How to Change Spark Plugs on Subaru

Changing spark plugs on a Subaru is a straightforward DIY task that can improve engine performance, fuel economy, and reliability. With the right tools and a bit of patience, most Subaru owners can complete this maintenance job in under an hour—no mechanic required.

Key Takeaways

• Regular spark plug replacement improves engine performance: Fresh spark plugs ensure efficient combustion, smoother idling, and better acceleration.
• Subaru engines often require special tools: Due to their boxer engine layout, accessing spark plugs may need a swivel socket and extension.
• Use OEM or high-quality replacement plugs: Subaru recommends specific heat ranges and gap settings—stick to NGK or Denso for best results.
• Check and replace one plug at a time: This prevents mixing up ignition coils and reduces the risk of cross-threading.
• Torque specifications matter: Over-tightening can damage threads; under-tightening may cause misfires. Always use a torque wrench.
• Inspect ignition coils during replacement: Look for cracks, carbon tracking, or corrosion—replace if damaged to avoid future issues.
• Reset the ECU after replacement: Clearing error codes and allowing the engine to relearn ensures optimal performance post-installation.

Why Changing Spark Plugs on Your Subaru Matters

If you own a Subaru—whether it’s a trusty Outback, a sporty WRX, or a rugged Forester—you know these vehicles are built for reliability and performance. But like any machine, they need regular maintenance to keep running smoothly. One of the most important yet often overlooked tasks is changing the spark plugs.

Spark plugs are the tiny but mighty components responsible for igniting the air-fuel mixture in your engine’s cylinders. Over time, they wear out, become fouled, or lose their ability to create a strong spark. When that happens, you might notice symptoms like rough idling, poor fuel economy, hesitation during acceleration, or even engine misfires. Replacing old spark plugs restores efficient combustion, improves throttle response, and can even boost your gas mileage by up to 30% in some cases.

The good news? Changing spark plugs on a Subaru is a job most DIYers can handle with basic tools and a little patience. Unlike some cars with cramped engine bays, many Subaru models—especially those with the horizontally opposed (boxer) engine—are actually quite accessible once you know where to look. This guide will walk you through the entire process, from gathering the right tools to testing your work, so you can keep your Subaru running like new.

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 changing spark plugs at specific intervals, but these can vary depending on your driving habits, fuel quality, and the type of plugs you’re using.

For most modern Subaru models with standard copper or nickel spark plugs, the recommended replacement interval is every 30,000 to 40,000 miles. However, many Subarus now come equipped with long-life iridium or platinum plugs, which can last up to 60,000 to 100,000 miles. Always check your owner’s manual for the exact recommendation for your vehicle.

But mileage isn’t the only factor. If you notice any of the following signs, it’s time to inspect or replace your spark plugs—even if you haven’t hit the recommended mileage:

– Engine misfires or hesitation during acceleration
– Rough idling or stalling
– Decreased fuel efficiency
– Difficulty starting the engine, especially when cold
– A noticeable drop in power or responsiveness

For example, if you drive a 2015 Subaru Forester with 70,000 miles and notice it’s sluggish on highway on-ramps, worn spark plugs could be the culprit. Similarly, a 2018 WRX owner who frequently drives in stop-and-go traffic may need to replace plugs sooner than someone who mostly cruises on the highway.

Checking Spark Plug Condition

Before replacing all four (or six, depending on your engine), it’s wise to inspect the current plugs. Remove one and examine the electrode tip. A healthy spark plug should have a light tan or gray color. If you see black, sooty deposits, it could indicate a rich fuel mixture. White, blistered tips suggest overheating, possibly from a lean mixture or incorrect heat range. Oil fouling—dark, wet deposits—points to worn piston rings or valve seals.

This visual inspection can help diagnose other engine issues, so don’t skip it. If one plug looks significantly different from the others, investigate further before replacing the rest.

