🔧 Outback - Thermostat Replacement

This repair replaces the engine coolant thermostat, which controls coolant flow between the engine and radiator. A stuck thermostat can cause overheating, slow warm-up, poor heater performance, or temperature swings.

Difficulty Level: Beginner-Intermediate | Estimated Time: 1.5-2.5 hours

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⚠️ Safety & Precautions

• ⚠️ Only work on a completely cold engine. Hot coolant can spray out and cause severe burns.
• ⚠️ Keep pets away from coolant. Engine coolant is toxic and has a sweet smell.
• ⚠️ Use jack stands if raising the front of your Outback. Never rely on a floor jack alone.
• ⚠️ Disconnecting the battery is not required for this repair.
• ⚠️ Subaru flat-4 engines can trap air in the cooling system, so careful bleeding is important after refilling.

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🔧 Required Tools

You'll need the following tools for this repair:

• 10mm socket
• 12mm socket
• 3/8-inch drive ratchet
• 3/8-inch drive extension 6-inch
• Torque wrench 5-30 Nm range
• Flat-blade screwdriver 6-inch
• Hose clamp pliers
• Drain pan 2-gallon minimum
• Spill-free coolant funnel kit (specialty)
• Floor jack rated 2-ton minimum
• Jack stands rated 2-ton minimum
• Wheel chocks
• Plastic scraper
• Shop towels
• Nitrile gloves
• Safety glasses

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Engine coolant thermostat - Qty: 1
• Thermostat gasket/O-ring - Qty: 1
• Subaru-compatible long-life coolant - Qty: 1-2 gallons
• Distilled water - Qty: 1 gallon if using concentrate coolant

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📋 Before You Begin

• 🅿️ Park your Outback on level ground, shift to Park, and set the parking brake.
• 🧊 Let the engine cool for several hours before opening the cooling system.
• 🚗 Chock the rear wheels with wheel chocks before lifting the front.
• 📌 The thermostat on your Outback is located at the lower front area of the engine, where the lower radiator hose connects to the thermostat housing.
• 📘 A thermostat is a spring-loaded valve that opens as the engine warms up, letting coolant circulate through the radiator.
• 🧰 A torque wrench is a tool that tightens bolts to an exact tightness so small aluminum parts are not damaged.
• 🧰 A spill-free coolant funnel is a funnel that locks onto the radiator/overflow fill neck and helps burp trapped air from the cooling system.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Raise and Secure the Front

• Use the floor jack rated 2-ton minimum to lift the front of your Outback at the front center jacking point.
• Place jack stands rated 2-ton minimum under the front side support points.
• Use wheel chocks behind the rear wheels.
• Gently shake the vehicle by hand to confirm it is stable before working underneath.

Step 2: Remove the Lower Engine Cover

• Put on safety glasses and nitrile gloves.
• Use a 10mm socket, 3/8-inch drive ratchet, and 3/8-inch drive extension 6-inch to remove the lower splash shield bolts.
• Lower the splash shield and set it aside with the bolts.
• Keep bolts grouped by location.

Step 3: Drain Some Coolant

• Place the drain pan 2-gallon minimum under the radiator drain area.
• Slowly loosen the radiator cap by hand only when the engine is cold.
• Use a flat-blade screwdriver 6-inch to carefully open the radiator drain plug if equipped.
• Drain about 1 gallon of coolant, or enough so the coolant level sits below the thermostat housing.
• Close the drain plug snugly with the flat-blade screwdriver 6-inch. Do not overtighten plastic drain plugs.

Step 4: Remove the Lower Radiator Hose

• Find the lower radiator hose where it attaches to the thermostat housing at the engine.
• Use hose clamp pliers to squeeze the spring clamp tabs and slide the clamp back onto the hose.
• Twist the hose gently by hand to break it loose.
• If stuck, use a flat-blade screwdriver 6-inch carefully between the hose and housing lip, without gouging the metal.
• Pull the hose off and aim any remaining coolant into the drain pan 2-gallon minimum.
• Twist first, then pull.

Step 5: Remove the Thermostat Housing

• Use a 10mm socket or 12mm socket with the 3/8-inch drive ratchet to remove the thermostat housing bolts, depending on the bolt head size installed.
• Support the housing with your hand as the final bolt comes out.
• Remove the thermostat housing from the engine.
• Note the thermostat position before removing it. The spring side faces into the engine.

