How to Change Spark Plugs on a Subaru

Changing spark plugs on a Subaru is a straightforward DIY task that can improve fuel efficiency, engine performance, and overall reliability. With the right tools and a bit of patience, even beginners can complete this maintenance job in under an hour.

This is a comprehensive guide about How to Change Spark Plugs on a Subaru.

Key Takeaways

• Regular spark plug replacement improves engine performance: Worn spark plugs can cause misfires, poor fuel economy, and rough idling—replacing them restores smooth operation.
• Use the correct spark plug type and gap: Always consult your Subaru owner’s manual or a trusted parts guide to ensure you’re using the right plugs and proper gap setting.
• Special tools make the job easier: A spark plug socket, torque wrench, and extension are essential for safely removing and installing plugs without damage.
• Work on a cool engine: Never change spark plugs on a hot engine—this prevents burns and reduces the risk of stripping threads.
• Label and organize wires or coils: Taking photos or labeling ignition coils before removal helps avoid mix-ups during reassembly.
• Inspect old plugs for engine clues: The condition of your old spark plugs can reveal issues like oil leaks, rich fuel mixture, or overheating.
• Torque to spec for safety: Over-tightening can damage threads; under-tightening can lead to poor heat transfer—always follow manufacturer torque specs.

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Why Changing Spark Plugs Matters for Your Subaru

If you own a Subaru—whether it’s a trusty Outback, a sporty WRX, or a versatile Forester—you likely appreciate its reliability and all-wheel-drive capability. But like any vehicle, Subarus need regular maintenance to keep running smoothly. One of the most important yet often overlooked tasks is changing the spark plugs.

Spark plugs are small but mighty components that ignite the air-fuel mixture in your engine’s cylinders. Over time, they wear out due to heat, carbon buildup, and electrical stress. When they fail, your Subaru may experience reduced fuel efficiency, sluggish acceleration, engine misfires, or even difficulty starting. In severe cases, a bad spark plug can damage the catalytic converter, leading to expensive repairs.

The good news? Replacing spark plugs is one of the most cost-effective and accessible DIY maintenance jobs you can do. Unlike some car repairs that require specialized equipment or advanced mechanical skills, spark plug replacement is well within reach for most Subaru owners—even those with minimal experience. With the right tools, a little patience, and this guide, you’ll have your engine firing on all cylinders in no time.

When Should You Change Spark Plugs on a Subaru?

Knowing when to replace your spark plugs is just as important as knowing how. Subaru recommends specific intervals based on your vehicle’s model, engine type, and driving conditions. Most modern Subarus with platinum or iridium spark plugs can go between 60,000 and 100,000 miles before needing replacement. However, this isn’t a one-size-fits-all rule.

For example, the 2.5L flat-four engine found in many Outbacks and Foresters typically uses iridium plugs with a 105,000-mile service life. Meanwhile, performance-oriented models like the WRX or STI may have shorter intervals—around 30,000 to 60,000 miles—due to higher engine stress and boost pressure.

But mileage isn’t the only factor. If you notice any of the following symptoms, it might be time to check or replace your spark plugs early:

– Rough idling or engine vibration at stoplights
– Hesitation or jerking during acceleration
– Decreased gas mileage
– Engine misfires (you might see a flashing check engine light)
– Difficulty starting the car, especially in cold weather

It’s also wise to inspect your spark plugs during routine maintenance, such as oil changes or tire rotations. A quick visual check can reveal a lot about your engine’s health. For instance, if the electrode is heavily eroded or covered in oil, it could indicate a deeper issue like worn piston rings or valve seals.

Ultimately, following your Subaru’s maintenance schedule is the best practice. Check your owner’s manual or consult a Subaru service guide for your specific model year and engine. And remember: replacing spark plugs proactively is far cheaper than dealing with engine problems down the road.

Tools and Materials You’ll Need

Before you pop the hood, make sure you have the right tools and parts on hand. Having everything ready beforehand saves time and frustration—especially when you’re elbow-deep in the engine bay.

