Air Pockets in the Cooling System
Air pockets can form in the cooling system after draining and refilling the coolant in the system. Air enters the cooling system after a coolant flush or replacing the water pump or lower radiator hose. These air pockets cause the engine to overheat, even after it appears to be filled correctly. It will continue to overheat until the stubborn air pocket is removed.
Air pockets are typically removed with patience, and depending on the vehicle, a special procedure or tool. Technicians can use a coolant fill funnel or a special tool that forces coolant into the system. Air rises to the top of the system and is more compressible than coolant; it can be difficult to remove without a special procedure.
Cooling system passages are designed to prevent any 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 through passages located beside each cylinder and into the cylinder head. Reverse cooling systems send cooled fluid to the cylinder head first.