🔧 Highlander - Spark Plug Replacement

On your Highlander’s V6, replacing spark plugs restores smooth starting, power, and fuel economy. The front bank is straightforward, but the rear bank sits under the upper intake manifold (also called the “plenum,” the top air passage feeding the engine), so there’s extra removal work.

Difficulty Level: Intermediate | Estimated Time: 2.5-4.5 hours

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⚠️ Safety & Precautions

• ⚠️ Work on a cold engine to avoid burns and protect aluminum threads.
• ⚠️ Disconnect the negative battery terminal to prevent accidental shorting and misfires while connectors are unplugged.
• ⚠️ Keep dirt out of the intake runners; cover openings immediately after removing the upper intake manifold.
• ⚠️ Do not over-tighten spark plugs; stripped threads in an aluminum head are a major repair.
• ⚠️ Label hoses/connectors as you go so nothing is missed on reassembly.

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🔧 Required Tools

You'll need the following tools for this repair:

• Safety glasses
• Nitrile gloves
• 10mm socket
• 12mm socket
• 14mm socket
• 3/8" drive ratchet
• 3/8" drive torque wrench (5-80 ft-lbs range)
• 5/8" (16mm) spark plug socket
• 3/8" drive extension set (3", 6", 10")
• 3/8" drive swivel joint
• Flat trim clip tool
• Needle-nose pliers
• Pick tool
• Shop vacuum
• Compressed air blow gun
• Magnetic pickup tool
• Flashlight

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs (OEM-type) - Qty: 6
• Upper intake manifold gasket set - Qty: 1
• Throttle body gasket - Qty: 1
• Dielectric grease - Qty: 1

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📋 Before You Begin

• 🅿️ Park on level ground, shift to Park, and set the parking brake.
• 🧯 Let the engine cool fully (at least 1 hour after driving).
• 🔋 Use a 10mm socket to disconnect the negative battery terminal and tuck it aside so it can’t spring back.
• 🧼 Use a shop vacuum and compressed air to clean around the ignition coils before removing anything (keeps dirt from falling into the plug wells).

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the engine cover and open up access

• Pull up to remove the engine cover (no tools on most covers). Use a flashlight to locate the three front ignition coils.
• Set bolts/clips in labeled piles.

Step 2: Remove the front-bank ignition coils

• An ignition coil is the pencil-shaped part that snaps onto the spark plug and makes the spark.
• Press the lock tab and unplug each coil connector by hand; if stubborn, gently help with a pick tool.
• Use a 10mm socket and 3/8" ratchet to remove the coil hold-down bolts.
• Twist each coil slightly, then pull straight up to remove it.

Step 3: Remove the front-bank spark plugs

• Blow out each plug well using a compressed air blow gun (so grit doesn’t fall into the cylinder).
• Use a 5/8" (16mm) spark plug socket, 3/8" ratchet, and a 6" extension to loosen and remove each spark plug.
• If a plug feels extremely tight, stop and work it back and forth gently with the ratchet to avoid thread damage.

Step 4: Install the new front-bank spark plugs

• Start each new spark plug by hand using the spark plug socket and a extension only (no ratchet at first) so it can’t cross-thread.
• Use a torque wrench to tighten each plug: Torque to 18 Nm (13 ft-lbs).
• Apply a small dab of dielectric grease (non-conductive moisture barrier) inside each coil boot.

Step 5: Reinstall the front-bank ignition coils

• Push each coil straight down until it fully seats on the plug.
• Install coil bolts with a 10mm socket and tighten: Torque to 9 Nm (80 in-lbs).
• Reconnect the coil connectors by hand until they click.

Step 6: Remove air intake ducting to reach the upper intake manifold

• Use a flat trim clip tool to remove intake duct clips (if equipped) and a 10mm socket to loosen any duct brackets.
• Use needle-nose pliers to release spring clamps on small hoses (if present), then gently twist hoses off.
• Move the ducting aside so you can access the upper intake manifold fasteners.

Step 7: Remove the throttle body from the upper intake manifold

• Unplug the throttle body connector by hand; use a pick tool gently if needed.
• Use a 10mm socket and ratchet to remove the throttle body bolts.
• Remove the throttle body and old gasket. Install the new gasket during reassembly.
• On reassembly, tighten throttle body bolts: Torque to 10 Nm (89 in-lbs).

Step 8: Remove the upper intake manifold (plenum) to access the rear bank

• Unplug any remaining sensors/hoses attached to the upper intake manifold; use needle-nose pliers for clamps and a flashlight to verify nothing is still attached.
• Use a 12mm socket and ratchet to remove the upper intake manifold bolts/nuts (a swivel joint helps with angled access; it’s a hinged adapter that lets the socket bend).
• Lift the upper intake manifold off carefully.
• Immediately cover the intake ports with clean shop towels (no tools required) so nothing can fall in.
• Replace the upper intake manifold gaskets with the new ones.

Step 9: Replace the rear-bank spark plugs

• Remove rear-bank coil connectors by hand (use pick tool gently if needed).
• Use a 10mm socket to remove coil bolts, then pull coils out.
• Blow out plug wells with compressed air blow gun.
• Use a 5/8" (16mm) spark plug socket, extensions, and a swivel joint to remove the rear spark plugs.
• Install new rear spark plugs by hand first, then use a torque wrench: Torque to 18 Nm (13 ft-lbs).
• Add a small dab of dielectric grease inside each coil boot, reinstall coils, and tighten coil bolts: Torque to 9 Nm (80 in-lbs).

Step 10: Reinstall the upper intake manifold and intake ducting

• Remove shop towels from intake ports and visually confirm nothing fell in using a flashlight.
• Set the upper intake manifold back in place with new gaskets installed.
• Hand-start all bolts/nuts, then tighten with a 12mm socket evenly in a crisscross pattern.
• Tighten upper intake manifold fasteners: Torque to 21 Nm (15 ft-lbs).
• Reinstall the throttle body with a new gasket and tighten with a torque wrench: Torque to 10 Nm (89 in-lbs).
• Reconnect all hoses and electrical connectors by hand until they click/seat.
• Reinstall intake ducting/clips using a 10mm socket, trim clip tool, and pliers as needed.

Step 11: Reconnect the battery

• Reconnect the negative battery terminal using a 10mm socket.

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✅ After Repair

• ✅ Start the engine and let it idle for 2-3 minutes. A slightly rough idle at first can happen after the battery disconnect.
• ✅ Listen for vacuum leaks (hissing). If you hear one, recheck intake hoses and that the upper intake manifold is seated correctly.
• ✅ If the check engine light turns on or it runs rough, recheck coil connectors (most common miss) and make sure each coil is fully seated.
• ✅ Do a short test drive. Recheck for any fuel/air smells and confirm smooth acceleration.

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💰 DIY vs Shop Cost

Shop Cost: $450-$900 (parts + labor)

DIY Cost: $70-$180 (parts only)

You Save: $280-$830 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 2.5-4.0 hours.

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