How do I find out where my antifreeze is leaking from?

To identify where your antifreeze is leaking, start with a quick visual check of the radiator, hoses, and expansion tank, then use a UV dye or a cooling-system pressure test to pinpoint the exact source.

Start with a quick visual check

This initial pass helps you spot obvious leaks and narrow down the probable causes before you start more invasive testing. Focus on the most common fault points in the cooling system.

• Radiator and radiator cap


• Radiator hoses and hose clamps


• Expansion (overflow) reservoir and its cap


• Water pump weep hole or pulley area


• Thermostat housing and intake manifold connections


• Heater core hoses and connections behind the dashboard


• All hose clamps and fittings for signs of seepage

Visually inspecting these components often reveals the source or at least reduces the list of suspects to a manageable few.

Tools and materials you may need

Having the right tools on hand makes it easier to diagnose without guessing. Gather these common items before you start probing for leaks.

• UV/fluorescent dye designed for cooling systems


• UV flashlight or black light


• Cooling system pressure tester (with adapters for your vehicle)


• Catch pan, rags, and protective gloves


• Funnel and fresh coolant (if you need to top up)


• Shop towels and a flashlight for hard-to-see areas

Having these tools ready helps you confidently identify leaks and avoid guessing about their location.

Using UV dye to locate leaks

UV dye makes even tiny leaks glow under ultraviolet light, helping you trace the exact path of the coolant without tearing apart components.

1. Check your coolant type and choose a dye compatible with your system; add the dye according to the product instructions.


2. Run the engine and bring it up to normal operating temperature to circulate the dye.


3. Shut off the engine when safe and use the UV light to inspect all suspect areas for glowing dye trails.


4. Follow the dye back to its source, which is typically a seam, a hose clamp, a cap, or a fitting.


5. Document the source and plan the proper repair or replacement.

UV dye testing is particularly valuable for small leaks or leaks that occur only under operating pressure. It can save time by revealing hidden seepage paths.

Verifying leaks with a cooling-system pressure test

A pressure test pressurizes the cooling system to its normal operating pressure, exposing leaks that might not drip when the engine is cold or just starting.

1. Ensure the engine is completely cool before removing the radiator cap or reservoir cap to avoid scalding.


2. Attach the pressure tester to the radiator fill neck or expansion tank, following the tester’s instructions.


3. Pump the tester to the vehicle’s recommended pressure (commonly around 13–15 psi, but check your service manual).


4. Observe the gauge and inspect all hoses, clamps, seams, the radiator, water pump area, and the heater core lines for any drop in pressure or visible leaks.


5. If a leak is found, repair or replace the faulty component and re-test to confirm the system holds pressure.

Pressure testing is a reliable method to confirm leaks and quantify the severity, especially when external signs are subtle or absent.

Safety considerations and cleanup

Antifreeze is highly toxic to pets and wildlife, and it’s also slippery when spilled. Follow these safety practices to protect yourself and the environment.

• Work in a well-ventilated area and wear gloves and eye protection.


• Keep pets and children away from exposed antifreeze spills.


• Clean up spills promptly with absorbent materials and dispose of used coolant according to local regulations.


• Never mix different types of coolant unless the manufacturer approves it; flush and refill as needed.

Proper handling minimizes health risks and prevents environmental contamination while you diagnose the issue.

When to seek professional help

If you notice signs of a head gasket issue (milky oil, white exhaust smoke), an overheating condition that returns after a simple top-up, or if you’re uncomfortable performing pressure tests or dye tracing, consult a qualified mechanic. Complex leaks or heater-core problems often require professional tools and expertise.

Summary

Finding where antifreeze leaks originate starts with a careful visual inspection of the radiator, hoses, and expansion tank. If the source isn’t obvious, use UV dye to trace leaks or a cooling-system pressure tester to reveal leaks under pressure. Prepare with the right tools, observe safety precautions, and address the fault promptly to prevent overheating and further damage. When in doubt, a professional diagnostic is the safest path to a reliable repair.

Where is my coolant going if I can't find a leak?

If you are losing coolant without visible leaks, it is likely going into the engine's combustion chamber, mixing with the engine oil, or evaporating due to a faulty radiator cap. Common signs of an internal leak include white smoke from the exhaust, milky or "milkshake"-like engine oil, and pressure issues in the cooling system. A faulty radiator cap will prevent the system from holding pressure, allowing coolant to escape as vapor and also causing overheating. 
Internal leaks

• Head gasket failure: This is a common cause, where the gasket between the engine block and cylinder head fails, allowing coolant to leak into the combustion chamber or oil.
  • Check for: White smoke from the exhaust that smells sweet, or engine oil that looks milky on the dipstick.
• Internal cracks: A cracked cylinder head or engine block can also allow coolant to leak internally. 

This video explains how a blown head gasket can cause coolant loss: 57sHVAC Mechanic YouTube · Nov 21, 2023
Evaporation

• Faulty radiator cap: If the radiator cap is not holding pressure, the coolant will boil at a lower temperature and evaporate out of the system.
  • Check for: The coolant level dropping, but no other signs of a leak, especially when the engine is hot. 

You can watch this video to learn how to check if your radiator cap is working properly: 51sBudget MechanicYouTube · Oct 2, 2022
Other possibilities

• Heater core leak: The heater core is a small radiator located inside the car's cabin. If it leaks, you may smell a sweet odor inside the car or see a wet spot on the passenger-side floor. 
• Slow external leak: Some leaks are only visible when the system is hot and under pressure, and may not be visible when the engine is cold. A professional can perform a pressure test to check for these hidden leaks. 

