2018 Subaru Forester Spark Plug Type, Gap & Torque Guide

The 2018 Subaru Forester is a rugged and dependable crossover, but keeping its horizontally opposed "Boxer" engine running at peak efficiency requires timely spark plug maintenance. Whether you have the naturally aspirated 2.5L engine or the potent 2.0L Turbo in the XT model, using the correct spark plugs, gaps, and torque specs is critical to preventing misfires, maintaining fuel economy, and ensuring long catalytic converter life.

Here is your comprehensive ASE-certified guide to 2018 Subaru Forester spark plugs.

Quick reference

Note: Subaru engines come factory-equipped with NGK spark plugs. For the best performance and longevity, sticking with the OEM NGK Laser Iridium plugs is highly recommended. Denso, Motorcraft, and ACDelco equivalents exist, but NGK is the gold standard for modern Subaru Boxer engines.

Why the right plug matters

Modern engines are highly sensitive to spark plug specifications, and the 2018 Forester is no exception.

• Heat Range: The heat range determines how fast the plug transfers heat away from the firing tip into the cylinder head. Using a plug that runs too hot can cause pre-ignition and catastrophic engine damage (especially on the 2.0L Turbo), while a plug that runs too cold will foul with carbon.
• Electrode Metal (Iridium vs. Platinum vs. Copper): The 2018 Forester requires Iridium spark plugs. Iridium is six times harder and eight times stronger than platinum. This allows for a much finer center electrode, resulting in a more concentrated spark, better combustion, and a lifespan that reliably meets the 60,000-mile replacement interval. Never downgrade to copper or standard platinum plugs in these engines.

How to gap a spark plug

Although most premium NGK iridium spark plugs come pre-gapped from the factory, you should always verify the gap before installation. Dropped boxes in shipping can easily close the gap.

1. Use the Right Tool: Use a wire-style gap gauge or a specialized feeler gauge. Avoid using cheap coin-style ramp gauges on iridium plugs, as sliding them across the tip can snap the delicate iridium center electrode.
2. Check the Gap: Slide the correct size wire between the center and ground electrode. It should pass through with a slight "drag."
3. Adjusting: If the gap needs widening, use the bending tool on your gauge to gently pry the ground strap upward. Never pry against the center electrode. If the gap is too wide, gently tap the ground strap on a hard surface to close it.

Replacement procedure tips

Replacing spark plugs on a Subaru Boxer engine can be notoriously tight because the cylinder heads sit mere inches away from the vehicle's frame rails.

• Make Room: You do not need to lift the engine. On the passenger side, remove the air intake box and ducting. On the driver side, remove the battery and unbolt the secondary air injection pump to gain access.
• Use the Right Extensions: You will need a low-profile 14mm thin-wall spark plug socket, a 1-inch extension, a 3-inch extension, and a high-quality universal joint (swivel). Build the tool inside the spark plug tube (insert socket, then extension, then ratchet).
• Coil-on-Plug Handling: The Forester uses individual coil packs bolted to the valve covers. Unplug the harness carefully (the plastic tabs become brittle with heat) and unbolt the 10mm retaining bolt to wiggle the coil out.
• Anti-Seize Debate: NGK explicitly states not to use anti-seize on their modern spark plugs because they feature a trivalent metal plating that prevents seizing. Adding anti-seize acts as a lubricant and can cause you to over-torque the plug, potentially damaging the aluminum cylinder head threads.
• Dielectric Grease: Apply a small dab of dielectric grease to the inside of the coil boot. This prevents moisture intrusion and stops the rubber boot from vulcanizing to the ceramic ribbing of the new spark plug.

Known issues

Unlike the infamous Ford 3-valve engines that suffer from broken 2-piece spark plugs, or early GM coil issues, the 2018 Subaru Forester's spark plug system is very reliable.

The most common "issue" is strictly mechanical access. Many owners or inexperienced technicians skip the 60,000-mile interval because the job is difficult due to the frame rail clearance. Neglecting this interval, especially on the 2.0L Direct Injection Turbo (FA20DIT), will eventually lead to misfires, hesitation under boost, and premature failure of the ignition coils.

Take your time, use a thin-wall magnetic spark plug socket, and thread the new plugs in entirely by hand to avoid cross-threading the aluminum block.

Need plugs replaced? Find a shop on Repairius.