Tools and Materials You’ll Need

Before diving in, make sure you have the right tools. Having everything ready beforehand will save time and frustration. Here’s a complete list of what you’ll need to change spark plugs on most Subaru models:

– **Spark plug socket (5/8” or 16mm)**: A deep socket with a rubber insert helps hold the plug securely.
– **Socket wrench with extensions**: A 6-inch extension is usually sufficient, but a 12-inch may help in tighter spaces.
– **Swivel joint or universal socket adapter**: Essential for accessing plugs at awkward angles in the boxer engine.
– **Torque wrench**: Critical for tightening plugs to the correct specification.
– **Spark plug gap tool**: To check and adjust the gap on new plugs if needed.
– **Dielectric grease**: Applied to the inside of ignition coil boots to prevent moisture and improve conductivity.
– **Compressed air or a small brush**: To clean debris from the spark plug wells.
– **New spark plugs**: Use OEM-recommended plugs (NGK or Denso are common for Subarus).
– **Gloves and safety glasses**: Protect your hands and eyes from dirt and debris.
– **Shop rags or paper towels**: For cleaning and wiping components.

Choosing the Right Spark Plugs

Subaru engines are precision-tuned, and using the wrong spark plugs can lead to poor performance or even engine damage. Always refer to your owner’s manual or the sticker under the hood for the correct part number. For most Subarus, NGK Iridium IX or Denso Iridium Power plugs are excellent choices. These offer longer life and better performance than standard copper plugs.

Avoid cheap, off-brand plugs. They may save you a few dollars upfront but can cause misfires, reduced fuel economy, and premature failure. Stick with reputable brands and ensure the heat range matches your engine’s requirements.

Also, check the gap on new plugs before installation. While many come pre-gapped, it’s wise to verify. Use a gap tool to measure the distance between the center and ground electrode. Most Subaru engines require a gap of 0.043 to 0.047 inches (1.1 to 1.2 mm), but confirm this for your specific model.

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

Now that you’re prepared, let’s get to the actual process. While the exact steps may vary slightly depending on your Subaru model (e.g., Impreza, Legacy, Crosstrek, or BRZ), the general procedure is consistent across most boxer-engine Subarus.

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 make components harder to remove. Open the hood and locate the engine cover (if present). Many Subarus have a plastic cover over the top of the engine held in place by clips or bolts. Remove it carefully and set it aside.

Next, disconnect the negative terminal of the battery. This is a safety precaution to prevent accidental electrical shorts while working near the ignition system. Use a wrench to loosen the clamp and gently wiggle the cable off the terminal.

Step 2: Locate the Ignition Coils and Spark Plugs

On most Subaru boxer engines, the spark plugs are located on the sides of the engine block, with two on each side (for a 4-cylinder) or three on each side (for a 6-cylinder like in the Outback or Legacy). Each plug is connected to an ignition coil—a small, rectangular component that sits directly on top of the plug.

You’ll notice that the coils are held in place by a single bolt or clip. Some models have coil packs that cover two plugs, while others have individual coils. Take a moment to familiarize yourself with the layout. Labeling the coils with tape or taking a photo can help you remember which coil goes where—though they’re usually identical and interchangeable.

Step 3: Remove the Ignition Coils

Using a socket wrench or screwdriver, remove the bolt or clip securing each ignition coil. Gently wiggle the coil back and forth while pulling upward to disconnect it from the spark plug. Be careful not to yank on the electrical connector—some models have a locking tab that must be pressed before disconnecting.

Once the coil is free, carefully lift it out of the spark plug well. Set it aside in a clean, dry place. Repeat this process for each coil, working one side of the engine at a time to avoid confusion.

Step 4: Remove the Old Spark Plugs

Now it’s time to remove the old spark plugs. Use your spark plug socket, extension, and swivel adapter to reach into the well. Attach the socket to the plug and turn it counterclockwise to loosen. Once loose, carefully unscrew it by hand to avoid dropping debris into the cylinder.

As you remove each plug, inspect it for signs of wear or damage (as discussed earlier). Place the old plugs in a labeled container so you can compare them later if needed.

Before installing new plugs, use compressed air or a small brush to clean any dirt or debris from the spark plug well. This prevents contaminants from falling into the engine when you install the new plug.

Step 5: Install the New Spark Plugs

Take your new spark plug and check the gap one more time. Adjust if necessary using the gap tool. Then, carefully thread the plug into the hole by hand to avoid cross-threading. Once it’s snug, use the spark plug socket and torque wrench to tighten it to the manufacturer’s specification.

For most Subaru engines, the torque specification is between 13 and 18 ft-lbs (18 to 24 Nm). Over-tightening can strip the threads in the aluminum cylinder head, which is expensive to repair. Under-tightening can lead to poor heat transfer and potential misfires. Always use a torque wrench—don’t guess.

