DIY! How to Check Motorcycle Radiator Cooling Path by a Radiator Leak Tester Kit

How to

With a water-cooled engine, it is essential to replace the cooling water regularly, but it is also important to maintain the condition of the cooling paths through which the cooling water flows, such as the radiator and hoses. Does the radiator hose swell and deform when the water temperature rises to near 100℃, or is it bleeding from the hose clamp part? Is the radiator cap working properly? You should also be careful about things like that. The radiator leak tester kit, which can objectively check the cooling system, is expensive but infrequently used equipment, and it is very reliable for water-cooled engine motorcycle.

The cooling water doesn't boil because it's pressurized.

The valve opening pressure is written on the radiator cap, in the case of this cap the main pressure valve is opened when the pressure in the cooling path exceeds 1.1kg/cm 2 (about 107kPa), the cooling water is released to the reservoir tank to release the pressure. Currently, since the unit of pressure is Pascal, the new annual cap is displayed by kPa (kilopascal).

A water-cooled engine that transfers the heat generated by the engine to the cooling water and dissipates the heat with a radiator. If your motorcycle has a water temperature gauge, don't worry if there is a needle between the low-temperature C "COLD" and the high-temperature H "HOT".

However, the water temperature will become high if there is a continuous high load on a circuit or when low-speed riding continues on a congested road where the wind does not hit the radiator sufficiently. Overheating, which is typical radiator trouble, is a condition in which cooling water boils in the cooling path, bubbles are generated, and cooling efficiency is significantly reduced by preventing the flow of cooling water. To avoid such conditions, the radiator cap has a valve to maintain the pressure in the cooling path higher than the atmospheric pressure.

You know that the boiling point of water changes according to the atmospheric pressure. At the summit of Mt. Fuji at an altitude of 3776m, the pressure drops from the altitude of 0m, so the boiling point drops and water boils at around 87°C. On the contrary, the boiling point increases as the air pressure is increased.

Using this principle, the radiator cap is equipped with a valve to increase the pressure in the entire cooling path.


  1. The boiling point of cooling water changes depending on the pressure in the cooling path.
  2. Increase the pressure with a valve with a built-in radiator cap.

Control the boiling point by setting the valve opening pressure

Not only as a "lid" for the radiator into which cooling water is injected, but also as a radiator cap that plays a role in increasing the boiling point of cooling water, it is a clever part with a simple structure. When observing the rear side of the cap, two ring-shaped rubber seals are set, and a seal with a small diameter acts as a pressurizer.

Even if the cooling water temperature rises by starting the engine, the cooling water does not circulate in the radiator until the thermostat is opened. When the thermostat is opened, cooling water spreads throughout the cooling path, including the radiator body, and the volume expands as the temperature increases.

The higher the pressure in the radiator or cooling path, the higher the boiling point of the cooling water. However, if it is completely sealed, the pressure increase associated with the rising temperature which becomes excessive, leading to damage to the radiator hose and radiator body. Therefore, when it comes to constant pressure or temperature, open the valve and let the cooling water escape. This pressure is determined for each radiator cap and is indicated by a number called valve opening pressure.

In the case of the cap shown in the image with the number 1.1, the valve opens when the pressure of the cooling path is at 1.1kg/cm2 (approx. 107kPa). When pressurized to 107kPa, the boiling point of cooling water rises to approximately 120°C, generating a margin of 20°C more than under atmospheric pressure. And since the cooling water does not lead to overheating unless it is boiled, the safety of the cooling path will be maintained.

Some sports-type radiator caps have high valve pressure values, which increase the boiling point of the cooling water by increasing the opening pressure, making it difficult to overheat.


  1. The cooling path is protected by opening the valve at the set pressure.
  2. High valve opening pressure to prevent overheating.
    Sport type radiator cap is effective.

The pressure is applied to the leak tester to diagnose the cooling path

The radiator leak tester kit is equipped with a pump to apply pressure and an adapter that supports the radiator cap of different shapes.

Set the adapter that fits the radiator cap on the motorcycle. Since the leak tester kit mainly corresponds to the radiator for the automobile, be careful to the adapter that attached to the tester it may not fit in the radiator cap for the motorcycle.

