🔧 Forester - Thermostat Replacement

Replacing the thermostat on your Forester helps restore proper engine warm-up and cooling system temperature control. The thermostat sits in the coolant outlet housing at the lower front area of the engine, so this job requires draining coolant, removing the housing, replacing the thermostat and gasket, then refilling and bleeding the cooling system.

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

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⚠️ Safety & Precautions

• ⚠️ Only work on the cooling system when the engine is completely cold; hot coolant can cause serious burns.
• ⚠️ Keep coolant away from pets and children; it is poisonous and can taste sweet.
• ⚠️ Support your Forester securely with jack stands if lifting it; never rely on a jack alone.
• ⚠️ Do not remove the radiator cap when the engine is hot.
• ⚠️ Battery disconnect is not required for this thermostat replacement.

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🔧 Required Tools

You'll need the following tools for this repair:

• 10mm socket
• 12mm socket
• 3/8-inch ratchet
• 3/8-inch extension
• Torque wrench 5-60 Nm
• Flat-blade screwdriver
• Pliers
• Plastic trim clip remover
• Drain pan 2-gallon minimum
• Coolant funnel kit
• Floor jack rated 2-ton minimum
• Jack stands rated 2-ton minimum
• Wheel chocks
• Nitrile gloves
• Safety glasses
• Clean shop towels

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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 super coolant or equivalent phosphate-based coolant - Qty: 1-2 gallons
• Distilled water - Qty: 1 gallon if using concentrate coolant

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📋 Before You Begin

• 🅿️ Park your Forester on level ground and let the engine cool fully, preferably overnight.
• 🧊 Set the heater temperature control to full hot before starting; this helps coolant circulate through the heater core during bleeding.
• 🛞 Place wheel chocks behind the rear wheels before lifting the front.
• 🧰 A coolant funnel kit is a spill-free funnel that attaches to the radiator neck and helps remove trapped air from the cooling system.
• ♻️ Plan to catch and recycle old coolant properly; do not pour it on the ground or into a drain.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Raise and Secure the Front

• Use wheel chocks behind the rear wheels.
• Use a floor jack rated 2-ton minimum to lift the front of your Forester high enough to access the lower engine area.
• Place jack stands rated 2-ton minimum under the proper front support points.
• Gently lower the vehicle onto the jack stands and give the vehicle a light shake to confirm it is stable.
• Never crawl under a jack-only vehicle.

Step 2: Remove the Lower Splash Shield

• Put on safety glasses and nitrile gloves.
• Use a plastic trim clip remover to remove the plastic clips holding the lower splash shield.
• Use a 10mm socket and 3/8-inch ratchet to remove any 10mm fasteners from the shield.
• Lower the splash shield and set it aside.

Step 3: Drain the Coolant

• Place a drain pan 2-gallon minimum under the radiator drain area.
• Slowly remove the radiator cap by hand only after confirming the engine is cold.
• Use a flat-blade screwdriver or your fingers to carefully open the radiator drain plug if accessible.
• If the drain plug is stuck, do not force it; use pliers to loosen the lower radiator hose clamp instead and drain from the hose carefully.
• Let coolant drain until flow slows to a drip.
• Coolant is slippery; wipe spills right away.

Step 4: Locate the Thermostat Housing

• Follow the lower radiator hose toward the engine by hand.
• The hose connects to the thermostat housing at the lower front of the engine.
• Use clean shop towels to wipe the area around the housing so dirt does not enter the cooling system.

Step 5: Remove the Lower Radiator Hose from the Housing

• Use pliers to squeeze the spring clamp tabs on the lower radiator hose.
• Slide the clamp back along the hose.
• Twist the hose gently by hand to break it loose.
• If needed, use a flat-blade screwdriver very carefully between the hose and housing to help release it without cutting the hose.
• Pull the hose off the thermostat housing and aim it into the drain pan 2-gallon minimum.

Step 6: Remove the Thermostat Housing

• Use a 12mm socket, 3/8-inch extension, and 3/8-inch ratchet to remove the thermostat housing bolts.
• Support the housing with your hand as the last bolt comes out.
• Remove the housing and note how the thermostat is installed before pulling it out.
• Take a phone photo before removal.

Step 7: Remove the Old Thermostat and Gasket

• Pull the old thermostat out by hand.
• Remove the old thermostat gasket/O-ring by hand.
• Use clean shop towels to wipe the sealing surfaces clean.
• Do not gouge or scrape the aluminum housing surface with metal tools.

Step 8: Install the New Thermostat

• Install the new thermostat gasket/O-ring onto the new engine coolant thermostat.
• Place the thermostat into the engine in the same orientation as the original.
• If the thermostat has a small jiggle valve or bleed pin, position it upward.
• A jiggle valve is a small loose pin or check valve that helps trapped air escape while filling coolant.

Step 9: Reinstall the Thermostat Housing

• Place the thermostat housing back onto the engine by hand.
• Start the bolts by hand first to avoid cross-threading.
• Use a 12mm socket, 3/8-inch extension, and 3/8-inch ratchet to snug the bolts evenly.
• Use a torque wrench 5-60 Nm with the 12mm socket to tighten the housing bolts to Torque to 10 Nm (7 ft-lbs).
• Snug is enough; aluminum strips easily.

Step 10: Reconnect the Lower Radiator Hose

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

Step 11: Close the Drain and Refill Coolant

• Use a flat-blade screwdriver to close the radiator drain plug if you opened it.
• Install the coolant funnel kit on the radiator filler neck.
• Pour in Subaru-compatible super coolant or equivalent phosphate-based coolant slowly.
• If using concentrate, mix it with distilled water to the coolant maker’s recommended ratio before filling.
• Fill until the funnel holds coolant above the radiator neck.

Step 12: Bleed Air from the Cooling System

• Keep the coolant funnel kit installed and partly filled.
• Start the engine and let it idle.
• Set the cabin heater to full hot and the blower to low.
• Watch the funnel; small bubbles are normal as air escapes.
• Use nitrile gloves to carefully squeeze the upper radiator hose a few times after it begins to warm.
• When the radiator fans cycle on and off and the heater blows hot, let the engine idle a few more minutes.
• Add coolant as the level drops, keeping the funnel from running empty.
• Do not rev a cold engine hard.

Step 13: Cap the Radiator and Fill the Overflow Tank

• Turn the engine off.
• Use the stopper from the coolant funnel kit to plug the funnel before removing it.
• Install the radiator cap by hand.
• Fill the coolant overflow reservoir to the “FULL” mark using Subaru-compatible super coolant or equivalent phosphate-based coolant.

Step 14: Reinstall the Splash Shield

• Lift the lower splash shield into position by hand.
• Use a plastic trim clip remover to help align and install the plastic clips.
• Use a 10mm socket and 3/8-inch ratchet to reinstall any 10mm fasteners snugly.

Step 15: 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 Forester slowly to the ground.
• Remove the wheel chocks.

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✅ After Repair

• ✅ Start the engine and check carefully around the thermostat housing and lower radiator hose for leaks.
• ✅ Watch the temperature gauge during the first full warm-up; it should rise normally and stay near the middle.
• ✅ Take a short test drive, then let the engine cool completely.
• ✅ Recheck the radiator and overflow reservoir levels when cold, then top off as needed.
• ✅ If the heater blows cold, the gauge climbs, or you hear gurgling, air may still be trapped and the cooling system should be bled again.
• ♻️ Take old coolant to a recycling center or auto parts store that accepts used coolant.

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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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