How to change spark plugs in a Subaru?
You can replace the spark plugs on most Subaru engines by removing each coil-on-plug, installing a new plug, and torquing to specification. It’s a doable DIY task with the right tools and careful attention to torque and gap specs.
This guide outlines a practical, model-aware approach for common Subaru engines (FA/FB four-cylinders, EJ-series four-cylinders, and H6 six-cylinders), including tool lists, step-by-step instructions, and safety tips. Always verify the exact torque and gap specs for your year and engine in the owner’s manual or service manual before you begin.
Understanding Subaru engines and spark plug basics
Most Subarus use a coil-on-plug ignition setup, with one ignition coil per cylinder. Four-cylinder engines (like many FA/FB and EJ models) have four plugs, while six-cylinder engines (the H6 family) have six plugs. Spark plugs for modern Subarus are typically iridium or platinum and are pre-gapped by the manufacturer; you should still confirm the correct gap for your specific model. Common gap recommendations are in the range of about 0.028–0.032 inches (0.7–0.8 mm), but always check the service manual for your engine. Replacement intervals are usually printed in the owner’s manual and can vary from around 60,000 to 100,000 miles depending on the plug type and engine family. Do not use anti-seize on most modern spark plugs unless explicitly directed by Subaru’s service documentation; instead, follow the recommended torque and lubrication guidance and use dielectric grease on the coil boots if desired.
Tools and parts you’ll need
Before you start, assemble the following tools and replacement parts to keep the job efficient and organized.
- Correct spark plugs for your engine (iridium or platinum, number equal to cylinders; e.g., 4 or 6).
- Spark plug socket (usually 5/8 inch hex) with a ratchet, extension, and a swivel if needed.
- Torque wrench capable of measuring in the low-to-mid range (typically around 18–22 Nm or 13–16 ft-lbs for many Subarus).
- Magnetic pickup tool or small magnet to help retrieve dropped plugs.
- Small flat-head screwdriver or pick for disconnecting ignition coil connectors and boot removal if required.
- Dielectric grease for the coil boots (optional but helpful); do not.Grease the plug threads.
- Rags and a spray to clean the area around the plug wells; compressed air if available.
- Anti-seize compound is generally not recommended for modern Subaru plugs unless specified by the service manual.
- Safety gear: gloves, eye protection, and a cool, parked vehicle with the parking brake engaged.
With these tools and parts on hand, you’re ready to begin the replacement process.
Step-by-step procedure (four-cylinder and six-cylinder Subaru engines)
Follow these steps in order to safely replace all spark plugs. If your car uses coil-on-plug assemblies, work on one cylinder at a time to avoid mixing up connectors.
- Ensure the engine is completely cool and disconnect the negative battery cable to prevent accidental short circuits.
- Identify the coil-on-plug assemblies: each cylinder will have its own coil sitting on top of the spark plug.
- Carefully unplug the electrical connector from each coil. Depress the locking tab and pull straight out to avoid damaging the connector.
- Remove the mounting bolt(s) for the coil using the appropriate driver, then gently lift the coil straight up to expose the spark plug. If the coil is sticking, use a small amount of penetrating oil around the base and wait a moment, then try again.
- Clean the area around the spark plug well to prevent debris from dropping into the cylinder. Use compressed air if available.
- Attach the spark plug socket to your ratchet with an extension and carefully thread the plug into the cylinder by hand at first to avoid cross-threading. Once the plug is seated, snug it with the ratchet, then torque to the specified value for your engine (check manual; typical range is around 18–22 Nm or 13–16 ft-lbs for many Subaru four-cylinder engines).
- Before installing the new plug, check and adjust the gap if required, following the manufacturer’s specification for your engine. If the plugs come pre-gapped, confirm the gap with a gauge as a precaution.
- Optionally apply a small amount of dielectric grease to the inside of the coil boot before reinstalling onto the new plug to aid future removal and seal the boot.
- Reinstall the coil onto the plug, align the bolt holes, and secure with the mounting bolt(s). Reconnect the electrical connector to the coil, ensuring it clicks into place.
- Repeat steps 3–9 for each remaining cylinder. Periodically pause to wipe away any oil or coolant seepage and ensure no tools or bolts are left behind in the engine bay.
- Reconnect the negative battery terminal, start the engine, and listen for smooth operation. If you see a check engine light or hear misfires, recheck the plug seating, gap, and coil connections.
After you’ve completed the replacement, take a few minutes to inspect for any loose connections or fuel smells and wipe down the engine bay to remove any stray oil or coolant.
Engine-specific notes
EJ-series and FA/FB four-cylinder engines
For most four-cylinder Subarus, the spark plugs are accessed from the top with coil-on-plug packs. The torque range commonly falls in the low-to-mid Teens of Newton-meters, and the gap is typically around 0.028–0.032 inches. Always confirm exact torque and gap in the service manual for your model year.
Six-cylinder H6 engines
Six-cylinder Subarus share the same coil-on-plug approach, but you’ll remove and replace six plugs. The torque and gap guidelines are similar to four-cylinder engines, but always verify the values in the official specs for your particular engine code and year.
Safety and maintenance tips
- Work in a well-ventilated area and keep the car secure with the parking brake engaged or wheels chocked.
- Always let the engine cool completely before starting; hot components can cause burns or damage.
- Do not mix up ignition coils or connectors. Label or photograph harnesses if you’re unsure about placement.
- Do not use anti-seize on modern spark plug threads unless the service manual specifies it for your engine.
- Test drive the vehicle after replacement and monitor for hesitation or misfires; address any issues promptly.
Common mistakes to avoid
- Cross-threading plugs by not starting threads by hand first.
- Over-tightening plugs, which can strip threads or damage the cylinder head.
- Using incorrect plug type or gap for your engine; always verify the part number and gap before installation.
- Forgetting to reconnect coil connectors or bolts, which can cause misfires or a no-start condition.
Summary
Changing spark plugs on a Subaru is a straightforward maintenance task that, with the right tools and careful attention to torque and gap specifications, can be completed at home. Key steps include identifying the correct plugs for your engine, removing each coil, accessing and replacing the plugs, and reassembling everything with proper torque. Always consult your owner’s manual or service manual for the exact specs for your model year and engine code, and consider professional help if you encounter restricted access or unfamiliar coil arrangements.
