2018 Subaru Outback Spark Plug Specs & Replacement Guide

Quick reference

Why the right plug matters

Choosing the correct spark plug for your 2018 Subaru Outback is crucial for optimal engine performance, fuel efficiency, and longevity. Spark plugs are not "one-size-fits-all," and several factors differentiate them, notably their heat range and electrode material.

• Heat Range: This refers to the plug's ability to dissipate heat from the combustion chamber. A "hot" plug retains more heat, while a "cold" plug dissipates more. Using a plug with an incorrect heat range can lead to pre-ignition, detonation, or excessive fouling. Subaru engineers precisely determine the ideal heat range for your Outback's engine to ensure efficient combustion and prevent damage.

• Electrode Metal:

  • Iridium: These are standard for your 2018 Outback. Iridium is an extremely durable and hard metal, allowing for very fine electrodes. This design provides a hotter, more concentrated spark, leading to better fuel economy, reduced emissions, and significantly longer service intervals (typically 60,000–100,000 miles). Both the 2.5L and 3.6L engines use iridium-tipped plugs from the factory.
  • Platinum: While not OEM for your 2018 Outback, platinum plugs offer good durability and performance, often outlasting copper plugs. They fall between copper and iridium in terms of cost and lifespan.
  • Copper: Copper plugs have a thicker copper core for excellent conductivity, but their electrodes wear down much faster, requiring more frequent replacement. They are generally not suitable for modern engines like those in your Outback due to their shorter lifespan and inability to meet emissions standards over extended periods.

Using the recommended iridium plugs ensures your Outback performs as intended, with reliable starts, smooth idling, and consistent power delivery.

How to gap a spark plug

While most iridium spark plugs come pre-gapped from the factory, it's always a good practice to double-check the gap before installation. This is especially true for iridium plugs, which can be easily damaged if mishandled during gapping.

1. Obtain a Gap Gauge: Use a wire-type spark plug gap gauge. Avoid using coin-style disc gauges on iridium plugs, as they can damage the fine iridium tip.
2. Check the Gap: Carefully slide the correct wire gauge (0.044 inches or 1.1 mm) between the center and ground electrodes. There should be a slight drag.
3. Adjust (if necessary): If the gap is incorrect, very gently bend the ground electrode. For iridium plugs, never bend the center electrode or pry against the iridium tip. If the gap is too small, use the gapping tool to gently push the ground electrode outwards. If it's too wide, carefully tap the ground electrode on a hard, non-marring surface until the correct gap is achieved. Extreme caution is advised to avoid damaging the delicate iridium tip.

Replacement procedure tips

Replacing spark plugs on a Subaru Outback, particularly the 2.5L and 3.6L Boxer engines, can be more involved due to their horizontally opposed cylinder layout. Accessing the plugs on the sides of the engine requires some specific attention.

• Coil-on-Plug Removal: Both engines utilize a coil-on-plug (COP) ignition system. To remove the spark plugs, you'll first need to disconnect the electrical connector from each ignition coil and then unbolt and remove the coil. Be gentle with the connectors, as older plastic can become brittle.

• Access: For the 2.5L and 3.6L engines, accessing the spark plugs, especially on the driver's side, can be tight. You may need specialized spark plug sockets (thin-walled) and various extensions and universal joints to reach them comfortably. Removing the battery and/or air intake components might provide better access on some cylinders.

• Anti-Seize Debate: There's an ongoing debate about using anti-seize compound on spark plug threads. Modern spark plugs often feature a special coating from the factory to prevent seizing and ensure proper torque readings. Applying anti-seize can act as a lubricant, causing you to overtighten the plug, potentially damaging the cylinder head threads. Subaru, like many manufacturers, generally recommends against using anti-seize on coated spark plugs. Follow the torque specifications precisely.

• Dielectric Grease on Boots: Apply a small amount of dielectric grease to the inside of the spark plug boots (the part that connects to the ceramic insulator of the spark plug). This helps with easier removal in the future, prevents moisture intrusion, and ensures a good electrical connection, insulating against voltage leaks.

• Torque Wrench is Essential: Given the aluminum cylinder heads, using a torque wrench is paramount. Overtightening can strip the threads in the head, leading to a costly repair. Undertightening can cause the plug to loosen, leading to misfires or damage. Stick to the specified 15.5 lb-ft (21 Nm).

Known issues

Fortunately, the 2018 Subaru Outback