2017 Subaru Forester: Spark Plug Type, Gap & Torque

Quick Reference

Why The Right Plug Matters

Your Subaru engine is a finely tuned machine, and the spark plugs are a critical component in its performance and efficiency. Using the correct plug ensures you get a strong, consistent spark at the right moment. But what makes a plug "correct"? It comes down to two key factors: heat range and electrode material.

• Heat Range: This refers to the spark plug's ability to dissipate heat from the combustion chamber. A plug that is too "hot" (doesn't dissipate heat well) can lead to pre-ignition, where the air-fuel mixture ignites before the spark, which can cause catastrophic engine damage. A plug that is too "cold" can accumulate carbon deposits, leading to misfires. The factory-specified NGK plugs for your Forester are designed with the precise heat range for optimal performance and engine longevity.

• Electrode Material: Both the 2.5L and 2.0L XT engines in the 2017 Forester come from the factory with Iridium-tipped spark plugs. Compared to older copper or even platinum plugs, iridium offers significantly better durability and performance. Its high melting point allows for a very fine center electrode, which requires less voltage to create a spark. This results in a more consistent and stable spark, improving fuel efficiency, throttle response, and cold starts. While they have a higher upfront cost, their long 60,000-mile service interval makes them a worthwhile investment.

How to Gap a Spark Plug

Properly gapping a spark plug is crucial for a healthy ignition system. The "gap" is the small distance between the center and ground electrodes. Even brand-new plugs should have their gap checked before installation, as they can be knocked out of spec during shipping.

To check the gap, you'll need a feeler gauge or a specific spark plug gapping tool. For the 2017 Forester, the plugs come pre-gapped, but it is always wise to verify. Slide the correct thickness gauge (0.043" for the 2.5L, 0.024" for the 2.0L XT) between the electrodes. It should slide through with just a slight drag. If the gap is too wide, you can gently tap the ground electrode on a clean, hard surface to close it slightly. If it's too narrow, you can use the tool to carefully bend the ground electrode open. Never pry against the fragile center iridium electrode, as this will damage the plug.

Replacement Procedure Tips

The 2017 Forester, like most modern vehicles, uses a coil-on-plug ignition system. This means each spark plug has its own dedicated ignition coil sitting directly on top of it, eliminating the need for a distributor and spark plug wires. This delivers a more efficient spark but adds a step to the replacement process.

1. Coil-on-Plug Removal: After removing the engine cover, you'll see the black ignition coils bolted to the valve covers. Before unbolting a coil, disconnect its electrical connector. Then, remove the single bolt holding the coil in place and gently twist and pull it straight up to remove it from the spark plug.

2. The Anti-Seize Debate: Subaru service literature does not recommend using anti-seize compound on the spark plug threads. The factory NGK plugs have a special trivalent plating that provides its own anti-seize and anti-corrosion properties. Applying additional anti-seize can act as a lubricant, leading you to over-torque the plug, which can stretch the threads in the aluminum cylinder head. It can also interfere with the plug's ability to ground properly. It is best to install the plugs clean and dry.

3. Dielectric Grease: While anti-seize is a no-go, applying a small amount of dielectric grease to the inside of the ignition coil boot (where it slips over the spark plug's porcelain top) is highly recommended. This grease prevents the boot from sticking to the plug over time and helps seal out moisture, preventing corrosion and ensuring a good connection.

Known Issues

Thankfully, the FB and FA series engines in the 2017 Forester are not known for problematic spark plug issues like those seen in some other manufacturer's engines. You won't have to worry about the infamous two-piece plug breakage that plagued Ford 3-valve engines or ionization-sensing issues seen in some GM V8s. Following the correct torque specifications and replacement interval is the best way to ensure a trouble-free experience.

Need plugs replaced? Find a shop on Repairius.