Education Station: Cooling

Education Station: Cooling

Welcome to the second installment of Education Station — the cooling system! We’re still trying to think of a better name.


The cooling system on a vehicle, like the name suggests, exists to keep your vehicle within its maximum operating temperature. Your engine is like Goldilocks; if things are too hot or too cold, nothing will work the way it should. It prefers everything just right. Your cooling system transfers the heat created during the engine’s operation through a liquid medium and then transfers it to the air.

There are two main types of cooling systems, air-cooled and liquid-cooled. Since the latter is much more prevalent in modern cars (old Porsches and Volkswagens are two of the more common examples of air-cooled systems), this feature will highlight the liquid-based cooling system. The cooling system relies on several major parts to keep everything working – the coolant, the thermostat, the radiator and pressure cap, the fan and the water pump. Basically, the water pump sends fluid into the engine block, where it returns through the cylinder head, through the thermostat and then to the radiator, where it disperses its heat into the air.

The Coolant

Since the temperatures in and around an engine can vary from below freezing to well over boiling, the fluid that runs through the cooling system must have a high range of tolerance – if it were to freeze, it wouldn’t flow and the car wouldn’t be able to dissipate heat; if it were to boil, it wouldn’t be able to transfer heat from the engine to the air.

While water is a very good way to transfer heat, its boiling and freezing points are too close to one another to work effectively in a combustion engine. So water is typically mixed with ethylene glycol, otherwise known as antifreeze, which can drop the freezing temperature of the mixture by about 60 degree Fahrenheit, well below what most cars experience in a given winter.

The boiling point, on the other hand, doesn’t raise by that much, so pressure must be added to the system. Just like how a pressure cooker operates, the pressure in the cooling system raises the boiling temperature well above what the liquid experiences as it flows through the engine block.

When it comes to the mixture of coolant and water, it’s usually a good rule of thumb to create a 50% water, 50% coolant mixture. Too much coolant, and it won’t be able to disperse heat as well, since the water does that job. Too much water, and you run the risk of freezing the lines and the block when the temperature dips low enough.

The Thermostat

The thermostat’s job is a very important one; it allows the engine to heat up quickly and then maintain an even temperature after that. The way it does this is by regulating the amount of water that flows through the system with an internal valve.

If you’ve never witnessed a thermostat in action, it’s actually pretty cool. Put a thermostat into a pot of boiling water, and you’ll watch its valve open by about an inch. It may look like magic, but it’s really all about wax. Inside the thermostat is a wax cylinder; as the engine heats up, the wax begins to melt and eventually expands as it becomes a liquid, pushing open the thermostat’s valve and allowing coolant to flow through the system unobstructed.

The thermostat’s valve remains closed when the car is still warming up, keeping the coolant within the engine block and cylinder head. This way, it won’t pass through the radiator and cool down when there still needs to be more heat in the system. Once the car is warmed up, however, the thermostat opens and allows coolant to flow to the radiator, cooling it off and keeping the temperature right within that sweet spot.

Here’s a picture of a generic thermostat:


The Radiator and Pressure Cap

The radiator is essentially a heat exchanger, in which heated fluids transfer that heat to a different medium, in this case the air. Modern cars typically use aluminum radiators due to aluminum’s proficiency at exchanging heat. It is constructed with a series of tubes running in parallel from the inlet to the outlet. Those tubes are connected to a large amount of aluminum fins, which pull the heat from the tubes and move it to the air, which is flowing through the radiator.

Radiators also commonly have a tank on each side, inside one of which is a transmission cooler. The hot oil from the transmission transfers the heat to the coolant, which is then dispersed through the radiator.

Earlier, there was mention of how the pressure in the cooling system raises the boiling point. The pressure cap regulates this pressure. It’s the cap on top of your radiator that warns you never to open it when warm; and that’s very good advice. When still hot, the system is under considerable pressure, and opening the cap will release that pressure violently, spewing hot (read: almost boiling) coolant all over you, your engine and anything within a few feet of you.

