🔧 Highlander - Spark Plug Replacement

Replacing the spark plugs on your Highlander helps restore smooth starting, strong acceleration, and proper fuel burn. On the 3.5L V6, the front bank is straightforward, but the rear bank requires removing the upper intake manifold to access the back three plugs.

Difficulty Level: Advanced | Estimated Time: 3-5 hours

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⚠️ Safety & Precautions

• ⚠️ Work only on a cold engine. Hot aluminum cylinder heads can burn you and are easier to damage.
• ⚠️ Disconnect the negative battery cable before removing ignition coils and intake components.
• ⚠️ Keep dirt, bolts, and tools out of the intake ports. Anything dropped inside can seriously damage the engine.
• ⚠️ Do not overtighten spark plugs. The cylinder head is aluminum and threads can strip.
• ⚠️ A torque wrench is required. A torque wrench tightens bolts to an exact force so parts are not too loose or too tight.

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🔧 Required Tools

You'll need the following tools for this repair:

• 10mm socket
• 12mm socket
• 14mm socket
• 5/8-inch spark plug socket
• 3/8-inch drive ratchet
• 3/8-inch drive torque wrench
• 1/4-inch drive ratchet
• 6-inch socket extension
• 10-inch socket extension
• Universal joint socket adapter
• Flathead screwdriver
• Needle-nose pliers
• Trim clip removal tool
• Magnetic pickup tool
• Compressed air blow gun
• Clean shop towels
• Masking tape
• Permanent marker
• Safety glasses
• Nitrile gloves

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs - Qty: 6
• Upper intake manifold gasket set - Qty: 1
• Throttle body gasket - Qty: 1
• Dielectric grease - Qty: 1 small tube

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📋 Before You Begin

• Park your Highlander on level ground and let the engine cool completely.
• Set the parking brake and remove the key from the vehicle.
• Open the hood and remove the negative battery cable using a 10mm socket.
• Use masking tape and a permanent marker to label hoses and electrical connectors before unplugging them.
• Have clean shop towels ready to cover the intake ports after the manifold is removed.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the Engine Cover

• Use both hands to lift the plastic engine cover straight upward from the rubber mounting grommets.
• Set the cover aside where it will not get stepped on.
• Pull straight up, not sideways.

Step 2: Disconnect the Negative Battery Cable

• Use a 10mm socket to loosen the negative battery terminal clamp.
• Move the cable away from the battery post so it cannot spring back and touch.
• This prevents accidental electrical shorts while you unplug ignition coils and sensors.

Step 3: Remove the Air Intake Tube

• Use a flathead screwdriver to loosen the hose clamp at the air filter box.
• Use a flathead screwdriver to loosen the hose clamp at the throttle body.
• Use needle-nose pliers to release any small hose clamps connected to the intake tube.
• Lift the intake tube out of the engine bay and set it aside.

Step 4: Remove Front Bank Ignition Coils

• The front bank is the row of three ignition coils closest to the radiator.
• An ignition coil is the small electrical unit that sits on top of each spark plug.
• Press the tab on each coil electrical connector and unplug it by hand.
• Use a 10mm socket to remove the hold-down bolt from each front ignition coil.
• Pull each coil straight upward and set it aside in order.

Step 5: Clean Around the Front Spark Plug Wells

• Use a compressed air blow gun to blow dirt away from each spark plug well.
• A spark plug well is the deep tube where the spark plug sits.
• Wear safety glasses while using compressed air.

Step 6: Remove the Front Spark Plugs

• Use a 5/8-inch spark plug socket, 6-inch socket extension, and 3/8-inch drive ratchet to loosen each spark plug.
• A spark plug socket has a rubber insert that grips the plug so it can be lifted out safely.
• Turn counterclockwise slowly until the plug is free.
• Lift the plug out carefully and compare it to the new plug.

Step 7: Install the Front Spark Plugs

• Place one new iridium spark plug into the 5/8-inch spark plug socket.
• Thread it in by hand using the 6-inch socket extension only, with no ratchet at first.
• This helps prevent cross-threading, which means starting the plug crooked in the threads.
• Once finger-tight, use a 3/8-inch drive torque wrench to tighten each spark plug to Torque to 18 Nm (13 ft-lbs).
• Start every plug by hand.

Step 8: Remove Throttle Body Electrical and Hose Connections

• Use your fingers to unplug the throttle body electrical connector.
• Use needle-nose pliers to release small hose clamps if any hoses must be moved for access.
• Label each hose with masking tape and a permanent marker before removal.

Step 9: Remove the Throttle Body

• Use a 10mm socket to remove the throttle body mounting bolts.
• Pull the throttle body straight off the intake manifold.
• Remove and discard the old throttle body gasket.
• Place the throttle body aside without pulling hard on any attached coolant hoses.

