It’s usually doable for a confident DIYer, but the level of difficulty depends on the engine in your Forester. Non-turbo 2.5L engines tend to be more straightforward, while turbocharged or newer engines can require extra disassembly and careful handling.
In this article we break down how accessibility varies by model year and engine, list the tools you’ll need, walk you through a step-by-step process, and share tips to avoid common pitfalls. We also outline when it might be smarter to leave the job to a shop.
Engine variants affect accessibility
The Subaru Forester has used several engine designs over its generations. Spark plug access ranges from relatively simple to moderately labor-intensive depending on whether you have a naturally aspirated 2.5L engine or a turbocharged 2.0L. The following notes help set expectations for common variants.
EJ25/2.5L naturally aspirated engines
In many Foresters equipped with the 2.5L NA engine, spark plugs sit on top of the cylinder heads beneath ignition coils. Access is usually reasonable, though some rear plugs may be harder to reach and mounting hardware for air intake or engine covers can complicate the job. Plan for a couple of hours for a full plug replacement on a typical 4-plug setup if you take your time and use the right extensions.
FA20/2.0L turbo and other turbo-era engines
Turbocharged Foresters often route air through intercoolers and additional intake pipes. To reach the plugs, you may need to remove the intercooler piping or part of the intake plenum. The job tends to take longer and requires extra care to avoid damaging hoses or unplugging the wrong connectors. Expect a higher level of complexity and a few more hours if you’re doing it yourself.
Tools and parts you’ll need
Having the right tools and parts on hand helps the job go smoothly and reduces the risk of damage to threads or coils.
- Long spark plug socket (usually 5/8" or 16mm) with extension and a ratchet
- Universal joint or flexible extension for tight spaces
- Torque wrench to match the manufacturer’s specification
- Flat-head or specialty screwdriver to help lift covers if needed
- Dielectric grease for ignition coil connectors
- Anti-seize compound for plug threads (use sparingly and only if recommended by Subaru)
- Replacement spark plugs specified for your engine (often iridium or platinum; check your owner’s manual or a trusted parts guide)
- Magnet or pick-up tool to retrieve dropped hardware
- Shop rag and compressed air (optional but helpful for cleaning plug wells)
- Basic hand tools: gloves, safety glasses, and a magnetic tray
With these tools, you’ll be prepared to complete the replacement in a clean, organized workspace.
Step-by-step replacement guide
The following sequence covers a typical 4-plug replacement on a Forester with a coil-on-plug design. If your model requires removing the intercooler or intake parts, adjust accordingly and refer to your service manual.
- Park on a level surface, engage the parking brake, and disconnect the negative battery terminal to prevent electrical shorts.
- Remove any engine covers or air intake ducts that block access to the spark plugs. For turbo models, loosen or remove intercooler piping as needed to reach the rear plugs.
- Identify the ignition coils and carefully unplug their electrical connectors. If coils are held by bolts, remove them and lift the coil assembly away from the plug.
- Clean the area around each plug well with a dry rag and, if available, blow out debris with compressed air to prevent debris from falling into the cylinder.
- Use the spark plug socket with extension to thread each old plug counterclockwise by hand until it loosens, then remove it with a slow, steady motion.
- Inspect the plug wells for signs of oil or fouling. If you notice oil on the plugs, address the underlying issue before installing new plugs.
- Thread the new plugs by hand to ensure they align properly, then seat them with a torque wrench to the manufacturer’s specification (do not overtighten).
- Re-seat the ignition coils, reconnect their electrical connectors, and ensure the coils are firmly in place.
- Reassemble any intercooler piping or intake components you removed, then reinstall engine covers if applicable.
- Reconnect the battery, start the engine, and listen for any misfires or unusual noises. If the engine runs smoothly, proceed to a short test drive.
- After driving, turn off the engine and recheck the area for leaks or loose hardware. If everything looks good, you’re done.
Following these steps carefully will help ensure a successful plug change without damaging threads or coils.
Common challenges and how to handle them
Subaru engines pose a few recurring challenges during spark plug replacement. Here are practical tips to overcome them.
- Tight access, especially on the rear bank or turbo models: use a flexible extension or wobble adapter and consider removing more intake components for space.
- Worn or stiff coil connectors: gently pry connectors with a flat tool and avoid yanking; apply a small amount of dielectric grease when reconnecting.
- Cross-threading risk: start plugs by hand to establish a straight thread path before using the torque wrench.
- Oil or carbon buildup in wells: clean wells and identify any valve cover or gasket issues before installing new plugs.
- Incorrect plug type or gap: always verify the correct plug type and gap for your engine in the manual or a trusted parts source.
These steps minimize common misfires, leaks, or damaged threads and help ensure a reliable replacement.
Maintenance considerations and tips
When to replace all plugs and choosing plug type
In most multi-cylinder engines, it’s wise to replace all plugs at the same time to maintain even performance and ignition timing. Subaru engines commonly use iridium or platinum plugs with longer service life than copper plugs, but intervals vary by model and plug type. Always follow the factory-recommended interval for your specific engine and plug choice.
Professional help vs. DIY
If you own a turbo Forester, have limited space around the engine, or feel uncertain about disconnecting intercooler plumbing, hiring a mechanic is a sensible option. A pro can complete the job efficiently and check for related issues (like coil packs or ignition wires) that could affect performance.
Safety considerations
Basic safety should guide every car maintenance task. Disconnect the battery before starting, wear eye protection, keep track of small fasteners, and avoid placing tools where they could fall into the engine bay. If the engine is hot, wait for it to cool to prevent burns.
When to call a pro
If you encounter stubborn coils, corroded connectors, severely recessed plugs, or if you’re unsure about your engine’s exact plug type and torque spec, a professional service can prevent common mistakes and ensure proper torque and sealing.
Bottom line
For most Forester models with a 2.5L naturally aspirated engine, spark plug replacement is a solid DIY project with the right tools and patience. Turbocharged variants and newer engines can be more challenging due to space constraints and intake/intercooler components. Always verify the specific engine type, plug spec, and torque values for your year, and consider professional help if accessibility or confidence becomes an issue.
Summary
- Accessibility varies by engine: easier on 2.5L NA, tougher on turbo and newer designs.
- Key tools: long spark plug socket, extensions, torque wrench, dielectric grease, and the correct plugs.
- Follow a careful step-by-step process to avoid cross-threading or damage to coils and wiring.
- Expect to spend 1–4 hours depending on engine and space availability; turbo models often require more time.
- When in doubt, consult the owner’s manual or a qualified technician to confirm plug type, spacing, and torque.