How does Subaru cooling system work?

Subaru relies on a traditional, closed-loop liquid cooling system driven by a mechanical water pump, thermostat, radiator, and electric fans to keep its boxer engines at an optimal temperature. Heat is removed via the radiator, while hot coolant also flows through the heater core to warm the cabin, all within a pressurized, expandable circuit that uses an expansion (degas) tank and a radiator cap to manage coolant volume and air.

Core components that make it work

The following parts form the backbone of Subaru’s cooling system, working together to absorb engine heat and release it to the air outside the vehicle.

• Engine block and cylinder head water jackets that hold the circulating coolant


• Water pump that circulates coolant through the loop


• Thermostat that governs coolant flow to the radiator based on temperature


• Radiator and cooling fans (electric or combined) to dissipate heat


• Expansion/degas bottle and radiator cap to manage pressure and coolant volume


• Hoses and clamps that route coolant between components


• Heater core and dashboard controls for cabin heating


• Coolant temperature sensor and ECU integration to regulate fan speed and operation

In practice, these parts work as a coordinated system: the engine warms up, the thermostat opens, the water pump pushes hot coolant to the radiator, fans help with air flow, and the cooled coolant returns to the engine. The heater core uses a branch of the same hot coolant to provide warm air inside the cabin when requested.

How the loop manages heat in real operation

When the engine runs, heat is transferred to the coolant, which then travels through channels in the engine block and cylinder heads. The thermostat acts as a valve, staying closed when the engine is cold to speed up heating, and opening once the coolant reaches a target temperature to allow steady circulation to the radiator. The radiator dissipates heat to the outside air, aided by fans, which may operate at different speeds depending on engine load and climate conditions. The coolant returning from the radiator continues through the engine to complete the loop.

Core flow path

The typical flow path begins at the water pump, moves through the engine’s jackets, passes through the thermostat to the radiator, then returns via the lower hose back to the pump. A branch diverts some hot coolant to the heater core for cabin warmth, and the expansion tank handles coolant expansion and air removal from the system.

Cabin heating, cooling, and electronics integration

Outside of engine cooling, Subaru designs route coolant through the heater core to provide warm air on demand. The vehicle’s climate control system coordinates with the cooling system so that engine temperature and cabin comfort are balanced. The coolant temperature sensor feeds data to the engine control unit (ECU), which can adjust fan speed and, in some models, mimic a two-stage cooling strategy to manage heat during high load or hot climates.

Maintenance, bleed, and common issues

Regular maintenance and proper bleeding are essential to prevent overheating and ensure long service life. Subarus use a specific coolant type and mix, and the system can trap air if not bled correctly. Below are typical maintenance steps and checks, followed by common problems to watch for.

Before performing maintenance or a bleed, consult the owner’s manual or a qualified technician for the exact procedure, coolant specification, and refill steps for your model year.

• Check coolant level when the engine is cold and, if applicable, when hot per manual instructions


• Inspect hoses for cracks, swelling, or leaks and replace as needed


• Inspect the radiator and cooling fins for obstructions, corrosion, or damage


• Test for leaks with a pressure test and check the expansion tank for cracks or improper sealing


• Ensure the water pump is operating and that the belt/drive is in good condition


• Use the correct Subaru-approved coolant and avoid mixing with incompatible products


• Bleed the system to remove air pockets to prevent hot spots and overheating

Common issues range from overheating and coolant leaks to a stuck thermostat or a failing water pump, as well as clogged radiators or malfunctioning cooling fans. Air in the system or a faulty coolant temperature sensor can also lead to improper fan operation or erratic temperature readings. If overheating occurs, it is important to stop safely, inspect for leaks, and have the system checked promptly to prevent head gasket damage.

Summary

In short, Subaru’s cooling system is a connected, heat-removing circuit that uses a water pump, thermostat, radiator, and fans to control engine temperature while also supplying heat for the cabin. Proper coolant type, confident bleeding, and routine checks of hoses, the radiator, and the expansion tank are essential to keep the system functioning smoothly and to avoid overheating or related engine damage.

Do subarus have coolant issues?

Overheating Engine
If your Subaru's engine temperature gauge consistently reads higher than normal, it might be due to low coolant levels caused by a leak. An overheating engine can cause significant damage and should be addressed immediately to avoid more severe issues.

How does the cooling system work step by step?

Coolant, driven by a water pump, is pushed through the cylinder block. As the solution passes through these channels, it absorbs heat from the engine. Leaving the engine, this heated fluid enters the radiator, where it is cooled by the air flow entering through the car's radiator grill.

How to burp a Subaru cooling system?

To burp a Subaru cooling system, fill the system with a coolant mixture, then run the engine with the radiator cap off and the heater on maximum heat. Use a no-spill funnel kit for easier filling and to prevent overflow, and squeeze the radiator hoses to help dislodge air bubbles. Allow the engine to reach operating temperature and rev it to force out any remaining air bubbles until no more bubbles appear in the funnel.
 
Step 1: Prepare the system

• With the engine cold, attach a no-spill funnel kit to the coolant reservoir or radiator cap opening. 
• Fill the funnel with the correct coolant mixture. 
• Turn the vehicle's heater to the highest heat setting and the fan speed to a low setting. 
• If your model has a radiator cap, you may need to jack up the front of the car to make the highest point in the system accessible for bleeding. 

Step 2: Run the engine and bleed the air

• Start the engine and let it idle. 
• As the engine warms up, the coolant level in the funnel will drop and you will see air bubbles rise to the surface. 
• Continue to add coolant to the funnel as needed to keep it full and prevent air from being sucked back into the system. 
• Squeeze the upper and lower radiator hoses to help dislodge trapped air. 

Step 3: Finalize the process

• Allow the engine to run until it reaches its operating temperature, which is indicated by the fans cycling on at least once or twice and the coolant temperature gauge showing a normal reading.
• Rev the engine to 3,0003 comma 0003,000 to 4,0004 comma 0004,000 RPM for a few minutes to force out any deeper air pockets.
• Continue to run the engine until no more air bubbles are seen in the funnel for at least five minutes.
• Turn off the engine and allow it to cool down before carefully removing the funnel.
• Remove the funnel, top off the coolant in the reservoir to the "full" line if necessary, and replace the radiator cap.
• Monitor the coolant level in the overflow tank for the next week, as it may need to be topped off as the system fully settles. 

How does the cooling system work on a Subaru?

The cooling system is vital. Your Subaru engine is kept at the correct operating temperature by liquid coolant pumped through internal galleries in the engine block and heads. Heat from the internal combustion process is transferred from the engine to the coolant.