2016 Subaru Forester Spark Plug Type, Gap & Torque

Quick Reference: 2016 Subaru Forester Spark Plugs

Why the Right Spark Plug Matters

Choosing the correct spark plug is critical for engine health, performance, and fuel economy. It’s not just about finding a plug that fits; it's about matching the plug's characteristics to the engine's demands.

• Heat Range: This is the most critical factor. A spark plug's heat range determines how quickly it dissipates heat from the combustion chamber. An engine that runs hot, like the Forester's 2.0L turbo, requires a "colder" plug that can transfer heat away quickly to prevent pre-ignition (detonation), which can cause severe engine damage. A "hotter" plug is used in lower-power engines to burn off carbon deposits more effectively. Using the wrong heat range can lead to fouling (if too cold) or engine-damaging knocking (if too hot).

• Electrode Material: The material of the center and ground electrodes dictates the plug's lifespan and performance. The 2016 Forester uses Iridium-tipped plugs from the factory.

  • Iridium/Platinum: These precious metals have extremely high melting points and excellent conductivity. This allows for a very fine-wire center electrode, which creates a more concentrated and efficient spark. The benefit is a very long service life—typically 60,000 to 100,000 miles—and stable performance. They are the OEM choice for a reason.
  • Copper: While once the standard, copper plugs have a much shorter lifespan (20,000-30,000 miles) and are not recommended for the 2016 Forester. The modern coil-on-plug ignition system is optimized for the resistance and firing characteristics of iridium plugs.

How to Gap a Spark Plug

Even pre-gapped plugs should be checked before installation. The gap is the precise distance between the center and ground electrodes. An incorrect gap can cause a weak spark, leading to misfires and poor combustion.

1. Use the Right Tool: A feeler gauge or a spark plug gap tool with wire loops is essential. Avoid the simple disc-style gauges, as they can put pressure on the fragile fine-wire electrode and damage it.
2. Check the Gap: Slide the correct wire gauge (e.g., 0.043" for the 2.5L) between the electrodes. It should have a slight drag, but you shouldn't have to force it.
3. Adjust Carefully: If adjustment is needed, only bend the ground strap (the curved metal piece over the center electrode). Use the tool's built-in lever or a pair of needle-nose pliers to gently pry it open or tap it lightly on a hard surface to close the gap. Never touch or put pressure on the center iridium electrode or the porcelain insulator.

Replacement Procedure Tips for Your Forester

The Subaru Boxer engine places the spark plugs on the sides of the engine, making access a bit different than on an inline or V-type engine. Both the 2.5L and 2.0L engines use a modern coil-on-plug system, where each spark plug has its own ignition coil sitting directly on top of it. There is no central distributor or old-fashioned spark plug wires.

1. Coil-on-Plug Removal: To access the plugs, you must first unbolt and remove the ignition coil. It's a single bolt (typically 10mm or 12mm). After removing the bolt, gently twist and pull the coil straight out. The rubber boot can sometimes create a seal, so a firm, steady pull is needed.

2. The Anti-Seize Debate: This is a hot topic. Modern NGK plugs have a special trivalent plating that acts as an anti-seize and prevents corrosion. NGK officially recommends not using additional anti-seize lubricant on these plugs. Applying it can act as a lubricant, leading you to over-torque the plug, which can stretch the threads in the aluminum cylinder head or even break the plug. It's best to follow the plug manufacturer's recommendation: install them clean and dry.

3. Dielectric Grease: While you should skip the anti-seize on the threads, applying a small amount of dielectric grease to the inside of the coil boot before reinstalling it is highly recommended. This prevents the boot from sticking to the spark plug porcelain in the future and helps seal out moisture, preventing corrosion on the terminal.

4. Torque is Critical: Use a torque wrench. Over-tightening can damage the cylinder head, while under-tightening can allow the plug to vibrate loose and cause poor heat transfer. The specified torque (15.6 lb-ft for the 2.5L, 12.5 lb-ft for the 2.0L) is designed to properly crush the new gasket washer, creating a perfect seal.

Known Issues

Fortunately, the 2016 Subaru Forester engines are not known for the major spark plug-related issues seen in some other vehicles. You don't have to worry about the infamous two-piece plug breakage common on some Ford 3-valve engines or issues with ionization-sensing ignition coils found on certain GM models. The primary challenge with the Forester is simply the tight access on the sides of the engine bay. A good set of socket extensions and a swivel joint will be your best friend for this job.

Need plugs replaced? The access on a Boxer engine can be tricky. Find a shop on Repairius.