKAR7B11) or Denso Long Life Iridium are the standard. Do not use copper or platinum plugs; your engine requires iridium for proper function and longevity.

Common Pitfalls & Tips:

  • Access: The biggest challenge is accessing the plugs, especially on the passenger side rear. You may need to remove the airbox on the passenger side and potentially the battery and ECU bracket on the driver's side for better access. Some wiring harnesses might need to be carefully unclipped and moved aside.
  • Stripping Threads: Always start the new plugs by hand to ensure they thread correctly. Cross-threading is a costly mistake.
  • Dropping Plugs: Use your magnetic spark plug socket to carefully guide the new plugs into place.
  • Torque: Do not overtighten. Use a torque wrench. Overtightening can damage the cylinder head threads.
  • Coil Packs: Be gentle when removing and installing the ignition coil packs. Ensure they click securely back into place.

When to See a Shop:

  • If you encounter a seized spark plug that won't budge with reasonable force.
  • If you're uncomfortable with the tight working space or lack the necessary tools.
  • If, after replacement, your check engine light comes on with misfire codes (P0301-P0304) or other related engine codes (e.g., P0420 for catalytic converter efficiency, which can sometimes be triggered by persistent misfires).
  • If your symptoms worsen or new ones appear after the DIY attempt.

This job is definitely doable for a DIYer with patience. Good luck!

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