2019 Subaru Outback Spark Plug Type & Gap

Quick reference

Why the right plug matters

Choosing the correct spark plug for your 2019 Subaru Outback is crucial for optimal engine performance, fuel efficiency, and longevity. Spark plugs are not a "one-size-fits-all" component; various factors like heat range, electrode material, and gap specification are precisely engineered for your specific engine.

The "heat range" of a spark plug dictates how well it dissipates heat from the combustion chamber. A plug that is too "cold" (dissipates heat too quickly) can lead to fouling, while a plug that is too "hot" (dissipates heat too slowly) can cause pre-ignition and engine damage. Subaru specifies a particular heat range to ensure stable combustion and prevent issues.

Electrode material also plays a significant role in a plug's lifespan and performance. The 2019 Subaru Outback primarily uses iridium spark plugs as original equipment.

• Iridium: This precious metal is incredibly hard and has a very high melting point, allowing for very fine electrodes. This results in a more concentrated spark and a much longer lifespan, typically 60,000 to 100,000 miles. Both the 2.5L and 3.6L Outback engines use iridium plugs from the factory.
• Platinum: While not as durable as iridium, platinum plugs still offer a longer lifespan than copper, often around 40,000 to 60,000 miles. They are less common as OEM in modern Subaru engines but can sometimes be suitable aftermarket options.
• Copper: These are the most common and least expensive type but have the shortest lifespan, typically 10,000 to 20,000 miles. Due to their rapid wear, copper plugs are generally not recommended for your Outback, as they would require much more frequent replacement than the factory-specified iridium plugs.

How to gap a spark plug

The spark plug gap is the distance between the center and ground electrodes. This seemingly small measurement is critical for a strong, consistent spark. Too large a gap can lead to misfires, while too small a gap can result in a weak spark.

For your 2019 Subaru Outback, the specified gap is 0.044 inches (1.1 mm). While some spark plugs claim to be "pre-gapped," it's always a good practice to verify the gap before installation, especially with iridium plugs, as their fine electrodes can be easily damaged.

To gap a spark plug, you'll need a wire-style or coin-style gap gauge. Avoid using ramp-style gauges, as they can damage fine-wire iridium electrodes. Gently slide the appropriate wire gauge into the gap. If adjustment is needed, carefully use the bending tool on the gauge to slightly open or close the ground electrode. Never bend the center electrode, especially on iridium plugs, as it can easily snap off.

Replacement procedure tips

Replacing spark plugs on a Subaru Outback, especially the horizontally opposed ("boxer") engine, can be more challenging than on an inline or V-configuration engine due to limited access. Both the 2.5L and 3.6L engines utilize a coil-on-plug (COP) ignition system.

1. Access: You'll likely need to remove components like the air intake box or battery to gain better access to the spark plugs, particularly on the passenger side. Ensure the engine is cool before starting any work.
2. Coil-on-plug removal: Each spark plug has its own ignition coil mounted directly above it. Disconnect the electrical connector by depressing the release tab and pulling. Then, gently twist and pull the coil straight up to remove it from the spark plug well. Avoid yanking on the wires.
3. Cleaning: Before removing the old plugs, use compressed air to blow out any debris around the spark plug wells. This prevents dirt from falling into the combustion chamber once the old plug is out.
4. Anti-seize debate: There is ongoing debate about using anti-seize on spark plug threads. Most modern spark plugs, especially those from NGK and Denso, have a special coating on the threads that acts as an anti-corrosion barrier and lubricating agent. Adding anti-seize can alter the torque readings, leading to over-tightening and potential damage to the cylinder head threads. Subaru, along with many spark plug manufacturers, generally recommends against using anti-seize on new spark plugs. If you choose to use it, apply a very thin coat and reduce the torque specification by approximately 10-20%.
5. Dielectric grease: Apply a small amount of dielectric grease inside the rubber boot of each ignition coil. This helps to seal out moisture, prevents the boot from sticking to the spark plug, and aids in electrical insulation. Do not apply dielectric grease to the spark plug threads or the electrode tip.
6. Torque: Always torque the new spark plugs to the manufacturer's specifications (15.0 lb-ft / 20.3 Nm for both engines). Over-tightening can strip cylinder head threads or damage the spark plug, while under-tightening can lead to compression leaks and misfires.

Known issues

Fortunately, the 2019 Subaru Outback engines (FB25DI and EZ36D) are not known for specific common spark plug issues that plagued some other manufacturers (like the Ford 3-valve 2-piece spark plug breakage). The primary challenges lie with access due to the engine design.

Need plugs replaced? Find a shop on Repairius.