🔧 Forester - Spark Plug Replacement

Replacing the spark plugs on your Forester helps restore smooth starting, clean idle, fuel economy, and proper power. The 2.5L flat-4 engine has two spark plugs on each side of the engine, so access is tight but manageable with the right extensions and patience.

Difficulty Level: Intermediate | Estimated Time: 1.5-2.5 hours

---

⚠️ Safety & Precautions

• ⚠️ Work only on a completely cool engine; the cylinder heads and exhaust are close to the spark plugs.
• ⚠️ Disconnect the negative battery cable before starting to avoid accidental electrical shorts.
• ⚠️ Do not force a spark plug. Cross-threading means the plug is going in crooked and can damage the aluminum cylinder head.
• ⚠️ Use only the correct spark plugs for your Forester. Do not guess on plug type or heat range.
• ⚠️ Replace spark plugs one at a time so ignition coil connectors and locations do not get mixed up.

---

🔧 Required Tools

You'll need the following tools for this repair:

• 10mm socket
• 12mm socket
• 14mm spark plug socket
• 3/8-inch drive ratchet
• 3/8-inch drive torque wrench
• 3-inch 3/8-inch drive extension
• 6-inch 3/8-inch drive extension
• 3/8-inch drive universal joint adapter
• Flat-blade screwdriver
• Needle-nose pliers
• Spark plug gap gauge
• Compressed air blow gun
• Battery terminal wrench 10mm
• Mechanic gloves
• Safety glasses

---

🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Spark plugs - Qty: 4
• Dielectric grease - Qty: 1 small tube

---

📋 Before You Begin

• 🚗 Park your Forester on level ground, shift into neutral, and set the parking brake.
• ❄️ Let the engine cool fully before touching the spark plug area.
• 🔋 Use a 10mm battery terminal wrench to disconnect the negative battery cable.
• 🧼 Blow loose dirt away from the ignition coil areas before removing parts. Dirt falling into a spark plug hole can damage the engine.
• 📏 Check each new spark plug gap with a spark plug gap gauge. The gap is the small space between the center tip and the curved ground electrode.
• ⚠️ Many iridium spark plugs come pre-gapped. Do not pry hard on the fine center electrode.

---

🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the Battery for Left-Side Access

• Use a 10mm battery terminal wrench to loosen the negative battery terminal, then move the cable aside.
• Use a 10mm socket to loosen and remove the positive battery terminal.
• Use a 10mm socket to remove the battery hold-down bracket.
• Lift the battery straight up and out. Batteries are heavy, so use both hands.
• Use a 10mm socket to remove the battery tray if it blocks access to the driver-side ignition coils.
• Take a photo before removal.

Step 2: Remove the Air Intake Parts for Right-Side Access

• Use a flat-blade screwdriver to loosen the intake tube clamp at the air box.
• Use a flat-blade screwdriver to loosen the intake tube clamp at the throttle body area if extra room is needed.
• Use needle-nose pliers to release any small hose clamps attached to the intake duct.
• Lift the intake duct out of the way carefully.
• If the air box blocks the passenger-side coils, use a 12mm socket to remove the air box mounting bolts and move the air box aside.

Step 3: Clean Around the Ignition Coils

• Put on safety glasses.
• Use a compressed air blow gun to blow dirt and grit away from the ignition coils and spark plug tube areas.
• The ignition coil is the small electrical part that sits on top of each spark plug and sends spark to it.
• Do one spark plug at a time from this point forward.

Step 4: Remove One Ignition Coil

• Use your fingers to press the electrical connector release tab on the ignition coil.
• If the connector is stuck, use a small flat-blade screwdriver gently on the release tab only.
• Use a 10mm socket to remove the ignition coil retaining bolt.
• Twist the ignition coil slightly by hand, then pull it straight out.
• Set the coil where it cannot fall or get dirty.

Step 5: Remove the Old Spark Plug

• Use a 14mm spark plug socket, 3-inch extension, 6-inch extension, universal joint adapter, and 3/8-inch ratchet as needed for access.
• A universal joint adapter lets the socket bend slightly so you can reach plugs in tight spaces.
• Turn the old spark plug counterclockwise slowly to loosen it.
• Once loose, spin it out by hand using the extension and spark plug socket.
• Pull the old spark plug out carefully so it does not drop.
• Slow turns prevent thread damage.

Step 6: Inspect and Prepare the New Spark Plug

• Use a spark plug gap gauge to verify the new spark plug gap is about 0.039-0.043 in.
• Do not apply anti-seize unless the spark plug manufacturer specifically requires it. Most modern plated spark plugs are installed dry.
• Apply a very small amount of dielectric grease inside the ignition coil boot only. Dielectric grease helps seal moisture but does not go on the metal spark plug threads.

Step 7: Install the New Spark Plug by Hand

• Place the new spark plug into the 14mm spark plug socket.
• Use only the extension by hand to start threading the spark plug clockwise.
• Turn it several full turns by hand before using a ratchet.
• If it feels tight right away, stop, remove it, and start again straight.
• Use a 3/8-inch drive torque wrench with the 14mm spark plug socket to tighten the spark plug to Torque to 17.5 Nm (12.9 ft-lbs).

Step 8: Reinstall the Ignition Coil

• Push the ignition coil straight onto the new spark plug until it seats fully.
• Use a 10mm socket to install the ignition coil retaining bolt.
• Tighten the ignition coil bolt snugly; do not overtighten the small bolt.
• Reconnect the electrical connector until it clicks.

Step 9: Repeat for the Other Three Spark Plugs

• Repeat Steps 3 through 8 for each remaining spark plug.
• Use the 14mm spark plug socket, extensions, universal joint adapter, and ratchet as needed for each plug.
• Use the 3/8-inch drive torque wrench on every new spark plug and tighten to Torque to 17.5 Nm (12.9 ft-lbs).
• Reconnect each ignition coil connector before moving to the next plug.

Step 10: Reinstall the Air Intake Parts

• Use a 12mm socket to reinstall the air box mounting bolts if removed.
• Position the intake duct back in place.
• Use needle-nose pliers to reconnect any small hose clamps that were removed.
• Use a flat-blade screwdriver to tighten the intake tube clamps securely.
• Make sure the intake duct is fully seated with no gaps.

Step 11: Reinstall the Battery

• Use a 10mm socket to reinstall the battery tray if removed.
• Place the battery back into position.
• Use a 10mm socket to reinstall the battery hold-down bracket.
• Use a 10mm battery terminal wrench to reconnect the positive battery terminal first.
• Use a 10mm battery terminal wrench to reconnect the negative battery terminal last.
• Make sure both battery terminals are tight and cannot twist by hand.

---

✅ After Repair

• ✅ Start the engine and let it idle for a few minutes.
• ✅ Listen for a smooth idle and check that the engine does not stumble or misfire.
• ✅ Check that the air intake tube is fully connected and no hoses were left loose.
• ✅ If the idle feels slightly different at first, let the engine idle until warm, then take a gentle test drive.
• ✅ Recheck the battery terminals after the test drive to make sure they are still tight.

---

💰 DIY vs Shop Cost

Shop Cost: $250-$450 (parts + labor)

DIY Cost: $45-$120 (parts only)

You Save: $130-$330 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 1-2 hours.

---

🎯 Ready to get started?

HowToo makes it easy: same-day/2-day shipping on every part, plus all the tools and specialty tools you need! Check out the parts and tools sections below to add everything to your cart.