Your Engine's Cooling System

Team BlueStar Inspections

A typical vehicle with a four-cylinder engine cruising down the highway at 55 miles per hour will produce nearly 5,000 controlled explosions per minute inside the engine as the spark plugs ignite the air and fuel mixture in each of the cylinders. This is what propels the vehicle down the road. These explosions produce an extreme amount of heat and will destroy an engine in a matter of minutes if not controlled. The engine's cooling system is designed to control and regulate these high temperatures.

Modern cooling systems have not changed much from older cooling systems, but they have become a lot more efficient and reliable at doing their job. The basic cooling system still consists of liquid coolant being circulated through the engine block and cylinder head (or heads in a V-type configured engine) and then forced out to the radiator to be cooled by the flow of air coming through the grille at the front of the vehicle.

The cooling system must maintain the engine at a constant temperature whether the outside air temperature is a hot 100 degrees Fahrenheit, or a frigid 30 degrees below zero. If the engine temperature is too low, fuel economy will suffer and emissions will rise. If the engine temperature gets too hot for too long, the engine will sustain damage. The engine operating temperature range for most cars is between 195 to 220 degrees Fahrenheit. The optimum temperature is around 212 degrees Fahrenheit. The higher temperature differential between the engine coolant and outside air makes heat transfer more efficient.The engine cooling system is comprised of engine coolant, passages inside the engine block and cylinder head(s), a water pump to circulate the coolant, a thermostat to control the temperature of the coolant, a radiator to cool the coolant, a fan to pull air through the radiator, a radiator cap to control the pressure in the system, and interconnecting hoses to transfer the coolant from the engine to radiator, and also to the vehicle's heating system where hot coolant is used to warm the cabin of the vehicle.

Engine coolant has a primary function of convective heat transfer for internal combustion engines. Coolant is a mixture of water, antifreeze, corrosion inhibitors, and lubricants. Coolant was developed to overcome the shortcomings of water as a heat transfer fluid. Many modern vehicles are equipped with extended or long-life coolant which is designed to last up to five years or 150,000 miles. Green coolant usually lasts two years or 30,000 miles. The correct coolant mixture and quality will prevent freezing in winter, prevent boiling in summer, prevent rust and corrosion of metal parts, become a good conductor of heat, and aid in preventing electrolysis.

A cooling system works by circulating liquid coolant through passages in the engine block and cylinder head(s). As the coolant flows through these passages, heat is transferred from the engine components to the coolant. The heated coolant then makes its way through a rubber hose to the radiator in the front of the engine compartment. As it flows through the thin tubes in the radiator, the hot liquid is cooled by the air flow entering the engine compartment through the grill in front of the vehicle. Once the fluid is cooled, it returns to the engine to absorb more heat. The water pump has the job of keeping the fluid circulating through the system when the engine is running.

A thermostat is placed between the engine and the radiator to make sure the coolant stays above a certain preset temperature to ensure the engine runs optimally. If the coolant temperature falls below this temperature, the thermostat blocks the coolant flow to the radiator, forcing the fluid instead through a bypass directly back to the engine. The coolant will continue to circulate like this until the optimal running temperature is achieved, at which point the thermostat will open and allow the coolant back through the radiator to be cooled.

The cooling system is designed to be pressurized to prevent coolant from boiling. However, too much pressure will cause hoses and other components to burst and leak, so a system is needed to relieve pressure if it exceeds a certain point. The job of maintaining the pressure in the cooling system belongs to the radiator or pressurized coolant recovery tank cap. The cap typically increases the cooling system pressure by 14 or 15 psi and raises the boiling point about 43 degrees Fahrenheit. The cap releases pressurized coolant into the coolant overflow tank. This fluid then returns to the cooling system after the engine cools down. Never remove the radiator cap immediately after stopping the engine because the pressurized coolant will immediately start boiling once the pressure is released. Burns and serious injury will almost certainly occur.

The coolant follows a path that takes it from the water pump, through passages inside the engine block where it collects the heat produced by the cylinders. It then flows up to the cylinder head(s) where it collects more heat from the combustion chambers. It then flows out past the thermostat (if the thermostat is opened to allow the fluid to pass), through the upper radiator hose, and into the radiator. The coolant flows through the thin tubes that make up the core of the radiator and is cooled by the air flow through the radiator. From there, it flows out of the radiator, through the lower radiator hose and back to the water pump. By this time, the coolant is cooled off and ready to collect more heat from the engine.

There are several rubber hoses that interconnect the components of the cooling system. The main hoses are called the upper and lower radiator hoses. These two hoses direct coolant between the engine and the radiator. Heater hoses supply hot coolant from the engine to the heater core. One of these hoses may have a heater control valve mounted in-line to block the hot coolant from entering the heater core when the air conditioner is set to maximum cool. Another hose, called the bypass hose, is used to circulate the coolant through the engine, bypassing the radiator, when the thermostat is closed. Some engines do not use a rubber bypass hose. Instead, they might use a metal tube or have a built-in passage in the front engine housing.

Mounted on the back of the radiator on the side closest to the engine is one or two electric cooling fans inside a housing that is designed to protect fingers and to direct the air flow. The fans are controlled by the vehicle's computer. A sensor monitors the engine temperature and sends the information to the computer. The computer determines if the fan should be turned on and actuates the fan relay if additional air flow through the radiator is necessary. The fans keep the air flowing through the radiator while the vehicle is moving slowly or stopped with the engine running. If the fans stopped working, the engine temperature would begin rising every time the vehicle came to a stop.

If the vehicle has air conditioning, an additional radiator, called the air conditioning condenser, is mounted in front of the engine cooling system radiator. The air conditioning condenser also needs to be cooled by the air flow entering the engine compartment. If the air conditioning is turned on, the system will keep one electric cooling fan running, even if the engine is not running hot. If there is no air flow through the air conditioning condenser, the air conditioner will not be able to cool the air entering the cabin of the vehicle.

An engine that is overheating will quickly self-destruct. Proper maintenance of the cooling system is vital to the life of the engine and the trouble-free operation of the cooling system. It is important to have an ASE certified technician perform an inspection of all cooling system components on a yearly basis. During the inspection, the technician should pressure test the radiator cap to ensure the cooling system is operating at the proper pressure level, run your vehicle to operating temperature to verify the engine thermostat is regulating the engine temperature properly, inspect the coolant level and visually inspect for any signs of coolant leaks, test the coolant protection and PH levels to determine if the coolant should be replaced, and visually inspect the cooling system hoses. Always make sure you use the coolant type and mixture recommended by your vehicle's manufacturer.