Here, you will find useful basics and important tips on the topic of cooling system rinsing in vehicles, radiator ventilation and checking leak-tightness of cooling system.

Cooling system contaminations and air enclosures lower cooling performance and can lead to major damages. Rinsing the cooling system and its careful ventilation are therefore imperative during repair measures. The same applies to testing cooling system air-tightness. On this page, we explain the technical background for these works and show how to do them efficiently.

Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their work. The information provided on this website is intended for use by suitably qualified personnel only.


If the cooling system is contaminated, then the coolant must first be drained and the cooling system must be rinsed.  


Contamination may be:

  • Oil (defective cylinder head gasket)
  • Rust (internal corrosion of engine)
  • Aluminium (internal corrosion of radiator)
  • Foreign particles (additives/sealant)
  • Foreign particles (defective water pump)


Depending on the degree of soiling, the cooling system is cleaned with hot water or with a special rinsing liquid. Depending on vehicle manufacturer and symptom, there are various approaches to rinsing. Audi specifies a special rinsing liquid for rinsing if the coolant is rusty brown and the heating power is insufficient. For the multiple rinsing process, the thermostat must be dismantled and the heating power must be measured before and after the rinsing.


With regard to its models Corsa, Vectra and Omega up to model year 1997, Opel/Vauxhall notes that a clogged radiator may be the cause of excessively high engine temperature. In that case, the system should be rinsed with hot water (> 50 °C) and, in addition to the radiator, all coolant-contacting parts (heat exchanger, cylinder head etc.) should be replaced. The degree of contamination and the vehicle manufacturer’s instructions thus specify the method and the rinsing agent to be used. It should at any rate be observed that due to the design (e.g. flat tube) of modern cooling systems not all components can be rinsed and therefore need to be replaced.


This applies in particular for the following components:

  • Thermostat
  • Radiator
  • Electrical valves
  • Cap
  • Heat exchanger


If the coolant level in the expansion tank cannot be checked due to the contamination (oil, rust), then the tank must likewise be replaced. Thermostat and the closing cap should be replaced as a rule. If special cooling system cleaners are used, then care must be taken that they do not attack sealing materials and do not get into the groundwater or are disposed of via the oil separator. The cleaning agents must be collected together with the coolant and be disposed of separately. After rinsing, the system must be filled with coolant following the vehicle manufacturer’s instructions, bled and checked for function and tightness.


Rinsing with chemical solution

The cooling system is rinsed with a chemical solution for removing contaminations.


03:21 min


Air in the cooling systems of motor vehicles has become a widespread problem. The air bubbles are caused by the positioning of the radiator or the expansion tank at the level of the engine or even below. Thus, the complete ventilation of the cooling system after repair or exchange of the coolant may be a serious problem. Air in the cooling system considerably reduces the circulation of the coolant and may lead to engine overheating and the consequent severe damage.


The radiator-vacuum filling unit can solve this problem. It can serve to:

  • Eliminating air bubbles
  • Checking for leaks
  • Quickly refilling the cooling system


The filling unit gets connected to the radiator or the expansion tank using the adapter. Then, connect a compressed air hose usually used to operate your pneumatic tools. Via a special valve, the cooling system is then evacuated and a high negative pressure generated. Then, connect the enclosed suction hose and add the fresh water/antifreeze mixture to the cooling system via a clean coolant tank (bucket, jug). With the help of the manometer, which measures the negative pressure, the tightness of the entire system can be checked at the same time.


To check the cooling system for leaks, the use of a pressure tester is recommended. The cooling system is pressurized with the aid of a hand pump. When observing the manometer, a pressure drop can indicate a leak in the cooling system. With the help of universal or vehicle-specific adapters, the pump can be connected via a quick coupling to almost all common passenger and commercial vehicles, agricultural and construction machinery.


For hard-to-find leaks, the cooling system can be filled with a tracer dye in advance.


Cooling system tightness check

Detecting a leak in cooling system using pressure test kit.


01:38 min


The photos show typical damage due to various causes.


All defects cause reduced performance of the radiator. Repair is not common in modern coolant coolers, because aluminium is difficult to weld and the small ducts might get clogged by the welding. Sealant must not be used, because it clogs and reduces the performance.


Limescale deposits and the use of sealants may clog the heat exchanger in the same way as the radiator. Such deposits can be removed by rinsing with certain cleaning agents. Note the vehicle manufacturer’s instructions.

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