Here’s a complete list of what you’ll need:

Essential Tools

• Spark plug socket (5/8″ or 16mm): This specialized socket has a rubber insert to hold the plug securely and prevent damage during removal.
• Socket wrench with extensions: A 3/8” drive ratchet with a 6” extension works well for most Subaru engines.
• Torque wrench: Critical for tightening new plugs to the manufacturer’s specification—usually between 13 and 20 ft-lbs for Subarus.
• Spark plug gap tool: Even if your new plugs are pre-gapped, it’s smart to double-check the gap before installation.
• Dielectric grease: Apply a small amount to the inside of the ignition coil boots to prevent moisture and ensure a good connection.
• Compressed air or a small brush: Use this to clean debris from the spark plug wells before removing the old plugs.

Replacement Parts

• New spark plugs: Always use OEM (Original Equipment Manufacturer) or high-quality aftermarket plugs recommended for your Subaru model. Popular brands include NGK, Denso, and Subaru Genuine Parts.
• Optional: Anti-seize compound: While not always recommended by Subaru, a tiny dab on the threads can help prevent seizing—especially in older engines or high-heat applications.

Safety Gear

• Gloves: Protect your hands from sharp edges and hot components.
• Safety glasses: Debris can fall into the engine bay, so eye protection is a must.

Optional but Helpful

• Work light: Subaru engine bays can be tight and shadowy—good lighting makes a big difference.
• Trim removal tool: Helps safely remove plastic covers without breaking clips.
• Digital camera or phone: Take photos before disconnecting coils or wires to help with reassembly.

Pro tip: If you’re working on a turbocharged Subaru like a WRX, consider investing in a spark plug socket with a longer reach or a swivel joint. The turbo layout can make access trickier, and a flexible tool saves frustration.

Step-by-Step Guide to Changing Spark Plugs on a Subaru

Now that you’re prepared, let’s walk through the process step by step. While specific details may vary slightly depending on your Subaru model (e.g., EJ25, FA20, or FB25 engine), the general procedure is consistent across most vehicles.

Step 1: Prepare Your Vehicle

Start by parking your Subaru on a flat, level surface and engaging the parking brake. Let the engine cool completely—working on a hot engine can cause burns and may warp components. Open the hood and locate the engine cover. Most Subarus have a plastic cover held in place by clips or bolts. Use a trim tool or your fingers to gently remove it.

Next, identify the spark plug locations. On a flat-four (boxer) engine—common in most Subarus—the spark plugs are located on the sides of the engine, two on the driver’s side and two on the passenger side. Each plug is connected to an ignition coil (also called a coil pack) that sits directly on top.

Before you begin, take a photo of the engine bay with your phone. This will help you remember how everything goes back together, especially if you’re new to this.

Step 2: Remove the Ignition Coils

Each ignition coil is held in place by a single 10mm bolt. Use your socket wrench to loosen and remove these bolts. Set them aside in a small container so they don’t get lost.

Once the bolts are out, gently wiggle the coil pack to break the seal. Do not pull hard—there’s a rubber boot underneath that can tear if forced. Carefully lift the coil straight up and set it aside. Repeat this process for all four coils.

Tip: Label each coil with tape or a marker (e.g., “Front Driver,” “Rear Passenger”) to avoid mixing them up. While the coils are interchangeable, keeping them in their original positions can help maintain consistent performance.

Step 3: Remove the Old Spark Plugs

With the coils removed, you’ll see the spark plug wells. Use compressed air or a small brush to blow out any dirt or debris. This prevents contaminants from falling into the cylinder when you remove the plug.

Attach your spark plug socket to the extension and ratchet. Carefully insert the socket into the well and engage the plug. Turn counterclockwise to loosen. Once it’s free, unscrew it by hand to avoid cross-threading.

As you remove each plug, inspect it closely. A healthy plug should have a light tan or gray insulator tip. Signs of trouble include:

– Black, sooty deposits (rich fuel mixture)
– Oil fouling (possible valve seal or piston ring issue)
– White, blistered insulator (overheating)
– Excessive electrode wear (plug is past its lifespan)

Place the old plugs in order so you can compare them later.

Step 4: Check and Gap the New Spark Plugs

Before installing the new plugs, check the gap using your gap tool. The correct gap varies by model—common specs range from 0.040” to 0.044” for most Subarus. Refer to your owner’s manual or the plug manufacturer’s website for exact values.

If adjustment is needed, gently bend the side electrode. Never force it—small movements make a big difference.

Once gapped, apply a tiny amount of dielectric grease to the inside of each coil boot. This helps create a moisture-resistant seal and makes future removal easier.

