2019 Subaru Forester Spark Plug Type & Gap Guide

Quick reference

Why the right plug matters

Choosing the correct spark plug for your 2019 Subaru Forester is crucial for optimal engine performance, fuel efficiency, and longevity. The "right" plug isn't just about fitment; it's about the right heat range and electrode material.

Heat Range

Spark plugs are designed with a specific "heat range," which refers to their ability to dissipate heat from the combustion chamber. A plug that's too "cold" (dissipates heat too quickly) can lead to fouling, where carbon deposits build up on the plug, causing misfires. A plug that's too "hot" (dissipates heat too slowly) can cause pre-ignition or detonation, leading to potentially severe engine damage. Subaru engineers, along with NGK, have carefully selected the ideal heat range for the FB25D engine, so sticking to the OEM-specified plug is always recommended.

Electrode Metal (Iridium vs. Platinum vs. Copper)

• Iridium: The 2019 Subaru Forester's 2.5L FB25D engine is designed for long-life iridium spark plugs. Iridium is an extremely hard and corrosion-resistant metal, allowing for very fine electrode tips. This finer tip concentrates the spark, improving ignition efficiency and extending the plug's lifespan significantly (typically 60,000 to 100,000 miles). They are more expensive but offer superior performance and durability.
• Platinum: Platinum plugs also offer a longer lifespan than copper plugs, typically around 40,000-60,000 miles. They use a platinum disc welded to the center electrode and/or ground electrode. While an excellent choice for many vehicles, they generally don't match the extreme longevity or performance of iridium for applications designed for iridium plugs.
• Copper: Copper spark plugs have a large copper core to dissipate heat efficiently, making them suitable for high-performance engines that require frequent spark plug changes. However, their nickel-alloy electrodes wear much faster than platinum or iridium, making them unsuitable for the long service intervals expected in modern vehicles like your Forester. Using copper plugs in your Forester would drastically reduce replacement intervals and could negatively impact performance and fuel economy.

How to gap a spark plug

For your 2019 Subaru Forester, the specified gap is 0.044 inches (1.1 mm). While modern iridium spark plugs are generally pre-gapped from the factory, it's always a good practice to verify the gap before installation. However, exercise extreme caution if adjusting the gap on iridium plugs, as their fine electrodes are very delicate.

1. Use the correct tool: Only use a wire-type spark plug gapping tool. Never use a ramp-style "coin" gauge, especially on iridium or platinum plugs, as it can damage the delicate electrode tip.
2. Check the gap: Gently slide the appropriate wire gauge between the center and ground electrodes. There should be a slight drag.
3. Adjust (if necessary, with extreme care): If adjustment is needed, gently bend the ground electrode only. Never pry against the center electrode. For iridium plugs, if the gap is significantly off, it's often safer to replace the plug than risk damaging the tip during adjustment.

Replacement procedure tips

Replacing spark plugs on a Subaru Boxer engine, including your 2019 Forester's FB25D, can be a bit more challenging than on an inline or V-engine due to the horizontally opposed cylinder layout. Accessibility to the plugs on the lower side of the engine is limited.

• Access: Many technicians recommend lifting the vehicle and accessing the lower plugs from underneath, or in some cases, temporarily moving components like the airbox or battery to gain better access from above. You