Step 6: Reinstall the Ignition Coils

Apply a small amount of dielectric grease to the inside of each ignition coil boot. This helps prevent moisture buildup and ensures a strong electrical connection. Carefully lower the coil over the new spark plug and press it down until it seats fully.

Reattach the coil bolt or clip and tighten it securely. Reconnect the electrical connector, making sure it clicks into place. Repeat for all coils.

Step 7: Reconnect the Battery and Test

Once all coils are reinstalled, reconnect the negative battery terminal and tighten the clamp. Replace the engine cover if you removed one.

Start the engine and listen for smooth operation. If the engine runs rough or you see a check engine light, double-check your connections. It’s also a good idea to use an OBD2 scanner to clear any stored error codes and reset the engine control unit (ECU). This allows the system to relearn optimal fuel and ignition timing.

Tips for Success and Common Mistakes to Avoid

Changing spark plugs on a Subaru is generally straightforward, but a few common mistakes can cause problems down the road. Here are some pro tips to ensure a smooth job:

– **Work on one plug at a time:** This prevents mixing up coils and reduces the chance of dropping tools or debris into the engine.
– **Don’t overtighten:** Aluminum cylinder heads are soft. Use a torque wrench and follow specs precisely.
– **Use dielectric grease sparingly:** A thin layer is enough—too much can trap heat or interfere with the connection.
– **Avoid cross-threading:** Always start threading the plug by hand. If it doesn’t go in easily, back it out and try again.
– **Check for oil leaks:** If you notice oil in the spark plug wells, it could indicate a leaking valve cover gasket—address this before replacing plugs.
– **Keep the area clean:** Dirt and debris can contaminate the combustion chamber if it falls into the well.

When to Call a Mechanic

While most Subaru owners can handle spark plug replacement, there are situations where professional help is wise. If you’re uncomfortable working with electrical components, lack the right tools, or encounter seized or broken plugs, it’s better to let a certified technician take over. Additionally, if your Subaru has a turbocharged engine (like the WRX or STI), extra care is needed due to higher combustion pressures and heat.

Maintaining Your Subaru After Spark Plug Replacement

Once your new spark plugs are in, keep up with regular maintenance to maximize their lifespan and your Subaru’s performance. Follow the recommended service schedule, use high-quality fuel, and consider using fuel system cleaners periodically to prevent carbon buildup.

Also, monitor your engine’s performance. If you notice any of the symptoms mentioned earlier—rough idle, hesitation, or reduced mileage—it could indicate another issue, such as a failing ignition coil, fuel injector problem, or sensor malfunction.

Conclusion

Changing spark plugs on your Subaru is a simple yet powerful way to maintain engine health, improve fuel efficiency, and ensure reliable performance. With the right tools, a bit of patience, and attention to detail, this DIY task can save you money and give you peace of mind knowing your vehicle is in top condition.

Whether you’re driving a daily commuter or a weekend adventure machine, fresh spark plugs help your Subaru deliver the smooth, responsive power it’s known for. So grab your tools, follow this guide, and enjoy the satisfaction of a job well done—and a better-running car.

Frequently Asked Questions

How often should I change spark plugs on my Subaru?

Most Subaru models with standard plugs need replacement every 30,000 to 40,000 miles. If your vehicle has long-life iridium or platinum plugs, you can wait 60,000 to 100,000 miles. Always check your owner’s manual for the exact interval.

Can I change spark plugs myself on a Subaru?

Yes! Most Subaru owners can change spark plugs at home with basic tools. The boxer engine layout is actually quite accessible once you know where to look. Just take your time and follow proper procedures.

What spark plugs does Subaru recommend?

Subaru typically recommends NGK or Denso spark plugs. Iridium or platinum plugs are preferred for their longevity and performance. Always match the heat range and gap specified in your owner’s manual.

Do I need a torque wrench to install spark plugs?

Yes. Using a torque wrench ensures you tighten the plugs to the correct specification (usually 13–18 ft-lbs). Over-tightening can damage the cylinder head, while under-tightening may cause misfires.

Why does my Subaru run rough after changing spark plugs?

This could be due to a loose connection, incorrect plug gap, or a damaged ignition coil. Double-check all connections, verify the gap, and inspect coils for cracks or carbon tracking. Resetting the ECU may also help.

Can I reuse old ignition coils when changing spark plugs?

Yes, as long as they’re in good condition. Inspect each coil for cracks, corrosion, or carbon tracking. If any show signs of damage, replace them to prevent future misfires.