Cover on cooling liquid reservoir

Leak tightness and leakage hole (vent) in vehicle water pumps

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What is a sliding ring sealing cartridge and how does it work? What function do sliding ring seals have when sealing water pumps in cars? What are ventilation holes or leakage holes on the pump housing for? Does the liquid in these holes in the pump indicate there is damage somewhere? Does the potential defect have to be repaired in a workshop, and does it mean a replacement part might be needed? What is a leak reservoir and when are elastomer seals, flat gaskets or liquid sealants used as housing seals on water pumps in cars? This article gives answers to questions about certain components, repairs and problems, special features of the water pump housing and how to vent it.

Sliding ring seal package

The sliding ring sealing cartridge is the actual sealing of the water pump. It essentially consists of two sliding rings and a spiral spring. The tribological pairing primarily consists of sliding rings that are made up of different materials. Depending on the durability requirements and operating conditions, hard carbon (graphite), aluminium oxide, tungsten carbide or silicon carbide may be used. The spiral spring presses the sliding rings onto one another in order to maintain the sealing effect in an unpressurised cooling system.
Sliding ring seal package, ready to install and disassembled
As with almost all designs in which two surfaces run against each other, lubricant is required in order to reduce the friction. In the sliding ring seal package, the cooling liquid in the cooling system is responsible for the lubrication and the cooling of both slide rings. As a result of the pressure in the cooling system and the rotation of the pump shaft, the cooling liquid enters between the sliding rings and enables lowwear liquid friction. To ensure the function and the attainment of the intended service life, it is always necessary for there to be a small flow of cooling liquid through the sliding rings.


This functional principle may lead to small leaks of cooling liquid on the outer face of the pump. This small leak is design-related and is not a cause for complaint.
Structure of the sliding ring seal package
1 Spiral spring
2 Sliding ring (fixed)
3 Sliding ring (rotating)
4 Bellows

Ventilation holes and leakage holes

The amount of cooling liquid that enters between the contact surfaces of the sliding rings and reaches the outer face is very small and is normally evaporated in the water pump itself. For this purpose, the pump housing features ventilation holes or leakage holes through which the cooling liquid can escape into the surrounding area. Glycol-based coolant agent contains colours and additives, meaning that coloured residue is formed on the outer face in the area of the leakage holes of the water pump.

Without the leakage holes, cooling liquid would accumulate between the sealing ring package and the pump bearing and it would penetrate into the pump bearing.
Ventilation hole and leakage hole

Leak reservoir

The visible cooling liquid residue on the leakage hole that was referred to above is frequently diagnosed as a water pump leakage by persons lacking the necessary professional knowledge. However, this minor leak is no reason to replace the water pump.

In order to prevent this misunderstanding, many engine manufacturers have switched to providing the water pumps with a reservoir at the leakage hole. The small quantities of cooling liquid that exit the water pump are collected in this reservoir. The cooling liquid therefore remains in the reservoir, where it cannot be seen from the outside, and it evaporates there.
Cover on cooling liquid reservoir

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