Why does my Subaru keep getting low on coolant?

Most often, a Subaru that keeps losing coolant is due to a leak somewhere in the cooling system or a cap that isn’t sealing properly. In rarer cases, persistent coolant loss can indicate more serious issues like a head gasket leak or a failing water pump that requires professional attention.

Understanding the problem

Below are the most common places coolant escapes in Subaru cooling systems and how they typically present themselves. Knowing these potential sources helps you narrow down the cause more quickly.

• External leaks at hoses, clamps, radiator, or the expansion tank are the most common. Puddles under the car or damp hoses can point to a source.


• Leakage through the heater core, which usually shows up as a damp cabin floor, a sweet smell in the cabin, or fogging of the windows.


• A cracked or leaking radiator or expansion tank can allow coolant to escape under pressure as the engine warms up.


• A faulty radiator cap or pressure system that doesn’t seal properly can lead to gradual coolant loss even if there are no obvious leaks.


• Water pump or thermostat housing gasket leaks can weep coolant externally, especially around the pump or along gasket seams.


• Head gasket failure can cause coolant to enter the combustion chamber or mix with oil, producing white exhaust, milky oil, or a dramatic drop in coolant level without obvious exterior leaks.

When you’re dealing with coolant loss, start by ruling out the easy-to-fix external leaks and cap issues before moving to more serious possibilities.

How to diagnose the issue

The following steps help you identify the source of coolant loss and determine whether you need professional service or a DIY fix.

1. Visually inspect the entire cooling system with the engine cold for obvious leaks around hoses, clamps, radiator, reservoir, and the water pump area. Look for wet spots, crusty dried coolant, or staining.


2. Inspect the radiator cap and expansion tank cap for cracks or a degraded seal. Replace if the cap is damaged or does not hold pressure properly.


3. Check the coolant level when the engine is cold and monitor consumption over time. A steadily dropping level suggests either a leak or an internal issue.


4. Perform a pressure test on the cooling system using a radiator pressure tester to locate hidden leaks. This test helps confirm if the system holds pressure and where it escapes.


5. Look for signs of internal leaks that could indicate a head gasket problem, such as milky oil, white exhaust smoke, coolant in the oil, or air bubbles in the radiator when the cap is removed.


6. Inspect the water pump for weeping at the weep hole or abnormal play in the pulley. A noisy pump or visible leakage is a red flag.


7. Consider adding a UV dye to the coolant and using a UV light to trace faint leaks that aren’t obvious to the naked eye.

Carrying out these diagnostic steps helps you distinguish between simple fixes, like a cap replacement, and more substantial repairs that may require professional service or engine work.

Model-specific considerations for Subaru

Head gasket history on older 2.5L engines

Subaru’s older 2.5L engines (notably certain EJ-series variants) have a documented history of head gasket issues that can manifest as similar coolant loss symptoms. If you own an older model in the high-risk range, particularly with high mileage, consider a professional diagnosis to rule out gasket problems.

Water pump and cooling system design in modern engines

Newer Subaru engines (including some FA/FB designs) have improved seals and cooling layouts, reducing the risk of certain leaks. However, wear over time, degraded hoses, or a failing water pump can still cause coolant loss. If you’ve ruled out basic leaks, a technician can assess pump seals and related components.

Prevention and maintenance tips

Proactive maintenance can prevent many common coolant-loss scenarios and help you spot problems before they become serious.

• Use the correct coolant type and a 50/50 mixture with distilled water if required by your model, following the owner’s manual or dealer guidance.


• Regularly inspect hoses, clamps, radiator, and the expansion tank for signs of wear, cracking, or staining. Replace worn parts promptly.


• Keep the cooling system clean and free of debris that can trap in the radiator fins and reduce cooling efficiency.


• Avoid mixing different coolant chemistries. If you’re changing brands, flush the system as recommended and refill with the specified coolant.


• Ensure the expansion tank and radiator cap seal properly and replace caps showing wear or poor sealing.


• Bleed the cooling system correctly after any work on the system or after refilling to remove air pockets that can mimic low-level readings or cause overheating.

Following these maintenance steps helps minimize coolant loss and extends the life of the cooling system.

What to do now

If you notice a persistent drop in coolant, unusual engine temperature rise, or signs of leaks, schedule a diagnostic with a qualified mechanic. Early testing and targeted repairs can prevent overheating and costly engine damage.

Summary

Coolant loss in a Subaru is most often caused by external leaks or a faulty cap, but it can also indicate more serious problems like a head gasket or water pump failure. Use a systematic diagnostic approach—inspect for leaks, test the cap, perform a pressure test, and watch for signs of internal engine issues. For older 2.5L engines, be mindful of gasket-related failures. Regular maintenance and using the correct coolant are key to preventing future incidents. If in doubt, seek professional help to avoid potential engine damage.

