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Radiator Leak: Symptoms, Causes And How To Fix

Of all the automotive systems, the engine cooling system is supremely important and perhaps the most fragile. In addition to hoses or coolant condition, the radiator assembly itself is a component that should be checked regularly. Any type of radiator failure, such as a radiator leak, threatens to cause irreparable damage to the engine itself. 

The engine may be irreparable damaged from a radiator leak
A radiator leak threatens to cause irreparable damage to the engine itself

What’s At Risk?

Almost every late-model gasoline or diesel-powered vehicle has a temperature gauge in the dash, and several temperature sensors; some temperature sensors are designed solely for that vehicle’s PCM. This is because engine temperature is critical to a vehicle’s performance and wellbeing, perhaps even more so than in past generations.

Countless engine and drivetrain components are made of aluminum alloy and are extremely heat-sensitive beyond a certain point. Head gaskets can fail; cylinder heads can warp. Even a relatively minor radiator leak can potentially lead to catastrophic engine failure if the engine is allowed to overheat for even a short period of time.

What Exactly Does The Radiator Do?

Saying that the radiator “cools” the engine is not exactly accurate; it removes heat. Think of the coolant flowing through a running engine as a kind of “heat sponge”, absorbing heat as it travels from one end of the engine to the other. Once the super-heated coolant enters the radiator, it releases the heat (called dissipation) through the fins of the radiator into the surrounding atmosphere.

For this to work, the coolant must maintain a predetermined pressure, typically around 16 PSI above the ambient atmospheric pressure. This pressure is important because it keeps the coolant from boiling – and vapor cannot do the job a liquid was designed to do.

Symptoms of Radiator Leak

Symptoms of a radiator leak are few but typically very noticeable. For instance, it might be a case of the driver noticing a puddle of coolant underneath the front of the vehicle when parked.

In other cases, a misinformed customer may simply mistake normal evaporator drainage for a coolant leak. But remember, a customer complaint of a “heater not blowing hot” should send up a red flag.

The vehicle’s temperature gauge is a great indication. Most drivers recognize that sweet spot where the needle rests – and stays – once the engine reaches operating temperature: usually just below the centerline. But even a slight movement above that setting may indicate a problem like a minor coolant leak. If the needle shoots over to “hot”, then all bets are off. At any rate, any unusual gauge movement calls for a closer inspection.

Common Causes Of Radiator Leak

First, consider where the radiator is located: at the front of the vehicle, where all the dust, dirt and road debris contact the moving vehicle head-on. As a result, debris tends to embed itself into the fins of the condenser and then the radiator, blocking airflow and raising pressures internally until failure becomes unavoidable. A seized or inoperable cooling fan may exhibit the same symptoms.

It’s important to perform coolant checks on a regular basis. Over time, elements within the coolant will break down, diminishing its ability to absorb and dissipate heat correctly.

In many cases, this also leads to corrosion of internal parts that can lead to blockages or leaks in the radiator along with the rest of the cooling system. This is why the strength of the coolant should be tested at regular intervals. In addition, avoid using the wrong coolant in a cooling system. Doing so can be just as destructive as using the wrong fluid in the transmission, so it pays to be familiar with the various types of coolants currently on the market.

Material fatigue and cracking remain a concern, particularly with higher mileage vehicles. Although vehicles with factory-installed towing packages utilize larger and more durable radiators, this is not the case for the majority of passenger cars and SUVs.

As previously stated, most radiator tanks are made from glass-reinforced nylon. Although these tanks are built to withstand temperature and pressure fluctuations over many thousands of miles (some longer than others), eventually the elements will catch up and gradually turn the plastic brittle. This is exceptionally noticeable during hose replacement on a higher-mileage vehicle, with a sickening crack of the radiator hose neck.

How A Failed Pressure Cap Can Cause A Radiator Leak

Some radiator leaks don’t directly involve the radiator at all. The radiator cap is more complicated than it may first appear. The cap contains valving than opens to allow pressurized coolant to be released into the overflow reservoir as it heats up and expands. Likewise, the cap allows the coolant to return to the radiator once the engine has cooled down and the pressure has decreased. A failed radiator cap can allow heated coolant to overfill the reservoir, a condition that looks suspiciously like a symptom of a blown head gasket. Likewise, a broken cap may not allow any coolant to flow to the reservoir, resulting in a blown hose or worse.

What Can Cause An Internal Radiator Leak

Many vehicles with automatic transmissions have transmission coolers integrated into the radiator. Occasionally, the internal seal between the transmission fluid and the coolant fails, resulting in a strawberry milkshake-looking fluid that should be removed immediately before the cooling system AND the transmission are ruined. In this case, the radiator must be replaced, and the entirety of both systems flushed.

How To Fix Radiator Leak

When the radiator itself leaks, it is most often due to a crack in the tank or severe corrosion. There is usually only one sensible recourse – replacement. An aftermarket epoxy may be an option for tiny cracks, but very little in the way of those types of repairs can be guaranteed to the customer. In some cases, a fusion weld can be achieved by melting nylon into the cracked portion of the tank’s base material. However, this repair has only a questionable success rate and is rarely cost-effective in the long run.

Radiator internal sealant is another approach, but there’s just one hitch: the sealant doesn’t know exactly where to go, so it goes everywhere. This means into cylinder head water jackets and heater cores, for example.

Tank replacement itself may make sense on an older copper or brass radiator, but with labor included, that may come to eclipse the price of a remanufactured replacement radiator. If the radiator is internally corroded to the point of leakage, the unit itself is not salvageable and must be replaced. Under these circumstances, a system flush and a new thermostat are strongly recommended.


When the vehicle is obviously losing coolant, it HAS to be going somewhere. The radiator remains the most prominent and accessible component of the vehicle’s cooling system. Therefore, a thorough visual inspection of the radiator, including the cap and overflow reservoir, is a great place to start. Keep in mind that a radiator leak is a symptom of a potentially much greater problem.

Read More: 4 Most Common Transmission Control Module Symptoms

Tim MillerFounderOBD Advisor

I’m Tim Miller from Denver, Colorado. I’m the founder of obdadvisor.com, an automotive blog about "Auto Diagnostic Tools and Repair Guides". My fan page is facebook.com/autozikcom. I've been working as an automotive mechanic and blogger for over 10 years writing articles to share my experiences and expertise.

Web: https://www.obdadvisor.comEmail: [email protected]
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