Components of Cooling System

Pressure cap:

The radiator cap actually increases the boiling point of your coolant by about 45 F (25 C). How does this simple cap do this? The same way a pressure cooker increases the boiling temperature of the water.

The cap is actually a pressure release valve, and on the car is usually set at 15 psi. The boiling point of water increases when the water is placed under pressure. When the fluid in the cooling system heats up, it expands, causing the pressure to build up. The cap is the only place where this pressure can escape so the setting of the spring defines the pressure in the cooling system.

When the pressure reaches 15 psi, the pressure pushes the valve opens, allowing coolant to escape from the cooling system. This coolant flows through the overflow tube into the bottom of the overflow tank. This arrangement keeps air out of the system.

When the radiator cools back down, a vacuum is created in the cooling system that pulls open another spring-loaded valve, sucking water back in from the bottom of the overflow tank to replace the water that was expelled.

In order to prevent the coolant from boiling, the cooling system is designed to be pressurized. Under pressure, the boiling point of the coolant is raised considerably. However, too much pressure will cause hoses and other parts to burst, 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 cap. It is designed to release pressure if it reaches the specified upper limit that the system was designed to handle. Prior to the ’70s, the cap would release this extra pressure to the pavement. Since then, a system was added to capture any released fluid and store it temporarily in a reserve tank. This fluid would then return to the cooling system after the engine cooled down.

Expansion Tank

As coolant gets hot, it expands. Since the cooling system is sealed, this expansion causes an increase in pressure in the cooling system, which is normal and part of the design. When coolant is under pressure, the temperature where the liquid begins to boil is considerably higher. This pressure, coupled with the higher boiling point of ethylene glycol, allows the coolant to safely reach temperatures in excess of 250 degrees.

The radiator pressure cap is a simple device that will maintain pressure in the cooling system up to a certain point. If the pressure builds up higher than the set pressure point, there is a spring-loaded valve, calibrated to the correct pounds per square inch (psi), to release the pressure.

When the cooling system pressure reaches the point where the cap needs to release this excess pressure, a small amount of coolant is bled off. It could happen during stop-and-go traffic on an extremely hot day, or if the cooling system is malfunctioning. If it does release pressure under these conditions, there is a system in place to capture the released coolant and store it in a plastic tank that is usually not pressurized.

Since there is now less coolant in the system, as the engine cools down, a partial vacuum is formed. The radiator cap on these closed systems has a secondary valve to allow the vacuum in the cooling system to draw the coolant back into the radiator from the reserve tank (like pulling the plunger back on a hypodermic needle).

There are usually markings on the side of the plastic tank marked Full-Cold and Full-Hot. When the engine is at normal operating temperature, the coolant in the translucent reserve tank should be up to the Full-Hot line. After the engine has been sitting for several hours and is cool to the touch, the coolant should be at the Full-Cold line.

The Pump

The pump is a simple centrifugal pump driven by a belt connected to the crankshaft of the engine. The pump circulates fluid whenever the engine is running. The water pump uses centrifugal force to send fluid to the outside while it spins, causing fluid to be drawn from the centre continuously. The inlet to the pump is located near the centre so that fluid returning from the radiator hits the pump vanes. The pump vanes fling the fluid to the outside of the pump, where it can enter the engine. The fluid leaving the pump flows first through the engine block and cylinder head, then into the radiator and finally back to the pump.

A water pump is a simple device that will keep the coolant moving as long as the engine is running. It is usually mounted on the front of the engine and turns whenever the engine is running. The water pump is driven by the engine through one of the following:

  • A fan belt that will also be responsible for driving an additional component like an alternator or power steering pump
  • A serpentine belt, which also drives the alternator, power steering pump, and AC compressor among other things
  • The timing belt, which is also responsible for driving one or more camshafts

The water pump is made up of a housing, usually made of cast iron or cast aluminium and an impeller mounted on a spinning shaft with a pulley attached to the shaft on the outside of the pump body.

A seal keeps fluid from leaking out of the pump housing past the spinning shaft. The impeller uses centrifugal force to draw the coolant in from the lower radiator hose and send it under pressure into the engine block. There is a gasket to seal the water pump to the engine block and prevent the flowing coolant from leaking out where the pump is attached to the block.

