2017 Subaru Outback Spark Plug Type & Gap

Quick reference

Why the right plug matters

Choosing the correct spark plug for your 2017 Subaru Outback is crucial for optimal engine performance, fuel efficiency, and longevity. Spark plugs are designed with specific heat ranges and electrode materials to match your engine's operating characteristics.

Heat Range: A spark plug's heat range refers to its ability to dissipate heat. A "hotter" plug dissipates heat slower, while a "colder" plug dissipates heat faster. Using a plug with an incorrect heat range can lead to pre-ignition, detonation, or fouling, all of which can damage your cherished Subaru engine.

Electrode Metal:

• Iridium: These are the most common type used in modern vehicles, including your Outback. Iridium is extremely hard and offers superior durability and a very fine electrode tip, providing an exceptionally long lifespan (typically 60,000 to 100,000 miles) and consistent, powerful spark.
• Platinum: Platinum plugs also offer good durability and performance, often lasting around 40,000-60,000 miles. They are a step up from copper but generally don't last as long as iridium.
• Copper: While excellent conductors, copper plugs have the shortest lifespan (around 20,000-30,000 miles) due to their softer electrode material. They are rarely used in modern vehicles from the factory.

Both the 2.5L and 3.6L engines in the 2017 Subaru Outback use iridium spark plugs from the factory, offering a long service interval and reliable performance.

How to gap a spark plug

While most iridium and platinum spark plugs come pre-gapped from the factory, it's always a good idea to double-check the gap before installation, especially with the more delicate fine-wire electrodes. For your 2017 Subaru Outback, the specified gap is 0.043 inches (1.1 mm) for the 2.5L engine and 0.044 inches (1.1 mm) for the 3.6L engine.

To check and adjust the gap:

1. Use a wire-style spark plug gap gauge. Avoid disc-style gauges, as they can damage the fine electrode tips of iridium and platinum plugs.
2. Carefully slide the correct feeler gauge between the center and ground electrodes. There should be a slight drag.
3. If adjustment is needed, gently bend the ground electrode only. Never bend or pry on the center electrode.

Replacement procedure tips for the Subaru Outback

Replacing spark plugs on a Subaru Boxer engine, like those in your Outback, can be more challenging than on many other vehicles due to the horizontally opposed cylinder design. The spark plugs are located on the sides of the engine, tucked away, making access a bit tighter.

1. Preparation: Ensure the engine is cool. Gather your tools: ratchets, extensions, universal joints, spark plug sockets (thin-walled 14mm for the 2.5L, 16mm for the 3.6L), torque wrench, dielectric grease, and anti-seize compound.
2. Access: You may need to remove air intake components or other ancillary items to gain better access to the spark plugs, especially on the passenger side.
3. Coil-on-Plug Removal: The 2017 Outback uses coil-on-plug ignition. Carefully disconnect the electrical connector from each ignition coil, then unbolt and gently twist and pull the coil straight up to remove it. Avoid pulling on the wire.
4. Spark Plug Removal: Use a spark plug socket with a rubber insert to prevent breaking the porcelain. Loosen the old plugs slowly. Take care not to drop anything into the spark plug wells.
5. Anti-Seize Debate: Some plug manufacturers advise against using anti-seize on modern nickel-plated spark plugs, as it can alter torque readings and potentially cause over-tightening. Subaru does not specify anti-seize for factory-installed spark plugs. If you choose to use it, apply a very thin amount to the threads only and reduce your torque specification by about 10-20%.
6. Dielectric Grease: Apply a small amount of dielectric grease inside the spark plug boot (the end that connects to the spark plug). This helps seal out moisture, prevents corrosion, and makes future removal easier.
7. Installation: Thread the new spark plugs in by hand first to avoid cross-threading. Once finger-tight, use a torque wrench to tighten them to the specified 15.2 lb-ft / 20.6 Nm. Overtightening can damage the plug or cylinder head; undertightening can cause misfires or allow the plug to vibrate loose.

Known issues

The 2017 Subaru Outback's FB25DI and EZ36D engines are generally reliable, and there are no widespread, specific spark plug-related "known issues" like the cracked porcelain or two-piece plug breakage seen in some other manufacturers' engines. However, like any vehicle, neglecting spark plug maintenance can lead to misfires, reduced fuel economy, and potential damage to ignition coils or catalytic converters.

Need plugs replaced? Find a shop on Repairius.