Tip Sheet

5 Simple steps to fix a radiator leak

A simple DIY task
PHOTO: Raymond Figuerres

A vehicle’s cooling system is essential in keeping an internal combustion engine in operating temperature. When the engine gets too hot, the system cools it down; when it’s starting from cold, the system is designed to get it up to optimum temperature more quickly.

Here’s a quick review of the cooling system’s components and their functions. The heart of the setup is the water pump. It circulates the coolant mixture through the engine block and the cylinder head/s through a series of hoses and metal pipes. After absorbing heat from the combustion in the cylinders, the coolant recirculates back to the pump, bypassing the radiator when it’s still cold. When it has attained operating temperature, the thermostat is thermo-mechanically opened to cool the coolant in the radiator.

The modern radiator is made from aluminum tube cores separated by cooling fins, with plastic tanks crimped at the ends. The plastic tanks are connected, on either ends, to an inlet hose coming from the engine, and an outlet hose leading to the water pump. The radiator exchanges the heat inside the core to air moving through the cooling fins; the air is provided by the radiator fans when the vehicle is on idle, or comes from the front of the car when the vehicle is moving at speed.

A shared component with the air-conditioning system is the heater core. Hot coolant is coursed through it when the A/C is set to heat the cabin. It’s a mini-radiator under the dashboard, ventilated by the A/C fans. The whole system is pressurized to 10-17psi, depending on the radiator-cap specification, to keep the boiling point of the coolant at around 127 degrees Celsius.

Excess pressure is relieved by the radiator cap, and expelled coolant is collected by the reservoir tank. As the engine cools down, negative pressure will suck some of the coolant back into the radiator. That way, it is a closed system; coolant is not wasted and does not need replenishment.

When you notice a puddle of coolant under your car, or the level on the reservoir tank is low, you have to investigate where the coolant is trickling from. A leaking cooling system will not be able to maintain its normal operating pressure, and will continuously lose coolant. You may notice it, and add on occasion coolant or water into the reservoir tank. As long as you don’t lose too much of the mixture, your engine will not overheat.

As the coolant level in the reservoir tank drops, air may be introduced inside the system, causing it to boil over and produce steam that is trapped within. Heat and pressure build up; cooling efficiency goes down. Sustained overheating will melt the head gasket, leading to an expensive engine teardown and rebuild.

Performing preventive measures will save you from any hassle. Observe the recommended drain and replacement interval for the coolant. The coolant has anti-corrosion additives that prevent rust or scale buildup in the engine block, the cylinder heads, and the radiator. This protection lasts for years, and will need to be refreshed to prevent oxidation and corrosion. Rust can block the tiny passages on the radiator core, decreasing its cooling capability. If the coolant is not renewed, erosion can eat through the thin walls of the core. Minor dribbles can become gushing flows if they are ignored.

There are available commercial products to plug a slow radiator leak. From the factory, these stop-leak products are plunked in to ensure airtight seals. Some engine rebuilders use them as a routine after a build. They are designed to solidify and block water from flowing out. They are recommended for tiny radiator or heater-core leaks, and for cracked gaskets that fail to sustain a seal. A ruptured hose, a cracked aluminum to plastic radiator seam, or a corroded core seal will be too much for this solution.

When using any of these stop-leak products, follow the instructions carefully and it could delay a permanent fix for a later time. 

1) Drain the old coolant.

Since neglecting to replace the water coolant mixture is the cause of the problem, it makes sense to drain it first. 

2) Flush the cooling system.

Using liberal amounts of distilled water, rinse the radiator and the whole system, making sure to activate the thermostat and the A/C heater. Tap water is not ideal because it contains minerals that will cause sedimentation.

You’re done when you’re draining clear, rust-free water. 

3) Pour in the new coolant mixture.

Use an appropriate ratio of distilled water and the manufacturer’s recommended coolant type.

4) Burp.

Make sure there is no air in the system. Park at a slight incline, or jack up the front with the engine higher than the rest of the vehicle while filling up the radiator.

5) Add your stop-leak product of your choice.

Run the engine up to operating temperature, and check the previous leaking location for oozing. Observe for a few days.

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PHOTO: Raymond Figuerres
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