The Ultimate Guide to Engine Cooling Systems

Heat can be hard on engines, but ultimately, it’s what gets them going. With more power and torque pushed through lower displacements, modern engines need to cope with higher heat levels. Inside cylinder heads, the combustion of air and fuel can reach thousands of degrees and only rises as loads and speeds increase. This can melt surrounding parts within a matter of seconds, if not managed properly.

Optimal working temperatures range between 195 and 220 degrees. This is the balance between metal-melting heat and what’s required to ignite fuel. Maintaining those temperatures is done with a myriad of parts that make up the engine cooling system. Efficiency is key, with parts tasked to remove excess heat before things get out of hand.

The Fundamentals of Engine Cooling

There are two main types of engine cooling systems:

  • air-cooling systems
  • liquid-cooling systems

The first are a rarity these days, and largely assigned to the obsolete section of the automotive market. The reason for this is the issues with overheating, as engines are cooled with airflow alone. Cylinder fins are assigned to dissipate excess heat across a larger area and helped on with fans forcing air in. Engine designs differ too, as each cylinder is placed further apart to increase airflow around all engine parts. The downsides of engines getting really hot really quickly also created boundaries as to how much power could be produced. Popular instances of air-cooled engines are the VW engine in the first Beetle, dating back to 1936, and of course the recognizable fins in any Harley V-twin.

For cars and trucks, things have moved on. Liquid cooling systems are the norm today, simply because they provide better efficiency. This also means engines can put out more power while remaining relatively cool, no matter how hard they work. In this type of system, water or liquid coolant is pushed through sleeves in the engine block and cylinder heads, picking up excess heat, and is then forced back to the radiator to be cooled. From here, the cycle repeats itself, ensuring a constant temperature. The system may be more complex than a simple air-cooled engine, but is a necessity in bigger engines required to generate more power.

The Parts in a Liquid Cooling System


Radiators are at the core of any liquid cooling system since this is where engine heat is dispersed. Radiators consist of an inlet tank that collects returning coolant, a system of multiple rows of narrow tubes interspersed with fins (to increase the cooling surface and aid in heat transfer), and an outlet tank that collects the cooled liquid now ready for another cycle.

Differing placement of inlet and outlet tanks has led to downflow and crossflow radiators. The first are more common in older cars, with tanks set horizontally and with a vertically placed radiator core. The coolant enters the top tank and pressure from the pump and gravity push it down into the outlet tank. Cooling efficiency is more of an issue of overall space, and the change to crossflow radiator tanks has more to do with changes in engine and vehicle designs.

Crossflow radiators have tanks set vertically on either side. Coolant frees itself from heat as it flows horizontally from one tank to the other. With the shift to front-wheel drive and transverse engines, cars have attained a wider footprint, allowing for larger crossflow radiators. As mentioned, more space allows for more efficient cooling.

The coolant expands as it reaches its boiling point of over 270 degrees. This causes an increase in pressure. As the whole system is sealed, excess pressure is released through the radiator cap, and prevents damage to the radiator itself and the connected hoses. After the engine cools, the cap is pulled back by a vacuum and this draws coolant from the coolant reservoir (or overflow) tank, bringing levels back to normal.

Hose Pipes and Water Jackets

Water jackets or cylinder sleeves line the cylinders. Cooled coolant or water arrives at the base of the cylinder, collects excess heat and exits at the top.  Coolant reaches the cylinders by way of rubber hose pipes that form the arteries of the cooling system.

Water Pumps and Fans

Moving coolant to where it does its thing is the task of the water pump. This draws cooled liquid for the radiator and pushes it into the cylinder water jackets. Pumps are often located at the front of the engine and powered by the crank pulley to which they are connected with timing or serpentine belts. There are also auxiliary water pumps that use heated coolant to power aircons and heaters.

Keeping engines cool is better done at highway speeds, but for slow-paced traffic or when the car is parked or at the lights cooling fans bring more forced air into the mix. Most fans today are electrically operated by info relayed to the ECU from coolant temperatures sensors also known as thermo time switches. Fans are turned on and run until coolant temperatures don’t fall below optimal working thresholds of around 200 degrees.


The thermostat is a spring-operated valve, set in its own housing just before the inlet valve in the radiator and the return hose pipes. Coolant that exceeds optimal working temperatures acts on the spring with increased pressure and opens the thermostat to allow the passage of coolant to the radiator. If the coolant and the engine are below this temperature, the thermostat remains closed and the coolant is redirected to the bypass system.

Signs That Things Are Getting Too Hot

You’ll instantly recognize when the cooling system is not functioning as it should. The engine gets hot just a few miles into your journey, and water temperature gauges easily surpass 200 degrees and quickly approach redlines of 270 degrees. Reasons for this may be defective time switches or broken fans, blown head gaskets, breaks or damage in the hose pipes, and leaking or cracked radiators. Faulty thermostats are another common issue, as well as rusted water pumps or loose timing belts.

Before this happens there are telltale signs that something’s wrong. Coolant stains under the vehicle are the first obvious sign, and the gradual loss of coolant means your car heats up quicker than usual and the engine struggles under higher loads. Coolant can leak from anywhere in the system. Also knocking engine sounds and misfires need to be taken seriously. In addition, white smoke from the tailpipes, and sweet smells in the cabin (especially with the A/C on) mean it’s time to visit your mechanic and prevent major engine failure.

What to Do in the Event Your Car Overheats

Obviously, the first thing to do is to find a safe place to pull over. Driving further will cause irreparable damage, with parts melting from excess heat and pipes and hoses bursting. Turn off the engine and wait at least 30 minutes before attempting to open the hood. Rushing into the whole issue can lead to serious burns. With the hood up, check the coolant levels in the overflow tank and if needed add how much is required. If the car doesn’t restart after this there’s the chance that there’s a blown head gasket, cracked cylinder heads, or seized pistons. Time to dig deep in your back pocket.

You can also limit heat buildup by putting less stress on the engine with auxiliary systems like the aircon. Turning it off prevents further heat generation. Instead, you can crank up the heater (hard in searing summer temperatures) but necessary to dissipate the hot air.