Radiator Fan

A radiator fan is used to draw the air towards the radiator and help in the cooling process. The radiator fan has four or more blades that spin rapidly to provide sufficient air that would cool the engine. It is usually mounted between the radiator and the engine so that the air can easily get to the radiator. Some cars have an additional fan in front of the radiator in order to draw more cool air into the engine. Especially when it is so hot and the vehicle isn’t moving fast enough, very little cool air reaches the radiator, and thus, the engine is not cooled properly.

Mounted on the back of the radiator, on the side closest to the engine, is one or two electric fans inside a housing that is designed to protect fingers and to direct airflow. These fans are there to keep the airflow going through the radiator while the vehicle is slowing down or when it’s stopped with the engine running. If these fans stopped working, every time you came to a stop, the engine temperature would begin rising.

On older systems, the fan was connected to the front of the water pump and would spin whenever the engine was running because it was driven by a fan belt instead of an electric motor. In these cases, if a driver would notice the engine begin to run hot in stop and go driving, the driver might put the car in neutral and rev the engine to turn the fan faster which helped cool the engine. Racing the engine on a car with a malfunctioning electric fan would only make things worse because you are producing more heat in the radiator with no fan to cool it off.

The electric fans are controlled by the vehicle’s computer. A temperature sensor monitors engine temperature and sends this information to the computer. The latter determines if the fan should be turned on and actuates the fan relay if additional airflow through the radiator is necessary.

If the car has air conditioning, there is an additional radiator mounted in front of the normal radiator. This “radiator” is called the air conditioner condenser, which also needs to be cooled by the airflow entering the engine compartment. You can find out more about the air conditioning condenser by going to our article on Automotive Air Conditioning.

As long as the air conditioning is turned on, the system will keep the fan running, even if the engine is not running hot. This is because if there is no airflow through the air conditioning condenser, the air conditioner will not be able to cool the air entering the interior.

Having a bad condenser will usually result in your air conditioning not being as cold as it should be, or it won’t even work at all.


The cooling system has a lot of plumbing. We’ll start at the pump and work our way through the system. The pump sends the fluid into the engine block, where it makes its way through passages in the engine around the cylinders. Then it returns through the cylinder head of the engine. The thermostat is located where the fluid leaves the engine. The plumbing around the thermostat sends the fluid back to the pump directly if the thermostat is closed. If it is open, the fluid goes through the radiator first and then back to the pump. There is also a separate circuit for the heating system. This circuit takes fluid from the cylinder head and passes it through a heater core and then back to the pump.

There are several rubber hoses that make up the plumbing to connect the components of the cooling system. The main hoses are called the upper and lower radiator hoses. These two hoses are approximately two inches in diameter and direct coolant between the engine and the radiator. Two additional hoses, called heater hoses, supply hot coolant from the engine to the heater core. These hoses are approximately one inch in diameter.

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 max-cool.

A fifth 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 hose. Instead, they might use a metal tube or have a built-in passage in the front housing.

These hoses are designed to withstand the pressure inside the cooling system. Because of this, they are subject to wear and tear and eventually may require replacing as part of routine maintenance. If the rubber is beginning to look dry and cracked or becomes soft and spongy, or you notice some ballooning at the ends, it is time to replace them.

The main radiator hoses are usually moulded to a shape that is designed to route the hose around obstacles without kinking. When purchasing replacements, make sure that they are designed to fit the vehicle.

There is a small rubber hose that runs from the radiator neck to the reserve bottle. This allows coolant that is released by the pressure cap to be sent to the reserve tank. This rubber hose is about a quarter-inch in diameter and is normally not part of the pressurized system. Once the engine is cool, the coolant is drawn back to the radiator by the same hose.


Cars operate in a wide variety of temperatures, from well below freezing to well over 100 F (38 C). So whatever fluid is used to cool the engine has to have a very low freezing point, a high boiling point, and it has to have the capacity to hold a lot of heat. Water is one of the most effective fluids for holding heat, but water freezes at too high a temperature to be used in car engines.

The fluid that most cars use is a mixture of water and ethylene glycol (C2H6O2), also known as antifreeze. By adding ethylene glycol to water, the boiling and freezing points are improved significantly. The temperature of the coolant can sometimes reach 250 to 275 F (121 to 135 C). Even with ethylene glycol added, these temperatures would boil the coolant, so something additional must be done to raise its boiling point.

The cooling system uses pressure to further raise the boiling point of the coolant. Just as the boiling temperature of the water is higher in a pressure cooker, the boiling temperature of the coolant is higher if you pressurize the system. Most cars have a pressure limit of 14 to 15 pounds per square inch (psi), which raises the boiling point another 45 F (25 C) so the coolant can withstand the high temperatures.


