2023 Subaru Forester Spark Plug Type & Gap

Quick reference

Why the right plug matters

Choosing the correct spark plug for your 2023 Subaru Forester is crucial for optimal engine performance, fuel efficiency, and longevity. The right plug ensures a strong, consistent spark, which is essential for efficient combustion. There are several key factors to consider:

• Heat Range: Spark plugs are designed with a specific "heat range" that determines how quickly they dissipate heat from the combustion chamber. A plug that's too hot can lead to pre-ignition and detonation, damaging the engine. A plug that's too cold can foul prematurely, leading to misfires. Subaru engineers specify a precise heat range for the FB25D engine to maintain optimal operating temperatures.
• Electrode Metal: The primary differences in spark plug types lie in their electrode materials:
  • Iridium: This is the most common and recommended material for modern vehicles like your Forester. Iridium is extremely hard and has a very high melting point, allowing for fine-wire electrodes that produce a strong spark and last a very long time (typically 60,000 to 100,000 miles).
  • Platinum: Platinum electrodes also offer good longevity (around 50,000 to 70,000 miles) and are often used in vehicles that originally came with platinum plugs. While good, they generally don't last as long or provide the consistent performance of iridium.
  • Copper: Copper plugs have a thicker center electrode and are excellent heat conductors, but they wear out much faster (10,000 to 20,000 miles). They are rarely recommended for modern engines designed for long-life iridium or platinum plugs.

For your 2023 Subaru Forester's 2.5L FB25D engine, the factory specifies NGK SILZKAR7B11 iridium plugs. Sticking with this type and brand is highly recommended to maintain peak performance and avoid potential issues.

How to gap a spark plug

While many iridium spark plugs come "pre-gapped" from the factory, it's always a good practice to verify the gap before installation, especially with fine-wire iridium plugs. The correct gap for your 2023 Subaru Forester is 0.044 inches (1.1 mm).

To gap a spark plug, you'll need a spark plug gap tool, typically a wire-style or coin-style gauge. Avoid using a ramp-style gauge on iridium plugs as this can damage the fine iridium tip.

1. Check the gap: Slide the correct wire gauge (0.044 inches) between the center and ground electrode. There should be a slight drag.
2. Adjusting the gap (if necessary): If the gap is too small, you can gently bend the ground electrode outwards using the gapping tool's adjustment feature. If the gap is too large, you'll need to carefully tap the ground electrode on a hard, flat surface to slightly close the gap. Exercise extreme caution to avoid bending or damaging the delicate iridium tip.
3. Recheck: Always recheck the gap after any adjustments.

Replacement procedure tips

Replacing spark plugs on a Subaru Boxer engine like the FB25D in your Forester can be a bit more involved due to the horizontally opposed engine design, which places the spark plugs in a more confined space.

1. Safety First: Always ensure the engine is cool before starting. Disconnect the negative battery terminal to prevent accidental electrical issues.
2. Access: You will need to access the spark plugs from the wheel wells, usually after removing the wheels and possibly some plastic shrouds for better access. This is where a universal joint or "wobble" extension for your ratchet will be invaluable.
3. Coil-on-Plug Removal: Your Forester uses a coil-on-plug (COP) ignition system. Each spark plug has its own ignition coil mounted directly on top of it. Carefully disconnect the electrical connector from each coil and then unbolt the coil (typically a single 10mm bolt). Gently pull and twist the coil to remove it from the spark plug well. Take care not to damage the rubber boot.
4. Cleaning: Before removing the old spark plugs, it's a good idea to blow out any debris around the spark plug well with compressed air. This prevents dirt from falling into the combustion chamber when the old plug is removed.
5. Anti-Seize Debate: There's an ongoing debate about using anti-seize on spark plug threads. Most modern spark plugs, especially those from NGK, have a special coating on the threads that acts as an anti-seize. Applying additional anti-seize can lead to over-torquing and damage to the cylinder head threads or the spark plug itself. Subaru and NGK generally recommend against anti-seize for their plugs. If you choose to use it, apply a very thin coat and reduce the torque specification by 10-20% to compensate for the reduced friction.
6. Dielectric Grease: It's a good practice to apply a small amount of dielectric grease inside the boot of the new ignition coil before reinstallation. This helps prevent moisture entry, insulates against voltage leaks, and makes future removal easier.
7. Torque Specification: Tighten the new spark plugs to the specified torque: 15.0 lb-ft (20.3 Nm). Using a torque wrench is crucial to prevent over or under-tightening, both of which can lead to problems.
8. Reassembly: Reinstall the ignition coils, ensuring they are seated properly and the electrical connectors are securely attached. Reinstall any shrouds and wheels, and finally, reconnect the negative battery terminal.

Known issues

Fortunately, the 2.5L FB25D engine in the 2023 Subaru Forester does not have widespread, notorious spark plug-related issues like the Ford 3-valve engine's breaking two-piece plugs or specific GM ignition coil problems. Following the recommended maintenance schedule and using the correct NGK iridium spark plugs will typically result in trouble-free operation.

Need plugs replaced? Find a shop on Repairius.