Once you have connected the hose of the pump to the radiator cap adapter, apply pressure slowly. By applying pressure from the radiator cap, even pressure is applied to the radiator body, the joint with the hose, the entire cooling path such as the thermostat, and the water pump.

Now that I understand the relationship between pressure and boiling point in the cooling path, the radiator leak tester kit will finally come into play. A brief description of how this tester kit works is to pump pressure into the cooling path to determine whether it is possible to maintain that pressure.

The valve opening pressure of the radiator cap of this motorcycle is 107kPa, at which time the cooling water has risen to nearly 120°C. However, when riding in the streets and riding gently, the temperature does not go up to a high temperature such as 120℃. In other words, if the water temperature does not reach 120°C, the pressure in the cooling path does not rise to 107kPa.

In that case, it is not possible to do so, to tell the truth, if it is good.

The joint between the hose and the pipe and also the crankcase can deteriorate over time. If a leak occurs at a pressure lower than 107kPa due to the laxity of the seals here and there, it will cause a leak in the cooling path and the overall pressure drops, then the boiling point also drops. If you are not aware of the state in which the boiling point has decreased, you may suddenly experience overheating.

When the radiator leak tester cap adapter is attached to the radiator, pressurize with a pump to 107kPa. If you can keep the pressure even if you wait for a while in this state, you can determine that there is no leak in the cooling path.

Do not pressurize unnecessarily because applying pressure above the opening pressure of the radiator cap will cause cooling water leakage. That's why the gauge is attached to the pump. The number on the outside is 1/100 of kPa, which is 107kPa, so it is good if the pressure can be maintained at 1.1.

When the pressure is increased, the cooling water dripped from the insertion portion of the radiator upper hose. As the cooling water leaks, the pressure decreases and the boiling point decreases. If the pressure of the cooling path is the same as the atmospheric pressure = if the pressure of the cooling path does not rise even if the water temperature rises, the cooling water boils at 100°C.

However, in the case of this motorcycle, cooling water leaked from the insertion part of the radiator body and the upper hose when pressurized with a pump. The fact that the cooling water did not leak while riding and leaked to 107kPa even though there was no experience of overheating it indicates that luckily the cooling water temperature had never reached 120°C before.

At the same time as a leak from the cooling path, the condition of the radiator cap can also be checked. By combining the adapter supplied with the tester kit, the cap body is pressurized at 107kPa to check whether the pressure can be kept or not. When pressure is higher than the valve opening pressure is applied, the spring behind the rubber seal is compressed and relieved. On the other hand, if the indicator value of the pump side gauge does not increase to 107kPa or temporarily reaches 107kPa, but the pressure cannot be maintained and it decreases, the performance of the cap body is determined to be degraded.

Loss of sealing, such as hardening and cracking of rubber seals, is mainly caused by deterioration over time. Manufacturers of radiator caps claim that caps are ideal consumable parts that should be replaced once a year. If you have a radiator leak tester kit, you can determine the status of the cap in detail, but by changing the radiator cap regularly even if you do not have a tester kit, you should worry less about overheating due to poor cap.

You can test the radiator cap itself by combining the adapters that come with the leak tester kit.

When pressurized with a pump, the pressure did not increase higher than 70kPa. This is considered that the seal of the cap is deteriorated. By the way, if the cooling path is 70kPa, the boiling temperature of the cooling water will be 110℃. It will be 10°C lower than when the radiator cap is healthy.

There are two large and small seals on the rear side of the radiator cap, and the small seal on the tip of the radiator cap is the main pressure valve that keeps the pressure of the cooling path. The seal surface is clearly marked with dent marks due to aging, it is considered that it is no longer possible to maintain the pressure because the elasticity of the rubber itself is also lost. Even if it's not actually overheating, the radiator cap needs to be replaced regularly.

Click here for a radiator leak tester kit


  1. You can check the cooling path with a radiator leak tester kit.
  2. Pressurizing the cooling path can detect cooling water leaks.
  3. Check over valve opening pressure of radiator cap to prevent overheating.
Copied title and URL