🔧 Highlander - Spark Plug Replacement

On your Highlander V6, there are 6 spark plugs—3 in the front bank (easy access) and 3 in the rear bank (under the upper intake manifold). Replacing them restores smooth idle, power, and fuel economy, and helps prevent misfires.

Difficulty Level: Intermediate | Estimated Time: 2.5-4.0 hours

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⚠️ Safety & Precautions

• ⚠️ Work on a cold engine to avoid burns and stripped threads.
• ⚠️ Disconnect the negative battery terminal before unplugging ignition coils.
• ⚠️ Do not over-tighten spark plugs—aluminum cylinder heads strip easily.
• ⚠️ Keep dirt out of the intake when the manifold is off; cover openings immediately.

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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 extension set (3", 6", 10")
• 3/8" drive universal joint
• 5/8" spark plug socket
• Torque wrench (3/8" drive, 10-80 N·m range)
• Flat-blade screwdriver
• Needle-nose pliers
• Trim clip removal tool
• Shop vacuum
• Flashlight
• Clean rags

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs (V6 set) - Qty: 6
• Upper intake manifold gasket set - Qty: 1
• Throttle body gasket - Qty: 1
• Dielectric grease (for coil boots) - 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.
• Use a 10mm socket to disconnect the negative battery terminal and tuck it aside so it can’t spring back.
• Take photos before unplugging hoses/connectors.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the engine cover

• Pull up to release the engine cover from its grommets (no tools on most covers).
• Set it aside so it won’t get stepped on.

Step 2: Replace the front bank spark plugs (radiator side)

• Unplug one ignition coil connector at a time (press the tab and pull).
• Use a 10mm socket to remove the coil hold-down bolt.
• Twist and pull the coil straight up to remove it.
• Use a shop vacuum to clean dirt from around the plug well.
• Use a 5/8" spark plug socket with a 3/8" ratchet and extension to remove the spark plug.
• Install the new spark plug by hand first using the spark plug socket and extension (no ratchet) so it doesn’t cross-thread.
• Tighten with a torque wrench: Torque to 18 N·m (13 ft-lbs).
• Apply a small amount of dielectric grease inside the coil boot (this helps prevent moisture and makes future removal easier).
• Reinstall the coil and tighten the bolt with a torque wrench: Torque to 9 N·m (80 in-lbs).
• Reconnect the coil connector until it clicks.
• Repeat for the other 2 front plugs.
• Do one cylinder at a time to avoid mix-ups.

Step 3: Remove the air intake tube (to access the rear bank)

• Use a flat-blade screwdriver to loosen the hose clamps.
• Unplug any attached sensor connector(s) you can see on the intake tube.
• Remove the intake tube and set it aside.

Step 4: Remove the upper intake manifold (rear bank access)

• Label and disconnect the vacuum hoses and breather hoses using needle-nose pliers (pinch clamps) as needed.
• Unplug the throttle body connector(s) you can access.
• Use a 10mm socket to remove the throttle body mounting bolts, then move the throttle body aside.
• Remove the upper intake manifold bolts/nuts using a 12mm socket (some fasteners may be in tight spots—use a universal joint and extensions).
• Lift the upper intake manifold off carefully.
• Immediately cover the exposed intake ports with clean rags so nothing can fall in.

Step 5: Replace the rear bank spark plugs (firewall side)

• Unplug one rear ignition coil connector.
• Use a 10mm socket to remove the coil bolt, then pull the coil out.
• Vacuum around the plug well using a shop vacuum.
• Remove the spark plug using a 5/8" spark plug socket, extensions, and a universal joint as needed.
• Install the new plug by hand first, then tighten with a torque wrench: Torque to 18 N·m (13 ft-lbs).
• Add a small amount of dielectric grease to the coil boot, reinstall the coil, and tighten: Torque to 9 N·m (80 in-lbs).
• Reconnect the coil connector until it clicks.
• Repeat for the other 2 rear plugs.

Step 6: Reinstall the upper intake manifold and throttle body

• Remove the rags from the intake ports.
• Replace the upper intake manifold gasket(s) with the new ones.
• Set the upper intake manifold back into place.
• Install bolts/nuts finger-tight first using a 12mm socket, then tighten evenly in a crisscross pattern.
• Tighten upper intake manifold fasteners with a torque wrench: Torque to 21 N·m (15 ft-lbs).
• Install a new throttle body gasket, reinstall the throttle body with a 10mm socket, and tighten: Torque to 10 N·m (89 in-lbs).
• Reconnect all vacuum hoses/breather hoses and electrical connectors.
• Double-check every hose clamp is fully seated.

Step 7: Reinstall the air intake tube and engine cover

• Reinstall the intake tube and tighten clamps using a flat-blade screwdriver.
• Reconnect any sensor connector(s) you unplugged.
• Reinstall the engine cover by pressing it into the grommets.

Step 8: Reconnect the battery

• Reconnect the negative battery terminal using a 10mm socket.
• Make sure the terminal is snug and doesn’t rotate by hand.

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✅ After Repair

• Start the engine and let it idle for 1–2 minutes. It should idle smoothly.
• Listen for vacuum leaks (hissing). If you hear one, recheck the intake tube clamps and manifold hoses.
• If the check engine light comes on, recheck coil connectors first (a loose connector is common).
• Take a short test drive, then recheck for any stored codes if you have a scan tool.

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💰 DIY vs Shop Cost

Shop Cost: $450-$850 (parts + labor)

DIY Cost: $80-$220 (parts only)

You Save: $370-$630 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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🎯 Ready to get started?

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