Assumption: Your RX350 has the factory 3.5L V6 and the thermostat is in the water inlet housing where the lower radiator hose attaches.

🔧 RX350 - Thermostat Replacement

The thermostat controls engine warm-up and operating temperature. If it’s stuck open you may get low heat and poor fuel economy; stuck closed can cause overheating. This job involves draining coolant, removing the thermostat housing, replacing the thermostat/gasket, then refilling and bleeding air from the cooling system.

Difficulty Level: Intermediate | Estimated Time: 2-4 hours

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⚠️ Safety & Precautions

• ⚠️ Only work on a fully cold engine; hot coolant can cause severe burns.
• ⚠️ Support the RX350 with jack stands; never rely on a floor jack alone.
• ⚠️ Keep coolant off belts and electrical connectors; clean spills immediately.
• ⚠️ Coolant is toxic to pets/people; drain into a sealed container and dispose properly.
• 🔋 Battery disconnect is not required, but keep hands/tools clear of the radiator fans (they can turn on automatically).

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🔧 Required Tools

You'll need the following tools for this repair:

• Floor jack (rated 3-ton minimum)
• Jack stands (rated 3-ton minimum)
• Wheel chocks
• Safety glasses
• Nitrile gloves
• Drain pan (at least 3-gallon)
• Spill-free coolant funnel kit
• 10mm socket
• 12mm socket
• 3/8" drive ratchet
• 3" extension (3/8" drive)
• Torque wrench (5–60 Nm range)
• Hose clamp pliers
• Pick tool
• Trim clip tool
• Flathead screwdriver
• Shop towels
• Flashlight

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Engine thermostat - Qty: 1
• Thermostat gasket / O-ring - Qty: 1
• Toyota / Lexus Super Long Life Coolant (pink, premixed) - Qty: 2 gallons
• Lower radiator hose clamp - Qty: 1

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📋 Before You Begin

• 🅿️ Park on level ground, set the parking brake, and chock the rear wheels.
• 🌡️ Make sure the engine is completely cold (ideally sit overnight).
• 📦 Lay cardboard under the front of the RX350 to catch drips.
• 🧰 Know your tools: a torque wrench is a tool that tightens bolts to an exact spec so you don’t strip or crack parts.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Raise the front safely

• Use floor jack to lift the front at the proper center jack point.
• Set the RX350 onto jack stands and keep wheel chocks in place.

Step 2: Remove the lower engine cover (if equipped)

• Use a trim clip tool to pop plastic clips.
• Use a 10mm socket and ratchet to remove the bolts.
• Keep clips/bolts grouped in a tray.

Step 3: Drain the coolant

• Place a drain pan (at least 3-gallon) under the radiator drain area.
• Slowly remove the radiator cap with a shop towel (turn to first stop, then off).
• Open the radiator drain cock using a flathead screwdriver if needed (some are hand-turn style).
• Let coolant drain fully into the drain pan.

Step 4: Remove the air intake duct for access

• Use a flathead screwdriver to loosen the hose clamps on the intake duct.
• Use a 10mm socket and ratchet to remove any duct mounting bolts.
• Lift the duct out and set it aside.

Step 5: Locate the thermostat housing

• Use a flashlight to find the lower radiator hose.
• Follow it to the engine; it attaches to the thermostat (water inlet) housing.

Step 6: Remove the lower radiator hose from the housing

• Use hose clamp pliers to compress the clamp and slide it back on the hose.
• Twist the hose gently by hand to break it free.
• If it’s stuck, use a pick tool carefully around the hose end to release it (don’t gouge the plastic/metal nipple).
• Pull the hose off and aim it into the drain pan for remaining coolant.

Step 7: Remove the thermostat housing

• Use a 10mm socket (sometimes 12mm socket) with a ratchet and extension to remove the housing bolts.
• Pull the housing straight off. Expect some coolant to spill—use shop towels.

Step 8: Replace the thermostat and gasket

• Remove the old thermostat and the old gasket / O-ring by hand.
• Clean the mating surfaces using shop towels. Do not scratch the sealing surface.
• Install the new thermostat in the same orientation as the old one.
• Install the new gasket / O-ring (do not reuse the old one).
• Match orientation before tightening anything.

Step 9: Reinstall the housing and torque bolts

• Reinstall the housing and hand-start all bolts.
• Use a torque wrench (5–60 Nm range) to tighten evenly.
• Torque to 10 Nm (89 in-lbs).

Step 10: Reinstall the lower radiator hose

• Push the hose fully onto the housing nipple by hand.
• Use hose clamp pliers to position the clamp back in its original spot.
• If the clamp feels weak or distorted, replace it with the new one.

Step 11: Reinstall the air intake duct and covers

• Reinstall the intake duct and tighten clamps using a flathead screwdriver.
• Reinstall the lower engine cover using a 10mm socket, ratchet, and trim clip tool.

Step 12: Refill and bleed air from the cooling system

• Make sure the radiator drain cock is closed (snug, not over-tight).
• Install a spill-free coolant funnel kit at the radiator fill neck.
• Slowly add Toyota / Lexus Super Long Life Coolant (pink, premixed) until full.
• Start the engine and let it idle. Set cabin heat to MAX temperature (highest heat) so coolant flows through the heater core.
• As the engine warms up, watch for air bubbles in the funnel and keep topping off.
• When the thermostat opens, the upper hose will get hot and the coolant level may drop—top off again.
• Once bubbles stop and the cooling fans cycle, shut the engine off and let it cool.
• Top off the radiator and then set the overflow reservoir to the FULL line.

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✅ After Repair

• 🔍 Check for leaks around the thermostat housing and lower hose with the engine running (use a flashlight).
• 🌡️ Watch the temperature gauge on a 10–15 minute drive; it should warm up normally and stay stable.
• 🧪 After the engine fully cools, recheck coolant level in the reservoir and top off if needed.
• 🧼 Rinse any spilled coolant off the underbody area using water and wipe dry.

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💰 DIY vs Shop Cost

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

DIY Cost: $60-$180 (parts only)

You Save: $270-$790 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 2-3 hours.

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🎯 Ready to get started?

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