Step 5: Install the New Spark Plugs

Carefully thread each new plug into the cylinder head by hand. This ensures you don’t cross-thread the delicate aluminum threads. Once it’s snug, use your torque wrench to tighten it to the manufacturer’s specification—typically 13–20 ft-lbs for Subaru engines.

Never overtighten. Aluminum threads can strip easily, leading to costly repairs. If you’re unsure, err on the side of slightly loose rather than too tight.

Step 6: Reinstall the Ignition Coils

Slide each coil pack back into place over the new spark plug. Press down firmly until you hear or feel it seat properly. Reinstall the 10mm bolts and tighten them securely—but again, don’t over-torque.

Reconnect any electrical connectors if they were disconnected (some models have clips that release when you lift the coil).

Step 7: Reassemble and Test

Replace the engine cover and secure all clips or bolts. Close the hood and start the engine.

Listen for smooth idle and acceleration. If everything sounds good, take your Subaru for a short test drive to ensure it’s running properly.

Tips for Specific Subaru Models

While the basic process is similar across models, some Subarus have quirks worth noting.

Subaru WRX and STI (Turbocharged Models)

Turbocharged engines run hotter and under more stress, so spark plug replacement is more critical. Use high-quality iridium plugs and consider replacing them every 30,000–60,000 miles. Access can be tighter due to the turbocharger and intercooler piping—use a swivel socket or flexible extension if needed.

Subaru Crosstrek and Impreza (FB-Series Engines)

These newer models have a more compact engine layout. The spark plugs are still accessible from the sides, but the coil packs may require a bit more wiggling to remove. Be patient and avoid forcing anything.

Older Subarus (EJ22, EJ25)

If you’re working on a 1990s or early 2000s Subaru, you may encounter more carbon buildup or seized plugs. Apply a penetrating oil like PB Blaster to stubborn plugs and let it sit for 15–20 minutes before attempting removal.

Common Mistakes to Avoid

Even experienced DIYers can make errors when changing spark plugs. Here are a few pitfalls to watch out for:

– Using the wrong spark plugs: Always match the heat range and thread size. Using a colder or hotter plug than recommended can cause engine damage.
– Cross-threading the plugs: Always start by threading the plug by hand. If it doesn’t go in smoothly, don’t force it.
– Over-tightening: This is the #1 cause of stripped threads. Use a torque wrench—don’t guess.
– Forgetting to reconnect coils: It’s easy to miss one during reassembly. Double-check all four.
– Ignoring the old plugs: Their condition can tell you a lot about your engine’s health. Don’t toss them without a look.

Conclusion

Changing spark plugs on a Subaru is a rewarding DIY task that pays off in better performance, improved fuel economy, and peace of mind. With the right tools, a bit of patience, and attention to detail, you can complete this job in under an hour—even if you’re not a mechanic.

Remember to follow your Subaru’s maintenance schedule, use the correct parts, and always torque to spec. And don’t forget to inspect your old plugs—they’re like a window into your engine’s soul.

By staying on top of this simple maintenance task, you’ll keep your Subaru running strong for years to come. So grab your tools, roll up your sleeves, and give your boxer engine the spark it deserves.

Frequently Asked Questions

How often should I change spark plugs on a Subaru?

Most modern Subarus require spark plug replacement every 60,000 to 100,000 miles, depending on the engine and plug type. Turbocharged models may need them sooner—around 30,000 to 60,000 miles. Always check your owner’s manual for the exact interval.

Can I change spark plugs myself, or should I go to a mechanic?

Yes, you can absolutely change spark plugs on a Subaru yourself. It’s a beginner-friendly job that requires basic tools and about 30–60 minutes of your time. Just follow the steps carefully and use the right parts.

What happens if I don’t change my spark plugs?

Worn spark plugs can cause misfires, poor fuel economy, rough idling, and difficulty starting. In extreme cases, they can damage the catalytic converter, leading to expensive repairs.

Do I need to gap new spark plugs before installing them?

Most new spark plugs come pre-gapped, but it’s always a good idea to check with a gap tool. Incorrect gap can cause poor ignition and engine performance.

Can I reuse old ignition coils?

Yes, ignition coils typically last much longer than spark plugs. As long as they’re functioning properly and not cracked or damaged, you can reuse them when replacing plugs.

What torque specification should I use for Subaru spark plugs?

Subaru recommends torque values between 13 and 20 ft-lbs, depending on the model and engine. Always consult your service manual or the spark plug manufacturer’s guidelines to avoid over-tightening.