After performing any work on the cooling system:

The cooling system must always be vented according to the manufacturer’s instructions

KS | Kolbenschmidt | Motorservice
Pierburg | Motorservice

Why does the cooling system have to be bled after replacing the water pump? How do you bleed the cooling system correctly? What can happen if there is air in the coolant circuit? How do hot spots occur? This article tells you want you need to be aware of after opening the cooling system.

State-of-the-art vehicles have one or more coolant circuits, which divide into different branches, and are closed or released depending on the operating state. In many cases, either several electrical water pumps are installed, or a combination of electrical and switchable mechanical water pumps.

If the system is not vented according to the specifications of the vehicle manufacturer, air can build up in all of these branches and components. This can lead to malfunctions, and in some cases, damage to the components or engine.

When this happens, the risk is not only that the water pumps could run dry. Air bubbles in the system can also impair heat transfer. This can cause heat to build up in some areas (hotspots), which could, for example, directly damage the circuit board of an electrical coolant pump, or cause the electronic components to age prematurely.



This not only applies to replacement of the water pump, as air enters the system when performing any work that requires the cooling system to be opened. Therefore, you must make sure to observe the manufacturer‘s instructions on venting the cooling system!


The venting process varies depending on the manufacturer, vehicle and model. Generally, venting takes place using a relevant program item in the diagnostic tester (e.g. BMW 120d, model year 2011-2019: Vehicle maintenance -> Venting the cooling circuit).

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