Step 10: Remove Upper Intake Manifold Brackets and Connections

• Use a 10mm socket, 12mm socket, and 14mm socket as needed to remove the upper intake manifold support brackets.
• Use needle-nose pliers to release vacuum hose clamps.
• Use a trim clip removal tool to release wiring harness clips attached to the manifold.
• Label every connector and hose with masking tape and a permanent marker.

Step 11: Remove the Upper Intake Manifold

• Use a 12mm socket, 10-inch socket extension, and 3/8-inch drive ratchet to remove the upper intake manifold bolts and nuts.
• Use a universal joint socket adapter if a rear fastener is hard to reach. This adapter lets the socket turn at an angle.
• Lift the manifold upward carefully.
• Immediately cover the open intake ports with clean shop towels.
• Do not drop anything inside.

Step 12: Remove Rear Bank Ignition Coils

• The rear bank is the row of three ignition coils closest to the firewall.
• Use your fingers to unplug each ignition coil connector.
• Use a 10mm socket and 1/4-inch drive ratchet to remove each coil hold-down bolt.
• Pull each coil straight upward and set it aside in order.

Step 13: Remove the Rear Spark Plugs

• Use a compressed air blow gun to clean each rear spark plug well.
• Use a 5/8-inch spark plug socket, 10-inch socket extension, universal joint socket adapter, and 3/8-inch drive ratchet to loosen each rear spark plug.
• Turn counterclockwise slowly until the plug is free.
• Use a magnetic pickup tool only if needed to retrieve a dropped fastener nearby, not inside the cylinder.

Step 14: Install the Rear Spark Plugs

• Place one new iridium spark plug into the 5/8-inch spark plug socket.
• Thread it in by hand using the 10-inch socket extension with no ratchet at first.
• After it seats by hand, use a 3/8-inch drive torque wrench to tighten each spark plug to Torque to 18 Nm (13 ft-lbs).
• Repeat for all three rear spark plugs.

Step 15: Reinstall the Ignition Coils

• Apply a small amount of dielectric grease inside the rubber boot of each ignition coil.
• Dielectric grease helps seal out moisture but does not go on the metal spark plug tip.
• Push each coil straight down until fully seated.
• Use a 10mm socket to install the coil hold-down bolts.
• Tighten each ignition coil bolt to Torque to 8 Nm (71 in-lbs).
• Reconnect each coil electrical connector by hand until it clicks.

Step 16: Install New Upper Intake Manifold Gaskets

• Remove the old intake manifold gaskets by hand.
• Use clean shop towels to wipe the gasket surfaces clean.
• Install the new upper intake manifold gasket set into the manifold grooves.
• Remove the shop towels from the intake ports only when you are ready to set the manifold back on.

Step 17: Reinstall the Upper Intake Manifold

• Lower the upper intake manifold straight into place by hand.
• Start all bolts and nuts by hand before tightening.
• Use a 12mm socket, 10-inch socket extension, and 3/8-inch drive torque wrench to tighten the manifold fasteners evenly.
• Tighten the upper intake manifold fasteners to Torque to 21 Nm (15 ft-lbs).

Step 18: Reinstall Brackets, Hoses, and Wiring

• Use a 10mm socket, 12mm socket, and 14mm socket to reinstall the manifold brackets.
• Reconnect every hose and connector using your labels as a guide.
• Use needle-nose pliers to reposition hose clamps.
• Use a trim clip removal tool to help align wiring clips if needed.

Step 19: Reinstall the Throttle Body

• Install a new throttle body gasket.
• Position the throttle body by hand.
• Use a 10mm socket and 3/8-inch drive torque wrench to tighten the throttle body bolts to Torque to 10 Nm (89 in-lbs).
• Reconnect the throttle body electrical connector by hand until it clicks.

Step 20: Reinstall the Air Intake Tube

• Set the air intake tube back between the air filter box and throttle body.
• Use a flathead screwdriver to tighten both hose clamps snugly.
• Reconnect any small hoses by hand and secure clamps with needle-nose pliers.

Step 21: Reconnect the Battery and Reinstall Engine Cover

• Use a 10mm socket to reconnect the negative battery cable.
• Tighten the terminal clamp snugly, but do not overtighten it.
• Push the engine cover straight down onto its rubber grommets by hand.

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✅ After Repair

• Start your Highlander and let it idle for several minutes.
• The idle may be slightly high or uneven at first while the throttle body relearns.
• Listen for hissing sounds, which can mean an intake hose or gasket is leaking air.
• Check that the check engine light stays off.
• Take a short test drive and confirm smooth acceleration.
• If the engine runs rough, shut it off and recheck every ignition coil connector, vacuum hose, and intake tube clamp.

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💰 DIY vs Shop Cost

Shop Cost: $650-$1,050 (parts + labor)

DIY Cost: $90-$220 (parts only)

You Save: $430-$830 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 3-5 hours.

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