🔧 Forester - Spark Plug Replacement

Replacing spark plugs restores smooth starting, idle, and fuel economy, and helps prevent misfires. On your Forester’s 2.5L engine, access is tight at the sides, so you’ll remove a few nearby components to reach each plug safely.

Difficulty Level: Intermediate | Estimated Time: 1.5-3.0 hours

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⚠️ Safety & Precautions

• ⚠️ Work on a completely cool engine to avoid thread damage and burns.
• ⚠️ Disconnect the negative battery terminal before unplugging ignition coils.
• ⚠️ Do not over-tighten spark plugs; aluminum cylinder head threads strip easily.
• ⚠️ Keep dirt out of the spark plug holes; contamination can damage the engine.

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🔧 Required Tools

You'll need the following tools for this repair:

• Safety glasses
• Nitrile gloves
• 10mm socket
• 10mm wrench
• 3/8" drive ratchet
• 3/8" drive torque wrench (5-30 ft-lbs range)
• 3/8" drive extension set (3", 6", 10")
• 3/8" drive wobble extension
• 3/8" drive universal joint
• 14mm thin-wall spark plug socket (3/8" drive)
• Flathead screwdriver
• Needle-nose pliers
• Trim clip removal tool
• Flashlight
• Compressed air blow gun

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Spark plugs (iridium, correct fit for your Forester) - Qty: 4
• Dielectric grease - Qty: 1

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📋 Before You Begin

• Park on level ground, shift to Park, and set the parking brake.
• Open the hood and let the engine cool fully (at least 1 hour).
• Use a 10mm wrench to disconnect the negative battery terminal and move it aside so it can’t spring back.
• Wobble extension = angled extension for tight spaces.
• Dielectric grease = silicone grease for coil boot sealing.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the intake ducting (passenger side access)

• Use a flathead screwdriver to loosen the hose clamp(s) on the intake snorkel/duct.
• Use a trim clip removal tool to pop any plastic clips holding the snorkel in place.
• Lift the ducting out and set it aside.

Step 2: Create space on the driver side (battery/washer area)

• If access is tight, remove the battery hold-down using a 10mm socket and 3/8" drive ratchet.
• Lift the battery out carefully (it’s heavy).
• If the washer filler neck blocks access, release its fastener(s) with a 10mm socket and move the neck aside (do not kink the hose). Use needle-nose pliers if a clamp needs to be slid back.

Step 3: Unplug and remove the ignition coil (do one cylinder at a time)

• Pick one coil to start with so connectors don’t get mixed up.
• Press the coil connector tab and unplug it (use a flashlight to see the tab clearly).
• Remove the coil retaining bolt with a 10mm socket and 3/8" drive ratchet.
• Gently twist and pull the coil straight out.
• Twist first; don’t yank the wiring.

Step 4: Clean around the spark plug well

• Blow out dirt around the plug hole using a compressed air blow gun.
• This prevents debris from falling into the cylinder when the plug comes out.

Step 5: Remove the old spark plug

• Install the 14mm thin-wall spark plug socket onto a 3/8" drive extension (use a universal joint or wobble extension if needed).
• Loosen and remove the spark plug with a 3/8" drive ratchet.
• If it feels extremely tight, stop and re-check that the socket is fully seated.

Step 6: Install the new spark plug (thread by hand first)

• Put the new plug into the 14mm thin-wall spark plug socket.
• Lower it into the hole and thread it in by hand using only the extension (no ratchet at first) to prevent cross-threading.
• Once seated, tighten with a 3/8" drive torque wrench: Torque to 20 Nm (15 ft-lbs).
• Do not use anti-seize on plated plugs.

Step 7: Reinstall the ignition coil

• Apply a very small amount of dielectric grease inside the coil boot.
• Push the coil straight onto the plug until it fully seats.
• Install the coil bolt with a 10mm socket and torque using a 3/8" drive torque wrench: Torque to 7.5 Nm (5.5 ft-lbs).
• Reconnect the electrical connector until it clicks.

Step 8: Repeat for the remaining 3 spark plugs

• Repeat Steps 3–7 for each cylinder.
• Use the universal joint and wobble extension as needed for the rearward plugs.

Step 9: Reassemble removed components

• Reinstall the washer filler neck (if moved) using a 10mm socket.
• Reinstall the battery and hold-down using a 10mm socket.
• Reconnect the negative battery terminal using a 10mm wrench.
• Reinstall the intake ducting and tighten clamps with a flathead screwdriver.

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✅ After Repair

• Start the engine and let it idle for 1–2 minutes. It should run smooth with no shaking.
• Listen for ticking or a “puffing” sound that could indicate a loose plug or coil not fully seated.
• If the check engine light is on or it runs rough, re-check coil connectors and coil seating first.

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💰 DIY vs Shop Cost

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

DIY Cost: $50-$120 (parts only)

You Save: $200-$330 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 1.5-2.5 hours.

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🎯 Ready to get started?

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