Is it normal to top up coolant every week?

If you are regularly topping off your coolant, you have a coolant leak somewhere in the system. It can be difficult to identify where the leak is coming from. Often it is best to have a professional diagnose a coolant leak, but there are a few things you can check before bringing your car into a shop.

How do I tell if my Subaru has a blown head gasket?

Common symptoms of a failing Subaru head gasket include engine overheating, white smoke from the exhaust, and milky-looking oil. Other signs can be a sweet smell from the coolant, bubbles in the coolant reservoir, external oil or coolant leaks, and a rough engine idle or misfires. 
Overheating and cooling system issues 

• Engine overheating: A failing head gasket can cause the engine to overheat frequently. 
• Coolant loss: You may notice coolant levels dropping without any visible external leaks, as the coolant could be leaking internally into the engine. 
• Bubbles in the coolant reservoir: Exhaust gases can leak into the cooling system, causing bubbles or gurgling noises in the coolant reservoir or radiator. 
• Pressurized hoses: Hoses may feel unusually firm or swollen after driving due to the extra pressure from combustion gases. 

Exhaust and engine performance 

• White smoke: Persistent white smoke from the exhaust pipe, especially with a sweet smell, is a classic sign of coolant burning in the combustion chamber. 
• Rough idle or misfires: Loss of compression from a blown gasket can lead to poor engine performance. 
• Poor engine performance: You may experience a noticeable decrease in power or engine misfires. 

Oil and fluid contamination 

• Milky oil: Check your engine oil on the dipstick; if it has a milky or "frothy" appearance, it indicates coolant and oil have mixed. 
• Oil in the coolant: The coolant reservoir may have a greasy film, or the coolant itself may look murky and dark. 

External leaks

• Visible leaks: You might see oil or coolant leaking from the engine, which can appear as stains on the ground under the car or on the engine itself.
• Smoke from leaks: An external leak can sometimes cause smoke when the fluid drips onto a hot exhaust component. 

If you notice any of these symptoms, it is important to have your vehicle inspected by a professional as soon as possible to prevent further, more costly damage.

Why is my coolant level low but no leaks?

A low coolant level without visible leaks can be caused by internal leaks (like a faulty head gasket), a bad radiator cap that lets coolant evaporate as vapor, or slow, pressure-dependent external leaks that only appear when the engine is hot. It's crucial to address this promptly, as it can lead to severe engine damage. 
You can watch this video to learn more about the causes of low coolant with no visible leaks: 54sOne Trick TechnicianYouTube · Dec 26, 2024
Potential causes and what to check
This video demonstrates how to check your car for a possible coolant leak: 1mZipZapDIYYouTube · Jun 10, 2014

• Bad radiator cap: A worn-out cap may not hold pressure, allowing coolant to escape as vapor without forming a puddle. 
  • Check: Visually inspect the cap and its seals for damage. Replacing the radiator cap is an inexpensive first step. 
• Internal head gasket leak: Coolant can leak into the engine cylinders or mix with the oil. 
  • Check: Look for milky-white oil on the dipstick or a sweet smell from the exhaust. A professional pressure test is often needed to confirm this. 
• Slow external leaks: Small leaks may only appear when the system is hot and under pressure. When the engine cools, it seals up and leaves no trace. 
  • Check: A mechanic can perform a pressure test to find these hidden leaks in the radiator, water pump, or hoses, which can be found by looking for mineral deposits or dried coolant residue. 
• Leaking heater core: This is an internal leak into the cabin of the car. 
  • Check: Look for a sweet-smelling, sometimes wet or fogged-up passenger-side floorboard. 

This video explains what to do when you find white smoke or signs of a coolant leak: 1mMaxTheCarGuyYouTube · May 15, 2025
Next steps

1. Check the oil: Use the dipstick to see if the oil is milky or foamy. If it is, this indicates a possible head gasket failure. 
2. Check your exhaust: Look for any white, sweet-smelling smoke coming from the tailpipe, which can be a sign of coolant burning in the cylinders. 
3. Replace the radiator cap: This is an easy and inexpensive test that can often solve the problem. 
4. Take it to a mechanic: A professional can perform a pressure test of the cooling system to accurately find small or hidden leaks that you cannot see. 

Why does my Subaru keep losing coolant?

Causes of Subaru Coolant Leaks
Damaged or worn out hoses: The hoses that carry the coolant through the engine can become worn out or damaged over time, leading to coolant leaks. Faulty radiator: The radiator is responsible for cooling the coolant fluid, and a faulty radiator can lead to leaks.