Step 6: Remove the Old Thermostat and Gasket

• Pull the old thermostat out by hand.
• Remove the old thermostat gasket/O-ring by hand.
• Use a plastic scraper and shop towels to clean the sealing surfaces on the housing and engine.
• Do not use metal scrapers on the aluminum sealing surfaces.
• Clean surfaces prevent leaks.

Step 7: Install the New Thermostat

• Install the new engine coolant thermostat into the engine with the spring side facing inward toward the engine.
• Install the new thermostat gasket/O-ring around the thermostat as designed.
• If the thermostat has a small jiggle valve or air bleed pin, position it at the top, near the 12 o’clock position.
• A jiggle valve is a small loose pin that lets trapped air pass through during filling.

Step 8: Reinstall the Thermostat Housing

• Position the thermostat housing squarely against the engine by hand.
• Start the bolts by hand first to avoid cross-threading.
• Use the 10mm socket or 12mm socket with the 3/8-inch drive ratchet to snug the bolts evenly.
• Use the torque wrench 5-30 Nm range to tighten the thermostat housing bolts to Torque to 6.4 Nm (4.7 ft-lbs).
• Do not overtighten. The housing and engine sealing surfaces are aluminum.

Step 9: Reconnect the Lower Radiator Hose

• Push the lower radiator hose fully onto the thermostat housing neck by hand.
• Use hose clamp pliers to move the spring clamp back to its original position.
• Confirm the clamp sits behind the raised bead on the housing neck.

Step 10: Refill the Cooling System

• Install the spill-free coolant funnel kit (specialty) onto the radiator fill neck.
• Pour in Subaru-compatible long-life coolant slowly through the funnel.
• If using coolant concentrate, mix it with distilled water to the correct 50/50 mixture before filling.
• Squeeze the upper radiator hose by hand several times to help push out trapped air.
• Fill the coolant overflow reservoir to the FULL mark using the same coolant mixture.

Step 11: Bleed Air from the Cooling System

• Leave the spill-free coolant funnel kit (specialty) installed and partially filled.
• Start the engine and let it idle with the heater set to full hot and fan on low.
• Watch for air bubbles coming up through the funnel.
• Use nitrile gloves to carefully squeeze the upper radiator hose as the engine warms.
• When the cooling fans cycle on and warm air blows from the cabin vents, let it run 3-5 more minutes.
• Shut the engine off and let it cool completely.
• Top off the radiator and reservoir as needed with Subaru-compatible long-life coolant.
• Air pockets can cause overheating.

Step 12: Reinstall the Lower Engine Cover

• Lift the lower splash shield back into place by hand.
• Use the 10mm socket, 3/8-inch drive ratchet, and 3/8-inch drive extension 6-inch to reinstall the splash shield bolts.
• Tighten the splash shield bolts snugly. If using a torque wrench, tighten small splash shield bolts to Torque to 5 Nm (44 in-lbs).

Step 13: Lower the Vehicle

• Use the floor jack rated 2-ton minimum to lift the front slightly off the jack stands.
• Remove the jack stands rated 2-ton minimum.
• Lower your Outback slowly to the ground.
• Remove the wheel chocks.

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✅ After Repair

• ✅ Check around the thermostat housing and lower radiator hose for leaks while the engine is running.
• ✅ Watch the temperature gauge during the first full warm-up. It should rise normally and stay steady.
• ✅ After the first drive, let the engine cool fully and recheck the coolant level in the radiator and overflow tank.
• ✅ Recheck again the next morning. Subaru cooling systems often release small trapped air pockets after the first heat cycle.
• ✅ Dispose of old coolant at an approved recycling location. Do not pour coolant onto the ground or into drains.

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💰 DIY vs Shop Cost

Shop Cost: $220-$420 parts + labor

DIY Cost: $35-$95 parts only

You Save: $185-$325 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 1-2 hours.

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🎯 Ready to get started?

HowToo makes it easy: same-day/2-day shipping on every part, plus all the tools and specialty tools you need! Check out the parts and tools sections below to add everything to your cart.