2006–2008 Subaru Forester

2006-2008 Subaru Forester — Spark Plug Replacement

🔧🔧 Moderate 60–120 minutes

Quick Video Tutorial

Parts You Need

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Total Cost

$0.00

Part Name	Part Number	Price	Buy
Spark plugs, NGK Laser Platinum FR5AP-11 (quantity 4)	—	—	Amazon
Dielectric/silicone paste for plug boots	—	—	Amazon
Spark plug wire set (optional, replace if boots are cracked or brittle)	—	—	Amazon

💡 Pro tip: Order all parts at once to save on shipping

Step-by-Step Instructions

Follow these steps carefully

Work on a cold engine and disconnect the battery

Make sure the engine has been off for several hours; spark plugs must be removed from a cold aluminum EJ25 head, because removing them hot can strip the threads. Open the hood and disconnect the negative battery terminal with a wrench to eliminate any chance of a spark near the ignition system. A cold engine and a disconnected battery are the two non-negotiable starting conditions for this job.

Clear access on the passenger side

On the 2006-2008 Forester the EJ25 is a boxer engine, so the four plugs stick out horizontally on the sides of the heads, tucked close to the frame rails. On the passenger side, remove the air filter box and its intake snorkel to expose the two plug wells; this usually takes only clips and one or two bolts. With the airbox out, you will see the two plug wire boots pointing at you through the tight space. The side is clear when both boots are visible and reachable.

Clear access on the driver side

On the driver side, remove the battery and its hold-down, and if needed unbolt the windshield washer bottle or move it aside, to reach the two rear driver-side plugs. The rearmost driver-side plug is the tightest of the four and may require a swivel joint on your extension. Keep every removed bolt in a labeled tray so reassembly is exact. You are ready when all four boots are reachable.

Remove the plug wire boots

Grasp each boot by its thick rubber body, twist it gently to break the seal, and pull it straight off the plug; never yank on the wire itself. On this Forester the plug wires run from a single coil pack, so label or note the wire routing order as you go. A torn boot causes a misfire you will have to chase later, so if one cracks, plan to replace the wire set. All four boots are off when each plug well is open.

Remove the old spark plugs

Slide a 5/8-inch spark plug socket with a short extension and a universal swivel joint into each plug well from the side. Break each plug loose with a ratchet, then back it out by hand with the extension, feeling for any binding that would signal thread damage. Use an extendable magnet to fish out any plug that separates from the socket inside the well. All four plugs are out when every well is empty.

Inspect and gap the new plugs

Inspect the old plugs for oil fouling or heavy deposits, which on an EJ25 can hint at valve cover gasket leaks into the plug wells. Take the new NGK Laser Platinum FR5AP-11 plugs and verify the gap is 0.044 inches with a wire-style feeler gauge, adjusting gently if needed. Correct plugs and correct gap matter on the EJ25 because the wrong heat range causes driveability issues. The plugs are ready when all four are gapped and clean.

Install the new plugs by hand first

Start every plug by threading it in with just the extension and your fingers, never with the ratchet, so you feel immediately if it cross-threads in the aluminum head. Once each plug is finger-tight and seated, snug it with the ratchet to the proper firmness; do not over-torque, because aluminum threads strip easily. Each plug is correct when it seats fully on its crush washer and turns no further by hand.

Reinstall the wires with dielectric grease

Apply a dab of silicone dielectric paste inside each plug wire boot, then push each boot firmly onto its plug until it clicks over the terminal. Route the wires back exactly as they were, keeping them off hot exhaust surfaces and away from sharp edges. Firm, fully seated boots with no visible gap complete this step.

Reassemble the airbox, battery, and washer bottle

Reinstall the air filter box and intake snorkel on the passenger side, making sure the box seats fully and its clips are latched. Reinstall the battery and hold-down on the driver side, then the washer bottle, tightening each fastener you removed. Reconnect the negative battery terminal and tighten it. The bay is done when everything you removed is back and secure.

Start, check for misfires, and test drive

Start the engine and let it idle, listening for a smooth idle with no shaking or flashing check-engine light that would indicate a misfire. If a misfire code appears, shut off and recheck the boot seating and wire routing on the affected cylinder. Take a short test drive with gentle acceleration, confirming normal power and no stumbling. The job is complete when the engine idles smoothly and pulls cleanly under load.

All Done!

Test your repair before closing everything up. If something doesn't work right, double-check your steps.

⚠️ Common Mistakes to Avoid

• Starting a plug with a ratchet instead of by hand, which cross-threads the aluminum head and turns a tune-up into a head repair.
• Over-torquing the new plugs, stretching or stripping the threads in the EJ25 heads.
• Pulling plug wires off by the wire instead of the boot, tearing the boot and causing a new misfire.
• Mixing up the plug wire routing order back to the coil pack, which makes the engine run badly.
• Installing the wrong plug type or gap on the EJ25; use the specified NGK plugs gapped to 0.044 inches.
• Forgetting to reconnect the negative battery terminal or leaving the airbox unlatched.

Tools You'll Need

5/8-inch spark plug socket 3/8-inch ratchet Socket extensions (short and medium) Universal swivel joint Extendable magnet Wire-style spark plug gap gauge Dielectric/silicone paste Wrench for battery terminal Gloves and safety glasses

⚠️ Safety First!

• Only remove spark plugs from a fully cold engine; hot aluminum threads in the EJ25 heads strip easily.
• Disconnect the negative battery terminal before touching the ignition components.
• Wear safety glasses; debris falls from the plug wells and frame rails while you work.
• Keep your hands clear of the cooling fans, which can cycle even with the engine off.