Usually, air ingresses into the cooling system by many reasons. One of them – failure of the expansion tank cap. If its thread is worn, air intake occurs. It accumulates in the expansion tank or other elements of the cooling system, and at the engine rpm increase, this air may force out the coolant.If the cap valve, designed for excessive pressure blow, is jammed, hoses, pipes and the radiator will break down due to time. This will lead to appearance of airlocks. According to their location, cooling system or accessory units and assemblies, such as heating radiator, may work improperly.
Also poor-quality priming of the cooling system after antifreeze refill may also cause an appearance of airlocks. Sometimes exhaust gases ingress into the cooling system. Usually it is caused by crack in the cylinder head or its gasket wear-out.