When the system is hot and under pressure, the cap regulates this by opening an internal valve when the pressure reaches its optimal psi. The valve allows extra coolant to move into an overflow tank (the tank with neon green liquid next to your radiator) without the allowance of outside air into the system. As the pressure decreases, a vacuum is created in the overflow tank, allowing that coolant to re-enter the system.

The following is a picture of a radiator from a Mercedes C63 AMG, and the picture after that is of a radiator overflow reservoir from a Honda Civic Si; the last picture is of a radiator cap from the same Honda:


The Fan

The fan is probably the simplest piece of the cooling system. The fan blows on the radiator, allowing for more air to flow over it. You can typically hear the fan turn on and off when idling at a stoplight or even during normal vehicle operation. It is activated by either a specific switch or by the engine’s computer, allowing the coolant to remain within its peak operating temperature. This type of fan setup is called an electric fan, and is more common on vehicles with front-wheel drive and transversely mounted engines. Rear-wheel drive vehicles use a viscous coupling (like the one found in a car’s differential) attached to the fan hub.

The following is a picture of a radiator fan found in the trunk of a Mercedes SL:


The Water Pump

The water pump is a pretty simple pump. Driven by a belt, the water pump circulates fluid whenever the engine is running. The inlet on the pump is near the center, and the spinning of the pump creates a centrifugal force, pushing the fluid to the outside, where vanes will push the fluid through the outlet and into the engine. The fluid leaving the pump flows first through the engine block and cylinder head, then through the radiator and back to the pump.

The following is a picture of a water pump from a Mercedes SL:


The Heater Core

The heater in your car is operated from this cooling system, as well. When you activate the heat, the hot coolant from the engine flows through a heater core, which looks similar to a radiator core. Air blows across this heater core, transferring the heat into the air and into your vehicle. This is a secondary means of cooling your engine, as well, which is why you should turn on the vehicle’s heater if the engine begins to overheat.

Issues That Can Arise with Cooling Systems


Overheating is a very broadly defined term. If your engine starts operating at temperature higher than it should be, and the temperature gauge rises towards the red, then the engine will begin to behave improperly. Once the heat of the engine surpasses the octane rating of the gasoline, the engine will begin to ping and lose power under load. The fuel will also start detonating early, which can cause hot spots that are able to melt right through pistons. To add more problems into the mix, head gaskets (gaskets between the cylinder head and the block) will crack and break, and the coolant hoses and radiator will occasionally leak or burst entirely under such intense heat and pressure. Overheating is not necessarily an issue by itself, but rather a symptom of a problem within one of the many pieces of the cooling system.

There are many causes for overheating, but the most common ones are a broken/stuck thermostat, air in the system (leading to prevention of coolant flow), a low coolant level, a leak somewhere in the system, a broken/inoperative fan or fan switch, a poorly operating water pump, a collapsed lower radiator hose, or even a defective radiator cap. Any of these problems can cause the coolant flow to cease within the system, causing the transfer of heat to become relatively ineffective.

Basically, any problem with the cooling system will create a lack of proper heat transfer, allowing the heat inside the engine to rise and create an overheating situation. Any time a car overheats, the immediate reason that comes to mind should involve the cooling system. From there, it is a matter of testing the individual parts to determine the proper course of action.

-Coolant Leaks

Leaking coolant is pretty easy to pick out, as its properties are unlike many of the other fluids that can possibly leak from a vehicle. Just as oil is dark, viscous and has a distinct smell, so too does coolant have its own unique properties. When a puddle of coolant appears underneath the car, it can be easily noticed by its neon-green color and distinct fishlike scent. If your garage starts smelling like a fishmonger, your best bet is to check your coolant level in the overflow tank. If it’s low, and you’ve got puddles under the car, something is leaking coolant. This can lead to overheating and all the lovely problems described above.

Coolant leaks without any visible puddles or spills can sometimes be attributed to leaky head gaskets, which then allow coolant to seep through into the crankcase and cylinders. This can be deduced by noticing a whitish smoke coming from the exhaust, especially after restarting the engine once the vehicle has been sitting for a while.

