🔧 Highlander - Spark Plug Replacement

On your Highlander’s V6, the front bank plugs are easy to reach, but the rear bank sits under the upper intake manifold (plenum). The job is mostly careful disassembly, keeping dirt out, and torquing the new plugs correctly to avoid misfires or thread damage.

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

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⚠️ Safety & Precautions

• 🧤 Work on a fully cool engine to avoid burns and damaged threads.
• ⚡ Disconnect the negative battery terminal to prevent accidental shorts.
• 🧼 Blow debris away from plug wells before removing plugs (dirt can fall into cylinders).
• 🚫 Don’t use anti-seize on Toyota iridium plugs (threads are plated; torque changes).

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🔧 Required Tools

You'll need the following tools for this repair:

• Safety glasses
• Nitrile gloves
• 10mm socket
• 12mm socket
• 1/4" drive ratchet
• 3/8" drive ratchet
• 1/4" drive extension set (3" and 6")
• 3/8" drive extension set (3", 6", and 10")
• Universal joint adapter (3/8")
• 5/8" spark plug socket (magnetic or rubber insert)
• Torque wrench (3/8" drive, 10-30 ft-lb range)
• Torque wrench (1/4" drive, inch-lb range)
• Flat-blade screwdriver
• Needle-nose pliers
• Trim clip removal tool
• Shop vacuum
• Compressed air blow gun
• Clean shop rags
• Masking tape and marker
• Dielectric grease

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Iridium spark plugs (correct spec for your Highlander) - Qty: 6
• Upper intake manifold (plenum) gasket - Qty: 1
• Throttle body gasket - Qty: 1

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📋 Before You Begin

• Park on level ground, set the parking brake, and open the hood.
• Use masking tape and marker to label any hose or connector you remove so it goes back to the same spot.
• Disconnect the battery: use a 10mm socket to remove the negative terminal and set it aside so it can’t spring back.
• Have shop rags ready to cover intake openings so nothing falls in.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Remove the engine cover and open access

• Lift off the plastic engine cover (it pulls up from rubber grommets; use your hands).
• Remove the air intake ducting as needed: loosen clamps with a flat-blade screwdriver and remove bolts with a 10mm socket.

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

• Unplug one ignition coil electrical connector (press the tab; use a flat-blade screwdriver gently if stuck).
• Remove the coil hold-down bolt with a 10mm socket, then pull the coil straight up.
• Blow out the spark plug well using compressed air blow gun.
• Remove the spark plug using a 5/8" spark plug socket with a 3/8" drive ratchet and extension.
• Install the new plug by hand first using the spark plug socket and extension only (no ratchet) to prevent cross-threading.
• Torque the spark plug: Torque to 18 Nm (13 ft-lbs) using a torque wrench (3/8").
• Apply a tiny smear of dielectric grease inside the coil boot (this helps prevent moisture issues).
• Reinstall the coil and bolt: Torque to 10 Nm (89 in-lbs) using a torque wrench (1/4").
• Reconnect the coil connector until it clicks.
• Repeat for the other front bank cylinders, one at a time. One coil at a time prevents mix-ups.

Step 3: Remove the throttle body (to reach the rear bank)

• Unplug the throttle body electrical connector (press the lock tab).
• Remove any small hoses from the intake/throttle area using needle-nose pliers for spring clamps (move clamps back first), then twist hoses gently to release.
• Remove the throttle body fasteners using a 10mm socket.
• Remove the throttle body and set it aside (don’t stress the wiring).
• Remove and discard the old throttle body gasket.

Step 4: Remove the upper intake manifold (plenum)

• Label and disconnect vacuum hoses/PCV hose and any electrical connectors on the plenum using masking tape.
• Remove the plenum bolts/nuts using a 12mm socket with extensions and a universal joint adapter as needed.
• Lift the plenum straight up and off.
• Immediately cover the lower intake openings with clean shop rags so nothing can fall in.
• Remove and discard the old upper intake manifold gasket.

Step 5: Replace the rear bank spark plugs (under the plenum)

• For each rear cylinder, unplug the ignition coil connector, remove the coil bolt with a 10mm socket, and remove the coil.
• Blow out the plug well using compressed air blow gun.
• Remove the spark plug using a 5/8" spark plug socket, 3/8" drive ratchet, and extensions.
• Hand-thread the new plug, then torque: Torque to 18 Nm (13 ft-lbs) using a torque wrench (3/8").
• Reinstall the coil and bolt: Torque to 10 Nm (89 in-lbs) using a torque wrench (1/4").
• Reconnect each coil connector until it clicks.

Step 6: Reinstall the plenum and throttle body

• Remove the shop rags from the intake openings.
• Install the new upper intake manifold gasket.
• Set the plenum back in place, then install bolts/nuts finger-tight first using a 12mm socket.
• Tighten the plenum fasteners evenly in a crisscross pattern using a 12mm socket and torque wrench (if you want, I can provide the exact spec once you confirm whether your plenum uses bolts only or bolts + nuts).
• Install the new throttle body gasket, reinstall the throttle body, and tighten fasteners evenly using a 10mm socket (torque with a torque wrench if available).
• Reconnect all hoses and electrical connectors using your labels.
• Reinstall the intake ducting using a 10mm socket and flat-blade screwdriver.

Step 7: Reconnect the battery and final recheck

• Reconnect the negative battery terminal using a 10mm socket.
• Do a final visual check for unplugged connectors, loose clamps, or leftover rags/tools.

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✅ After Repair

• Start the engine and let it idle for 2–3 minutes. It may idle slightly rough for a few seconds while the throttle relearns.
• Listen for hissing (vacuum leak) around the plenum and intake tube.
• If you get a check engine light or misfire, recheck coil connectors and any hoses you removed.

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💰 DIY vs Shop Cost

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

DIY Cost: $70-$170 (parts only)

You Save: $280-$780 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 2.5-4.5 hours.

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🎯 Ready to get started?

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