2020 Subaru Forester Spark Plug Guide

Quick Reference: 2020 Subaru Forester Spark Plugs

Why the Right Spark Plug Matters

The humble spark plug is a critical component in your Forester's ignition system, responsible for igniting the air-fuel mixture in the combustion chamber. Using the correct plug is essential for engine performance, fuel economy, and longevity. Key factors include:

• Heat Range: This refers to the plug's ability to dissipate heat. A plug that is too "hot" can lead to pre-ignition (detonation), while a plug that is too "cold" can foul with carbon deposits. Subaru specifies a precise heat range for the FB25DE engine to ensure optimal combustion.
• Electrode Material: Modern engines like the one in your Forester use long-life spark plugs with precious metal electrodes.
  • Iridium: The 2020 Forester comes from the factory with iridium-tipped spark plugs. Iridium is an extremely hard, dense metal that resists wear far better than platinum or copper, allowing for a very long service interval—typically 60,000 miles in this application. Its fine-wire center electrode also requires less voltage to create a spark, improving ignition efficiency.
  • Platinum: While a durable material, platinum is generally less resistant to wear than iridium. Double-platinum plugs are a good long-life option but may not last as long as the OEM iridium plugs.
  • Copper: Standard copper plugs are excellent conductors but have a very short lifespan, often needing replacement every 20,000-30,000 miles. They are not recommended for the 2020 Forester, as using them would be a significant downgrade in both performance and longevity.

How to Gap a Spark Plug

Properly gapping your spark plugs is crucial. Even pre-gapped plugs should be checked before installation, as they can be knocked out of spec during shipping. The gap is the distance between the center electrode and the ground electrode.

1. Use the Right Tool: A feeler gauge or a dedicated spark plug gapper tool is necessary. Avoid the cheap "coin-style" gappers, as they can put pressure on the fragile fine-wire iridium electrode and damage it.
2. Measure the Gap: Gently slide the correct gauge (0.032" or 0.8mm for the 2020 Forester) between the electrodes. It should slide through with a slight drag. If it's too loose or too tight, the gap needs adjustment.
3. Adjust Carefully: To adjust the gap, never pry against the center electrode. Instead, gently bend the ground electrode (the curved piece of metal over the top) up or down using the adjustment slot on your gapper tool or a pair of needle-nose pliers. Make tiny adjustments and re-measure until the gap is perfect.

Replacement Procedure Tips for the 2.5L Forester

The FB25DE engine in the Forester is a boxer engine, meaning the cylinders are horizontally opposed. This makes accessing the spark plugs more challenging than on an inline or V-type engine, but it is manageable with the right tools.

• Coil-on-Plug Removal: This engine uses a modern coil-on-plug ignition system, meaning each spark plug has its own ignition coil mounted directly on top of it. There are no distributor or spark plug wires. To access the plug, you must first unbolt and carefully remove the ignition coil. Gently twist and pull the coil boot straight off the spark plug to avoid damage.
• The Anti-Seize Debate: NGK, the OEM supplier for Subaru, specifically advises against using anti-seize compound on their modern spark plugs. The plugs feature a special trivalent plating that acts as a release agent and provides corrosion protection. Applying anti-seize can alter the torque reading, leading you to over-tighten the plug, which can stretch the plug's metal shell and potentially cause catastrophic engine damage.
• Dielectric Grease: While anti-seize on the threads is a no-go, applying a small amount of dielectric grease to the inside of the ignition coil boot is highly recommended. This prevents the boot from sticking to the spark plug porcelain over time and helps seal out moisture, preventing misfires.

Known Issues

Unlike certain engines notorious for spark plug problems (like the early Ford 3-valve Triton engines with their two-piece plugs that break in the head), the Subaru FB25DE engine has no widespread, known issues related to spark plug failure or removal. The design is robust and straightforward. Following the correct torque specification (15 lb-ft) is the most critical step to prevent thread damage to the aluminum cylinder heads.

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