🔧 Highlander - Spark Plug Replacement

Replacing the spark plugs on your Highlander helps restore smooth starting, steady idle, and proper fuel economy. The 2.7L inline-4 engine has four spark plugs located under the ignition coils on top of the engine, so this is a beginner-friendly job with careful handling.

Difficulty Level: Beginner | Estimated Time: 1-2 hours

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⚠️ Safety & Precautions

• ⚠️ Work only on a completely cool engine to avoid burns and aluminum-thread damage.
• ⚠️ Disconnect the negative battery cable before unplugging ignition coils.
• ⚠️ Do not let dirt fall into the spark plug wells; dirt can enter the engine once the plug is removed.
• ⚠️ Start each new spark plug by hand first. Cross-threading means the plug goes in crooked and can damage the cylinder head.
• ⚠️ Do not use anti-seize on modern iridium spark plugs unless the plug manufacturer specifically requires it.

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🔧 Required Tools

You'll need the following tools for this repair:

• 10mm socket
• 3/8-inch ratchet
• 3/8-inch extension 6-inch
• 5/8-inch spark plug socket
• Torque wrench 3/8-inch drive
• Needle-nose pliers
• Compressed air nozzle
• Spark plug gap gauge
• Dielectric grease
• Clean shop towels
• Nitrile gloves
• Safety glasses

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs - Qty: 4
• Ignition coil boots - Qty: 4, optional if cracked, swollen, oil-soaked, or carbon-tracked

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📋 Before You Begin

• Park your Highlander on level ground, shift to Park, and apply the parking brake.
• Let the engine cool fully before removing the spark plugs.
• Open the hood and remove the key from the ignition.
• Use a 10mm socket to disconnect the negative battery cable, then move the cable aside so it cannot touch the battery post.
• A spark plug socket is a deep socket with a rubber insert or magnet that holds the spark plug straight while removing and installing it.
• A torque wrench tightens bolts or plugs to a measured tightness so you do not over-tighten them.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the Engine Cover

• Use your hands to lift the plastic engine cover straight upward if equipped.
• If the cover has fasteners, use a 10mm socket to remove them.
• Set the cover aside in a safe spot.
• Take a photo first.

Step 2: Locate the Ignition Coils

• Look across the top of the engine for four ignition coils.
• Each ignition coil is a small black electrical unit held by one 10mm bolt.
• The ignition coil sends high voltage to the spark plug.

Step 3: Clean Around the Coil Area

• Put on safety glasses.
• Use a compressed air nozzle to blow dust and loose dirt away from the tops of the ignition coils.
• Use clean shop towels to wipe around each coil.
• This prevents dirt from falling into the spark plug tubes.

Step 4: Unplug the First Ignition Coil

• Work on one cylinder at a time so parts do not get mixed up.
• Press the electrical connector tab by hand and pull the connector straight back.
• If the connector is stuck, use needle-nose pliers gently on the connector body, not the wires.
• Do not pull on the wires.

Step 5: Remove the First Ignition Coil

• Use a 10mm socket, 3/8-inch ratchet, and 3/8-inch extension to remove the coil hold-down bolt.
• Twist the ignition coil gently by hand to loosen the rubber boot.
• Pull the coil straight up and out.
• Inspect the rubber boot for cracks, oil, white marks, or black burn lines.

Step 6: Clean the Spark Plug Well

• Use a compressed air nozzle to blow out the spark plug well before removing the plug.
• The spark plug well is the deep tube where the plug sits.
• Keep your face away from the well while blowing air.

Step 7: Remove the Old Spark Plug

• Use a 5/8-inch spark plug socket, 3/8-inch extension, and 3/8-inch ratchet.
• Turn counterclockwise slowly to loosen the spark plug.
• Once loose, spin it out gently and lift it straight up.
• If the plug feels tight after breaking loose, stop and turn it back in slightly, then loosen again slowly.
• Slow and straight prevents damage.

Step 8: Check the New Spark Plug

• Use a spark plug gap gauge to confirm the gap is about 0.044 inch.
• Most iridium plugs come pre-gapped, so do not forcefully pry on the small center electrode.
• Compare the new plug to the old plug to make sure the length and thread area match.

Step 9: Install the New Spark Plug by Hand

• Place the new spark plug into the 5/8-inch spark plug socket.
• Attach the 3/8-inch extension only, without the ratchet.
• Lower the plug straight into the spark plug well.
• Turn clockwise by hand using the extension until the plug is fully seated.
• If it does not spin in smoothly, remove it and start again.

Step 10: Torque the New Spark Plug

• Attach the torque wrench to the 3/8-inch extension and 5/8-inch spark plug socket.
• Tighten the spark plug to Torque to 18 Nm (13 ft-lbs).
• Do not over-tighten; the cylinder head is aluminum and can be damaged.

Step 11: Reinstall the Ignition Coil

• Apply a small pea-sized amount of dielectric grease inside the end of the ignition coil boot.
• Dielectric grease helps seal moisture out of the boot but does not go on the spark plug threads.
• Push the ignition coil straight down over the new spark plug until fully seated.
• Install the coil hold-down bolt using a 10mm socket and torque wrench.
• Torque to 8 Nm (71 in-lbs).

Step 12: Reconnect the Ignition Coil Connector

• Push the electrical connector onto the ignition coil by hand until it clicks.
• Lightly tug the connector body to make sure it is locked.

Step 13: Repeat for the Other Three Spark Plugs

• Repeat Steps 4 through 12 for the remaining three ignition coils and spark plugs.
• Use the same 10mm socket, 5/8-inch spark plug socket, 3/8-inch extension, ratchet, compressed air nozzle, and torque wrench.
• Replace all four spark plugs as a set.

Step 14: Reinstall the Engine Cover

• Set the engine cover back in place by hand.
• If fasteners were removed, use a 10mm socket to reinstall them snugly.
• Do not over-tighten plastic cover fasteners.

Step 15: Reconnect the Battery

• Place the negative battery cable back on the negative battery post.
• Use a 10mm socket to tighten the terminal nut until snug.
• Make sure the terminal does not twist by hand.

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✅ After Repair

• Start your Highlander and let it idle for 1-2 minutes.
• The idle may briefly fluctuate after the battery was disconnected; this usually settles quickly.
• Listen for misfires, ticking, or popping sounds.
• Confirm the check engine light is not flashing.
• Take a short test drive and make sure acceleration feels smooth.
• If the engine runs rough, shut it off and recheck that all four ignition coil connectors are fully clicked in.

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💰 DIY vs Shop Cost

Shop Cost: $220-$420 (parts + labor)

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

You Save: $175-$300 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 1-1.5 hours.

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🎯 Ready to get started?

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