2024 Subaru Forester: Spark Plug Type, Gap & Torque

Quick reference

Why the right plug matters

The spark plug is a small but critical component in your Forester's ignition system. It's responsible for igniting the air-fuel mixture in the combustion chamber, and using the correct plug is essential for engine performance, fuel economy, and emissions. The most important factors to consider are 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" can lead to pre-ignition and detonation, while a "cold" plug can foul with carbon deposits. The engineers at Subaru have selected the optimal heat range for the 2.5L engine in your Forester, so it's crucial to stick with the recommended part number.
• Electrode material: The center and ground electrodes of a spark plug can be made from a variety of materials, each with its own advantages and disadvantages.
  • Iridium: Iridium is a precious metal that is extremely hard and has a very high melting point. This makes it an ideal material for spark plug electrodes, as it allows for a very fine-wire center electrode that provides a more focused and consistent spark. Iridium plugs also have a very long service life, typically 60,000 to 120,000 miles.
  • Platinum: Platinum is another precious metal that is often used in spark plugs. It is not as hard as iridium, but it still provides a very long service life and a consistent spark. Platinum plugs are a good mid-range option, with a typical service life of 60,000 to 80,000 miles.
  • Copper: Copper is a good conductor of electricity, but it is not as durable as iridium or platinum. Copper plugs have a shorter service life, typically 20,000 to 30,000 miles, but they are also the most affordable option.

How to gap a spark plug

All spark plugs come pre-gapped from the factory, but it's always a good idea to check the gap before installation. The gap is the distance between the center electrode and the ground electrode, and it's critical for proper ignition. To check the gap, you'll need a feeler gauge or a spark plug gap tool. Insert the appropriate wire or feeler into the gap. It should be a snug fit, but you shouldn't have to force it. If the gap is too wide, you can gently tap the ground electrode on a hard surface to close it up. If the gap is too narrow, you can use the hook on your gap tool to gently bend the ground electrode open.

Replacement procedure tips

The 2.5L engine in your 2024 Forester uses a coil-on-plug ignition system, which means that each spark plug has its own individual ignition coil. This makes changing the plugs a bit more involved than on an older engine with a distributor and spark plug wires, but it's still a job that can be done by a DIYer with basic hand tools.

• Coil-on-plug removal: To access the spark plugs, you'll first need to remove the ignition coils. Each coil is held in place by a single bolt. Once the bolt is removed, the coil can be gently twisted and pulled straight up and out of the spark plug well.
• Anti-seize debate: There is some debate in the automotive community about whether or not to use anti-seize on spark plug threads. The concern is that the anti-seize can act as a lubricant, which can lead to over-tightening of the plugs. However, many technicians still recommend using a small amount of anti-seize on the threads to prevent them from seizing in the cylinder head. If you do choose to use anti-seize, be sure to use it sparingly and reduce the torque spec by about 10%.
• Dielectric grease on boots: It's a good idea to apply a small amount of dielectric grease to the inside of the spark plug boot before reinstalling the ignition coil. This will help to seal out moisture and prevent the boot from sticking to the spark plug.

Known issues

While there are no major known issues with the spark plugs on the 2024 Subaru Forester, there are a few general issues to be aware of:

• Cracked porcelain: The porcelain insulator on a spark plug can crack if it is dropped or mishandled. A cracked insulator can cause a misfire, so it's important to inspect the plugs carefully before installation.
• Broken plug shells: The metal shell of a spark plug can break if it is over-tightened. This is a serious problem, as it can be very difficult to remove the broken piece of the plug from the cylinder head.

Need plugs replaced? Find a shop on Repairius.