A thermostat allows the engine to heat up quickly, and then to keeps the engine at a constant temperature. It does this by regulating the amount of water that goes through the radiator. At low temperatures, the outlet to the radiator is completely blocked — all of the coolants is recirculated back through the engine. Once the temperature of the coolant rises to 180 and 195 F (82 – 91 C), the thermostat starts to open, allowing fluid to flow through the radiator. By the time the coolant reaches 200 to 218 F (93 – 103 C), the thermostat is open all the way.

A thermostat is placed between the engine and the radiator to make sure that the coolant stays above a certain preset temperature. 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 it reaches the design temperature, at which point, the thermostat will open a valve and allow the coolant back through the radiator.

The thermostat is simply a valve that measures the temperature of the coolant and, if it is hot enough, opens to allow the coolant to flow through the radiator. If the coolant is not hot enough, the flow to the radiator is blocked and fluid is directed to a bypass system that allows the coolant to return to the engine. This bypass system allows the coolant to keep moving through the engine to balance the temperature and avoid hot spots. Because flow to the radiator is blocked, the engine will reach operating temperature sooner and, on a cold day, will allow the heater to begin supplying hot air to the interior more quickly.

Since the ’70s, thermostats have been calibrated to keep the temperature of the coolant above 192 to 195 degrees. Prior to that, 180-degree thermostats were the norm. It was found that if the engine is allowed to run at these hotter temperatures, emissions are reduced, moisture condensation inside the engine is quickly burned off, which extends engine life, and combustion is more complete, which improves fuel economy.

The heart of a thermostat is a sealed copper cup that contains wax and a metal pellet. As the thermostat heats up, the hot wax expands, pushing a piston against spring pressure to open the valve and allow coolant to circulate.

The thermostat is usually located in the front, top part of the engine in a water outlet housing that also serves as the connection point for the upper radiator hose. Its housing attaches to the engine, usually with two bolts and a gasket to seal it against leaks. This gasket is usually made of a heavy paper or uses a rubber O-ring. In some applications, there is no gasket or rubber seal. Instead, a thin bead of special silicone sealer is squeezed from a tube to form a seal.

There is a mistaken belief by some people that if they remove the thermostat, they will be able to solve hard to find overheating problems—but this couldn’t be further from the truth. Removing the thermostat will allow uncontrolled circulation of the coolant throughout the system. It is possible for the coolant to move so fast that it will not be properly cooled as it races through the radiator, so the engine can run even hotter than before under certain conditions.

Other times, the engine will never reach its operating temperature. On computer-controlled vehicles, the computer monitors engine temperatures and regulates fuel usage based on that temperature. If the engine never reaches operating temperatures, fuel economy and performance will suffer considerably.

Antifreeze Mixture

In western countries, if the water used in the radiator freezes because of cold climates, then ice formed has more volume and produces cracks in the cylinder blocks, pipes, and radiator. So, to prevent freezing antifreeze mixtures or solutions are added in the cooling water. The ideal antifreeze solutions should have the following properties :

  • It should dissolve in water easily.
  • It should not evaporate.
  • It should not deposit any foreign matter in the cooling system.
  • It should not have any harmful effect on any part of the cooling system.
  • It should be cheap and easily available.
  • It should not corrode the system.

No single antifreeze satisfies all the requirements. Normally following are used as antifreeze solutions :

  • Methyl, ethyl and isopropyl alcohols.
  • A solution of alcohol and water.
  • Ethylene Glycol.
  • A solution of water and Ethylene Glycol.
  • Glycerin along with water, etc.

The coolant that courses through the engine and associated plumbing must be able to withstand temperatures well below zero without freezing. It must also be able to handle engine temperatures in excess of 250 degrees without boiling. It’s a tall order for any fluid, but that’s not all—the fluid must also contain rust inhibitors and a lubricant.

The coolant in today’s vehicles is a mixture of ethylene glycol (antifreeze) and water, with a recommended ratio of 1:1. In other words, one part antifreeze and one part water. This is the minimum recommended for use in automobile engines. Less antifreeze and the boiling point would be too low. In certain climates where the temperatures can go well below zero, it is permissible to have as much as 75% antifreeze and 25% water, but no more than that. Pure antifreeze will not work properly and can cause a boil-over.

