2021 Subaru Outback Spark Plug Type & Gap

Quick reference

Why the right plug matters

Choosing the correct spark plug for your 2021 Subaru Outback is essential for optimal engine performance, fuel efficiency, and longevity. Spark plugs are critical components in the ignition system, responsible for igniting the air-fuel mixture in the combustion chamber. Using the wrong type, gap, or heat range can lead to various issues, from misfires and reduced power to catalytic converter damage.

Heat Range: The heat range refers to the spark plug's ability to dissipate heat. A "cold" plug dissipates heat quickly and is suitable for high-performance or forced induction engines, preventing pre-ignition. A "hot" plug retains more heat and is better for lower-output engines, preventing carbon fouling. Both the 2.5L and 2.4L turbo engines in the 2021 Outback require specific heat range plugs to operate efficiently.

Electrode Metal:

• Iridium: This is the most common and recommended electrode material for modern engines, including your Outback. Iridium plugs offer exceptional durability and a very fine electrode tip, leading to a more consistent and powerful spark. They have a significantly longer lifespan than platinum or copper plugs.
• Platinum: Platinum plugs are a step up from copper, offering better durability and a longer lifespan. They are often used as OEM plugs in some vehicles but are generally superseded by iridium for extended service intervals.
• Copper: While excellent conductors, copper plugs have a shorter lifespan due to softer electrodes that wear down more quickly. They are generally not recommended for modern engines designed for iridium or platinum plugs, as they will require much more frequent replacement and may not provide the same ignition performance.

How to gap a spark plug

The spark plug gap is the distance between the center and ground electrodes. An incorrect gap can lead to misfires, poor fuel economy, and even damage to the ignition system. Always verify the gap using a wire-type spark plug gapping tool, even with pre-gapped plugs.

1. Obtain the correct gap specification: Refer to the "Quick Reference" table above for your 2021 Subaru Outback engine.
2. Use a wire gauge tool: Slide the correct size wire from your gapping tool between the electrodes. It should have a slight drag.
3. Adjust if necessary: If the gap is too small, use the gapping tool to gently bend the ground electrode outwards. If the gap is too large, gently tap the ground electrode on a hard surface to close the gap. Never pry on the center electrode as this can damage the iridium tip.
4. Re-check: Always re-check the gap after any adjustments.

Replacement procedure tips

Replacing spark plugs on a Subaru Outback, especially the horizontally opposed ("boxer") engines, can be a bit more involved due to limited access. Here are some tips:

• Warm, not hot, engine: Work on an engine that is warm to the touch, but not hot. This helps prevent stripping threads in the cylinder heads.
• Coil-on-plug removal: Both Outback engines utilize a coil-on-plug ignition system. Carefully disconnect the electrical connector from each ignition coil and then unbolt the coil. Gently pull the coil straight up to remove it from the spark plug. Avoid yanking on the coil or connector.
• Access: The horizontally opposed engine design means the spark plugs are located on the sides of the engine, often requiring extensions and swivel universal joints to reach them. You may need to remove the airbox or other components for better access on one or both sides.
• Anti-seize debate: There's ongoing debate about using anti-seize on spark plug threads. Most modern spark plugs have a special coating from the factory that negates the need for anti-seize. Applying anti-seize can actually alter the torque reading, potentially leading to over-tightening. If you do use it, apply a very thin coat and reduce the torque specification by about 10-20%.
• Dielectric grease on boots: Apply a small amount of dielectric grease inside the spark plug boot (the end that connects to the spark plug). This helps lubricate the boot for easier removal in the future and prevents moisture entry, improving electrical insulation.
• Torque wrench is critical: Always use a torque wrench to tighten spark plugs to the manufacturer's specifications. Over-tightening can strip cylinder head threads or damage the spark plug, while under-tightening can lead to compression leaks and misfires.

Known issues

While the 2021 Subaru Outback engines are generally reliable, there aren't widespread "known issues" specifically related to spark plugs like the Ford 3-valve engine's propensity for breakage. However, incorrect spark plug installation or using the wrong type can still lead to problems. Always ensure genuine Subaru-recommended or equivalent aftermarket iridium plugs are used.

Need plugs replaced? Find a shop on Repairius.