Air Pockets in the Cooling System
Air pockets form in the cooling system after draining and refilling the coolant in the system. Air also enters the system while replacing the water pump or lower radiator hose. Sometimes even after the cooling system appears bled and filled, the engine continues to overheat. It takes monitoring and patience. Air pockets can be stubborn, and the engine will continue to overheat until the air is removed.
Air rises to the top of the system and is more compressible than coolant. It may require a special procedure or a coolant fill funnel. Some vehicles require a special tool that forces coolant into the system.
Cooling system passages are designed to prevent air or steam pockets from developing. It's usually after a repair that this problem arises. It depends on the design of the cooling system.
There are two types of cooling systems found on today's vehicles, the series, and the parallel system. A series system flows around the cylinders, then to the back of the block before flowing to the cylinder heads. In parallel systems, the coolant flows parallel through passages located beside each cylinder and through the cylinder head. Reverse cooling systems send cooled fluid to the cylinder head first.