Can I drive with a coolant leak if I keep topping it up?

No, you should not drive with a coolant leak, even if you keep topping it up, because it can cause serious and expensive engine damage like overheating, head gasket failure, or a complete engine melt-down. While driving a very short distance to a repair shop might be possible if the leak is minor, it's risky and depends on closely monitoring the temperature gauge and pulling over if it rises. 
Risks of driving with a coolant leak

• Overheating: The engine can overheat, leading to a breakdown. 
• Major engine damage: A complete engine meltdown or a blown head gasket are expensive repairs that could require an engine replacement. 
• Corrosion: Coolant leaks can damage electrical components. 
• Stranding: You could be left stranded on the side of the road. 

What to do instead

• Don't drive: Avoid driving the vehicle as much as possible to prevent further damage. 
• Check for the leak: Look for puddles under your car, which are often a sweet-smelling, brightly colored liquid (green, orange, or pink). 
• Get professional help: Have a mechanic diagnose and fix the leak as soon as possible. 
• If you must drive a short distance:
  • Let the engine cool completely before adding coolant. 
  • Use the correct coolant or water in an emergency, but remember that water lacks anti-freeze and anti-corrosion properties and should only be a temporary solution. 
  • Drive slowly, keeping a constant eye on the temperature gauge. 
  • If the temperature gauge starts to climb, pull over immediately, shut off the engine, and let it cool down completely before adding more coolant. 
  • Only attempt this for the shortest possible distance to the nearest repair shop. 

What is the most common cause of a coolant leak?

The most common cause of a coolant leak is wear and tear on hoses and connections, which leads to cracks or loose fittings. Other frequent culprits include a corroded or damaged radiator and a failing water pump. 
Common causes of coolant leaks

• Damaged hoses: Rubber hoses degrade over time, becoming brittle, cracked, or soft, which allows coolant to escape. The lower radiator hose is particularly susceptible due to higher pressure. 
• Loose or worn connections: The clamps and connections where hoses meet other parts of the system can become loose or the gaskets can fail, leading to leaks. 
• Corroded radiator: The radiator can rust or corrode from road salt, debris, and sediment buildup, creating small holes where coolant can leak out. 
• Failing water pump: The seals on a water pump can wear out, and the gasket that seals it to the engine can also fail, causing coolant to leak from the pump's body. 
• Damaged radiator cap: A faulty or worn-out radiator cap can fail to seal the system properly, allowing coolant to leak out under pressure. 
• Blown head gasket: A more serious cause is a failed head gasket, which can cause internal or external coolant leaks. 

How to tell where a coolant leak is coming from?

To find a coolant leak, first, visually inspect the engine bay for puddles or drips and signs of dried coolant. If the source isn't obvious, use a coolant pressure tester to pressurize the system, making leaks easier to spot. For slow or hard-to-find leaks, add a UV dye to the coolant and use a black light to pinpoint the glowing fluid.
 
Method 1: Visual and pressure testing
This video demonstrates how to use a pressure tester to find a coolant leak: 58sGarage GurusYouTube · Oct 17, 2024

1. Prepare the area: Park your vehicle on a large piece of cardboard to catch any drips, which can help trace the leak's origin. 
2. Perform a visual inspection: Check for any obvious signs of leaking coolant while the engine is cold. Look for puddles under the car, wet spots on hoses, or dried, crusty residue on components like the radiator, hoses, water pump, and thermostat housing. 
3. Use a pressure tester: 
   • Allow the engine to cool completely before beginning. 
   • Attach the pressure tester to the radiator or coolant reservoir, following the tool's instructions. 
   • Pump the system to the recommended pressure (typically 10-15 PSI). 
   • Look and listen for any leaks. You can also spray soapy water on connections to see if bubbles form. 
   • If the pressure drops but there's no visible leak, you may have an internal leak. 

Method 2: UV dye test 
This video explains how to use UV dye to find coolant leaks: 47sRatchets And WrenchesYouTube · Oct 8, 2022

1. Add dye: Once the engine is cool, add a small amount of UV-reactive coolant dye to the radiator or overflow tank according to the product's instructions. 
2. Circulate the dye: Run the engine for a while or drive the vehicle to allow the dye to circulate throughout the system. 
3. Inspect with a black light: Use a UV black light to scan the cooling system components. The dye will glow brightly where there is a leak. 

Common leak locations to inspect

• Radiator: Check the seams where the tanks connect to the radiator core. 
• Hoses: Inspect the upper and lower radiator hoses, as well as the heater hoses that go to the firewall. 
• Thermostat housing: Look for leaks where the thermostat housing connects to the engine. 
• Water pump: Check the area around the water pump, often located behind the engine's pulleys. 
• Heater core: A leak here may be indicated by wetness on the passenger-side floorboard. 
• Head gasket or block: If you can't find an external leak, check for internal signs like a milky-looking substance on the oil dipstick or white smoke from the tailpipe. 

Safety precautions

• Never open a hot cooling system: The system is under high pressure when hot, and pressurized, hot coolant can cause severe burns. Always let the engine cool down completely. 
• Wear safety gear: Wear safety glasses and gloves when working with the cooling system.