Antifreeze is poisonous and should be kept away from people and animals, especially dogs and cats, who are attracted by the sweet taste. Ethylene glycol, if ingested, will form calcium oxalate crystals in the kidneys, which can cause acute renal failure and death.

Radiator Material – copper vs Aluminium

 When you’re planning out the stages of your car buildup, the cooling system probably isn’t a particularly thrilling part of the investment. So if you’re in the market for a new radiator, you’ve probably noticed that there are a myriad of models available to fit your car. Will a two-row aluminium work as well as a four-core copper-brass radiator? Which material cools better, and why? We spoke with the folks at Griffin Radiators, who deconstructed the marketing hype and broke down a little cooling science for your benefit (as well as ours).


Just like your engine, a radiator needs air to function. It’s a water-to-air heat exchanger, so it requires air to flow past a sufficiently large network of tubes that contain flowing engine coolant. The tubes make contact with thin metal fins to further increase the surface area available for cooling. Of course, more surface area means that more of the coolant’s heat can be dissipated. The ideal radiator, then, would be built from highly conductive metal with large diameter tubes and maximum tube-to-fin contact, and it would be able to pass air efficiently with minimal restriction. Copper-brass conducts heat considerably better than aluminium does. Bigger tubes and more fins increase the surface area. So why don’t we build a five-core copper-brass radiator with huge tubes and a bunch of cooling fins? The limitations are material strength, weight, and airflow.

Copper-brass alloy isn’t as strong as aluminium, so its tubes are more susceptible to blowing out under even the relatively mild pressure generated by a cooling system. Building a copper-brass radiator with a larger, more efficient 1-inch tube diameter requires thickening the tube wall to 0.015 inch-twice as thick as is necessary on a 51/48-inch-diameter tube. That means the larger tubes weigh over three times as much as the smaller tubes-not good! The compromise comes from building the tubes out of aluminium. An aluminium radiator using 1-inch-wide tubes with 0.016-inch wall thickness is 60 per cent lighter than the same copper-brass radiator. The 1-inch-wide tubes increase tube-to-fin contact and cooling capacity by roughly 25 per cent over a radiator built with 11/42-inch tubes. The net result? Griffin claims that a two-row aluminium radiator with 1-inch tubes will cool as well as a five-row copper-brass radiator with 11/42-inch tubes. That frees up some extra room under the hood, and the two-row design allows less restricted airflow through the core. More air equals more cooling.

Sure, the theory works, but is it enough to justify shelving your stock copper-brass radiator for a slick, shiny aluminium piece? We’ve certainly been able to cool our big-blocks just fine using a stock-appearing four-core copper-brass radiator-like U.S. Radiator’s Desert Cooler. In fact, our U.S. Radiator-cooled 455ci Pontiac has yet to eclipse 200 degrees F. Would the Pontiac run any cooler with a trick aluminium radiator? Griffin feels that a well-designed aluminium radiator cools better. These days, aluminium radiators are the trend in the aftermarket as well as in OEM production. But it’s hard to say how much better an aluminium radiator will cool a unique car. Griffin explained that aluminium radiators do have more distinct advantages in racing, where damage resistance and ultra-high-pressure cooling systems are commonplace. They can handle a 30-psi pressurized cooling system, and a special high-temperature epoxy reinforcement process provides additional strength to the welded tubes. That’s a bit much to ask from a soldered copper-brass radiator.

Custom-fit aluminium radiators are still pretty steeply-priced, but universal-fit aluminium racing radiators are very competitive with copper-brass replacements. Griffin’s racing radiators are MIG-welded, and while they don’t look as nice as the company’s TIG’d custom-fit radiators, they should function just as well provided you’re up to a little fabrication work to install them. The real penalty is a paltry 30-day warranty, compared to the two-year guarantee on all Griffin’s custom-fit radiators. Provided there are good engine-to-frame grounds to prevent electrolysis, and assuming you change your coolant every year, Griffin says either model should keep you cool for years.


So what should you take from this? If you’re beating the living daylights out of your car, whether it’s in the form of a 140-mph blast on the Silver State Challenge or a 7,000-rpm downshift before turn five of Elkhart Lake Raceway, you may need to exploit the high-coolant-pressure handling and vibration-fatigue resistance of a stout aluminium radiator. But whether you like the high-tech look of aluminium or the resto/sleeper stealth of copper-brass, either (when properly selected) should be able to cool your street car, provided it’s coupled with a good fan and shroud.


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