To prevent the combustion engine in the car from overheating, constant cooling is necessary. The water pump ensures a constant flow in the cooling system and a low engine temperature. However, the coolant pump can become defective, especially if the car has a high mileage. In our guide, we reveal the symptoms of a defective coolant pump and why the pump breaks down. You will also receive valuable tips on how to change the water pump.

Function of the water pump in the car

The water pump ensures that the engine is always supplied with sufficient cooling water. This is important so that the engine does not overheat. Almost every modern car with an internal combustion engine today has a corresponding cooling system with cooling water as the coolant. The heat generated during engine operation can thus be quickly dissipated.

Pump puts water into circulation

As soon as the engine is running, the water pump ensures a constant circulation of the coolant. The coolant thus reliably flows through the radiator at the front of the vehicle. Here, the temperature of the coolant is significantly lowered. The cooled coolant is then returned to the engine. If necessary, the heat exchanger of the heater is integrated into the cooling circuit.

The mechanically driven coolant pump

In most modern vehicles, a cooling water pump is installed as a mechanically driven pump. The pump is often flanged directly to the engine block and is driven from the outside by a belt. Depending on the vehicle model and engine, the pump is driven via the V-ribbed belt or, in some cases, also via the timing belt.

Coolant pump with electric motor

In addition to the water pump with mechanical drive, there are also variants that are driven purely electrically. Here, an electric motor provides the necessary revolutions of the pump, completely independent of the speed of the combustion engine. The performance of the water pump can thus be better adapted to the requirements of the car’s operation.

Water pump defective – change or repair?

If the water pump is defective, the question arises: Replace or repair? The clear answer is: replacing the defective water pump is urgently recommended. Only rarely is it even possible to repair a broken coolant pump. In addition, numerous components of the water pump are usually worn out when it breaks down.

Changing the water pump is less time-consuming than repairing the component. In addition, a suitable pump can usually be obtained relatively cheaply as a spare part. However, the coolant pump can differ within a model generation depending on the engine. This should be taken into account when choosing a new or good used pump.

Typical symptoms of a defective water pump

If the water pump is defective, typical symptoms often appear. If the pump fails completely, the temperature in the cooling system rises rapidly. The driver is informed of this via the instrument cluster. A warning light may also indicate overheating. In this case, the car should be stopped and the engine switched off.

Water pump is leaking

In addition to a complete failure of the water pump, leaking water pumps occur especially in older vehicles. In this case, the failure of the water pump is announced at an early stage by leaking coolant, which often evaporates immediately due to the high temperatures. A drop in the coolant in the expansion tank then often indicates the defective water pump.

Water under the car or noise

If there are major leaks in the water pump, symptoms such as coolant clearly dripping, and small pools of coolant under the car are also possible. Noises may also appear even before the coolant pump fails. For example, a defective water pump announces itself with a grinding or hammering noise.

Symptoms of a defective water pump at a glance:

• Engine temperature rises suddenly
• Overheating of the cooling system
• Warning message in the vehicle cockpit
• Warning light for the coolant temperature lights up
• Noise from the pump area
• Loss of cooling water in the cooling system
• Cooling water under the car

Possible causes for the defect of the water pump

If the water pump is defective, there are several possible causes. Especially with high mileage, progressive wear is often responsible for the defective water pump. When the engine is running, the water pump is always driven, so it wears out. The breakage of internal components is also possible.

Loss of water due to worn seal

Apart from wear and tear, the ageing of the materials is often the decisive factor for a defect in the water pump. Due to the constant stress caused by high temperatures, the seals of the water pump can become brittle and porous. As a result, more and more coolant leaks out. Damage due to dirt in the coolant or incorrect additives is also possible.

Causes for the defect of the water pump in the overview:

• Wear of the water pump at high mileage
• Impeller inside is broken
• Material ageing on seals
• Foreign body in the coolant
• Cooling additives without manufacturer’s approval

Changing the water pump

If the water pump is defective, it must be replaced with a suitable spare part. Diagnosis and replacement should be carried out promptly, as otherwise the engine cooling system can no longer fulfil its function. Skilled car mechanics are happy to replace the water pump themselves. The exact procedure for replacing the coolant pump can vary depending on the vehicle and engine.

Replacing a water pump with V-belt drive in 5 steps

1. Diagnosis: Before starting to change the water pump, it should be checked whether there is actually a defect in the pump. It is also possible that there is a defect in the thermostat that releases the coolant circuit to the radiator when the operating temperature is reached. Leaks in the water pump can often be detected by pressing the cooling circuit.
2. Prepare for replacement: In order to be able to change the water pump, various preparations must be made depending on the vehicle model. Parts in the engine compartment often have to be dismantled in order to reach the water pump. If the coolant pump is driven by a V-belt, the belt must be loosened and removed from the pump.
3. Remove the water pump: Once everything is well prepared, the removal of the defective coolant pump can begin. Since coolant can leak out, a sufficiently large container should be placed underneath to collect it. Then loosen the screw connections of the coolant pump. Then remove the water pump.
4. Install the coolant pump: After removing the defective part, the new coolant pump can be installed. Before doing so, it is a good idea to make a brief comparison between the two components. This should ensure that the right water pump is installed. The new water pump is installed in the reverse order.
5. Complete the exchange: After installing the water pump, the drive belt is tensioned again. The belt can also be replaced with a new one at the same time. Then fill up the coolant as specified by the vehicle manufacturer and bleed the cooling system.

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