How to Address Some of These Issues

-The Problem of the Weakest Link

In many occurrences, customers will come in for one part of the cooling system to be fixed. For example, let’s use a water pump. If the water pump was old and damaged and succumbing to the pressures of the cooling system, then the whole cooling system was rendered inoperable. A cooling system is only as good as its weakest link. If the water pump is replaced, the stress can then transfer to the next weakest part, like the thermostat or the hosing. It’s not terribly uncommon for a customer to return months, weeks or even days after the fix, only to suffer more problems with the cooling system. If you can afford it at the time, replacing multiple pieces of the cooling system tends to best solve the problem. If your hoses are starting to look ratty, and all you replace is the thermostat, odds are you’ll end up with leaks somewhere in your system sometime soon. A thorough look at your cooling system by a trusted mechanic will give you a list of issues that can (and should) be remedied, especially if you daily drive your car or subject it to heavy loads (towing, mountain driving, etc.).

-European Cars and Their Issues

Cars whose makers originate in European countries (Mercedes, BMW, Audi, etc.) are set up a bit differently. As the European winters are decidedly less harsh than American winters (and a lot less harsh than Chicago winters), and since the European summers aren’t as hot as American ones, their cars are not built to deal with such a massive fluctuation in temperature. European car manufacturers typically build parts of the cooling system from a plastic and fiber polymer resin, which is subject to cracking much quicker than other cars from other manufacturers. Typically, one would be lucky to get 70,000-100,000 miles out of many parts within the cooling system on a European vehicle. This isn’t so much a warning as it is a general notion for European car owners to consider when their car’s mileage gets to that range.

-Coolant Flushing

Eventually, the coolant will run its course and begin to lose its ability to dissipate heat effectively. At this point, it is in the owner’s best interest to flush the cooling system and replace the coolant with a new mixture. This is recommended every two to four years, but two years is optimal if you are aiming for longevity with your vehicle.

-Most Other Issues

In most situations, the only course of action to fix a problematic piece of the cooling system is to replace it. Cracked hoses cannot be repaired completely, and to attempt to fix a thermostat is pointless, as it’s a relatively cheap part.

If the problem with the system can be linked to air in the system, the fix should be as easy as opening a bleeder valve or loosening a heater hose.

In specific regard to thermostats, there are many on the market that come with different temperature ratings (the temperature at which the wax will melt and the valve will open). Putting a thermostat with a different rating in your car might cause the temperature to get either higher or lower than the suggested operating temperature for the engine. While some racing thermostats allow the valve to open earlier, this is only really useful in cars that are driven on the track and are therefore subject to heavy loads and higher overall engine temperatures.*

Radiators are subject to the same replacement procedures. If your car is under heavy loads more often than a typical daily driver would be, it might be in your best interest to consider a larger radiator to help disperse the heat faster and more thoroughly. Aftermarket fan switches can also aid this issue; by causing the fan to start at a lower engine temperature, it will keep the temperatures a bit lower under heavy load.* Sometimes, though, something as simple as bug and dirt accumulation can cause a radiator to perform poorly; in that situation, it is best to remove the radiator and give it a thorough cleaning to see if that eradicates the issue.

Since the water pump is pulley-and-belt-driven, belts can wear and should be replaced as well. An entire system overheating is sometimes easily fixed with a belt replacement or a pulley readjustment or replacement (if an uneven pulley causes unnecessary wear on the belt).

Replacing the head gasket (or temporarily sealing it with an added sealer product) is paramount to the operation of your vehicle, if the problem comes from the head gasket itself. It may be a bit more expensive than the other procedures in this section, but it keeps your engine in operating condition. If coolant were to leak through the head gasket and foul up the oil in the crankcase, bad things can and will happen to your engine.

* In regards to heat and the engine, it’s a tricky mixture. You want your engine to run hot to better atomize the fuel and maximize efficiency, but if you go over a certain edge, you’ll start detonating and ruining your engine. In race applications, it’s best to install cooling system parts that create a lower operating temperature, so you can run with more power and end up at that fine line between hot and too hot. If you were to run a race application without aftermarket cooling parts, the higher power would overheat the engine easily. Engines run their best when they’re at that point just between hot and too hot.

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