🔧 Highlander - Spark Plug Replacement

On your Highlander’s V6, the front-bank spark plugs are easy to reach, but the rear-bank plugs sit under the upper intake manifold (the “plenum,” the top air passage). This job is mostly careful disassembly, keeping dirt out, and tightening the new plugs to the correct torque.

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

Assumption: V6 requires upper intake manifold removal for rear plugs.

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⚠️ Safety & Precautions

• ⚠️ Work on a fully cool engine to avoid burns and prevent thread damage in the cylinder heads.
• ⚠️ Disconnect the negative battery cable using a 10mm wrench to prevent accidental shorts and to protect sensitive electronics.
• ⚠️ Keep debris out of the intake—cover open ports with clean rags as soon as the manifold comes off.
• ⚠️ Do not use anti-seize on modern plated spark plugs; it can cause over-tightening and thread damage.

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🔧 Required Tools

You'll need the following tools for this repair:

• Safety glasses
• Nitrile gloves
• Floor jack (rated 3-ton minimum)
• Jack stands (rated 3-ton minimum)
• Wheel chocks
• 10mm wrench
• 10mm socket
• 12mm socket
• 14mm socket
• 5/8" spark plug socket
• 3/8" drive ratchet
• 3/8" drive torque wrench (10-30 ft-lbs range)
• 6" extension (3/8" drive)
• 10" extension (3/8" drive)
• Universal joint swivel (3/8" drive)
• Flat trim tool
• Small pick tool
• Needle-nose pliers
• Magnetic pickup tool
• Shop vacuum
• Flashlight
• Shop rags

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs (OEM-style, pre-gapped) - Qty: 6
• Upper intake manifold (plenum) 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.
• Open the hood and let the engine cool completely.
• Disconnect the negative battery terminal using a 10mm wrench, then set the cable aside so it can’t spring back.
• Use a shop vacuum to remove loose dirt around the intake and ignition coils so nothing falls into the engine.

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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 (it’s held by rubber grommets).
• Use a 10mm socket and 3/8" drive ratchet to remove any small covers or brackets that block access to the intake tube.

Step 2: Remove the air intake tube

• Use a 10mm socket to loosen the hose clamps on the intake tube.
• Unplug any intake tube connectors using a small pick tool (lift the lock tab gently).
• Remove the tube and set it aside with a shop rag covering the open throttle area to keep dirt out.

Step 3: Replace the front-bank spark plugs (easy side first)

• Unplug each ignition coil electrical connector using a small pick tool (a coil is the “spark plug transformer” that sits on top of each plug).
• Remove the coil hold-down bolt using a 10mm socket, then pull the coil straight up.
• Vacuum around the spark plug well using a shop vacuum.
• Remove the spark plug using a 5/8" spark plug socket, 6" extension, and 3/8" drive ratchet.
• Install the new plug by hand using the 5/8" spark plug socket and extension only (no ratchet at first) to avoid cross-threading.
• Tighten the spark plug using a 3/8" drive torque wrench: Torque to 18 Nm (13 ft-lbs).
• Apply a small dab of dielectric grease inside the coil boot.
• Reinstall the coil and tighten the coil bolt: Torque to 9 Nm (80 in-lbs).
• Repeat for the remaining front-bank cylinders.

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

• Unplug sensors and connectors on the plenum using a small pick tool.
• Disconnect small vacuum/breather hoses using needle-nose pliers (move spring clamps back, then twist the hose gently to free it).
• Remove the throttle body fasteners using a 10mm socket and 3/8" drive ratchet.
• Remove the plenum bolts/nuts using a 12mm socket and 3/8" drive ratchet (use a magnetic pickup tool if anything feels like it may drop).
• Lift the plenum off carefully. Immediately cover the intake ports with shop rags.
• Tip: Lay bolts out in the pattern removed.

Step 5: Replace the rear-bank spark plugs

• Remove rear ignition coils the same way as the front using a 10mm socket.
• Vacuum each spark plug well using a shop vacuum.
• Remove each rear plug using a 5/8" spark plug socket, 10" extension, and universal joint swivel.
• Install each new plug by hand first, then torque using a 3/8" drive torque wrench: Torque to 18 Nm (13 ft-lbs).
• Reinstall coils with a small dab of dielectric grease in each boot and tighten coil bolts: Torque to 9 Nm (80 in-lbs).

Step 6: Reinstall the plenum and throttle body

• Remove the old plenum gaskets and install the new upper intake manifold gasket set.
• Set the plenum back in place without pinching any hoses or wiring.
• Install plenum fasteners finger-tight first, then tighten evenly using a 12mm socket.
• Tighten the plenum fasteners using a 3/8" drive torque wrench: Torque to 21 Nm (15 ft-lbs).
• Install a new throttle body gasket, reinstall the throttle body using a 10mm socket, and tighten: Torque to 10 Nm (89 in-lbs).
• Reconnect all vacuum lines and electrical connectors (listen/feel for the “click”).

Step 7: Reinstall the intake tube and reconnect the battery

• Reinstall the intake tube and tighten clamps using a 10mm socket.
• Reconnect any intake sensors.
• Reconnect the negative battery cable using a 10mm wrench.
• Reinstall the engine cover by pressing it down into the grommets.

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✅ After Repair

• Start the engine and let it idle for 2–3 minutes. The first start may take a couple extra cranks.
• Listen for a hissing sound (vacuum leak). If you hear one, recheck the plenum area hoses and gasket seating.
• Verify smooth idle and no check engine light. If a light appears, recheck coil connectors (one missed connector is very common).
• Take a short test drive, then recheck that nothing is loose around the intake tube and throttle body.

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💰 DIY vs Shop Cost

Shop Cost: $350-$650 (parts + labor)

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

You Save: $180-$580 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 2.0-3.0 hours.

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🎯 Ready to get started?

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