Which Way Does Coolant Flow Through An Engine? Coolant Pathway!

It may not seem important but it does matter to understand the cooling system and its correct pathway throughout your car’s engine bay. Considering that coolant takes care of the overheating problem, this is one of the most important factors to take into account for maintaining the health of your engine.

Knowing about the flowchart for the coolant to and from the engine helps you diagnose a lot of issues and keep the engine safe. Let’s hop in and understand all about the engine cooling system.

Brief Overview of Engine Coolant

First, engine coolant is also known as antifreeze. Most people think coolant only helps in keeping the engine safe from overheating. However, it also works the other way. The coolant or antifreeze in your car keeps the engine from freezing at lower temperatures.

So, what is a coolant or antifreeze? It is an equivalent mixture of water and a chemical compound named ethylene glycol. Sometimes, less toxic propylene glycol is also used to reduce the effect of toxicity on humans and the environment. 

What Makes Engine Coolant Special?

One of the special properties of these chemicals’ interaction with water is that they are not just soluble but completely miscible with water in any ratio. 

Therefore, the effectiveness of the coolant is determined for various parts of the world by varying the mixture ratio.

It absorbs heat from the surrounding and can also dissipate the heat to become cool when needed. 

Another important feature of the engine coolant is that not many physical parameters change with the change in the internal temperature of the coolant except for over-pressurizing.

In earlier times, coolant from the system simply used to leak to the ground or inside the engine head through gaskets. 

But, most modern cooling systems have an additional reservoir where the overflooded hot coolant is stored and resupplied to the system when the temperature falls.

Read Also: Can Low Coolant Cause Car to Shake? (Identify The Reason)

Understanding the Engine Cooling System

You got to know about the coolant, now it’s time for understanding the entire engine cooling system. Gasoline power vehicles use internal combustion engines which get extremely hot because of the burning air-fuel mixture.

Now, extreme temperature due to the heat generated inside the engine block is not good for the piston, cylinder, and other components. Overheating can actively hurt the engine and damage them permanently.

Again, extremely cold temperatures are also not good for the engine and its effectiveness. Fuel economy and engine performance reduce rapidly due to lower temperatures.

Therefore, engineers discovered a system to maintain an optimum temperature in the engine. This is known as the engine cooling system. This system is composed of some major components which are stated below.


For the engine, the radiator serves as a heat transfer device. It typically consists of multiple small-diameter hoses with fins attached.  These are made of aluminum. 

Its job is to transfer heat from the hot coolant coming from the engine to the surrounding air. Besides, it has an exit port, an intake port, a sealed lid, and a drainage plug.

Water Pump

You may have seen a water pump in your house to pull out groundwater. An engine cooling system’s water pump acts in the same way. It is driven by a pulley connected to the engine’s crankshaft using a belt. 

As the engine runs, the water pump uses pulling force to suck up coolant from the engine’s lower radiator hose and pushes it through the engine components such as the engine block and cylinder head.

The water pump contains a centrifugal impeller, which spins rapidly and creates a suction force that draws coolant into the pump’s inlet port. These blades push the coolant through the pump’s outlet hose and into the engine block.

The impeller blades are engineered in a way to create a steady flow of coolant, even at low speeds. 

Thus, it ensures the engine is cooled properly. Hence, the water pump is one of the most crucial components in your engine cooling system.


The thermostat has two main jobs to do and is entirely dependent upon the temperature sensor. You’ll find it located between the engine and the radiator, usually in a housing connecting to the engine block. 

It contains a temperature-sensitive valve that opens or shuts depending upon coolant temperature. When the engine is cold, the thermostat is closed, blocking the flow of coolant to the radiator and allowing the engine to warm up quickly. 

This situation mostly happens in cold regions where the engine must be warmed up. As the engine warms up, the thermostat valve begins to open, allowing coolant to flow through the engine to and from the radiator.

Once the coolant reaches a certain temperature, the thermostat valve is fully open. Hence, it allows maximum coolant flow to the radiator to ensure an optimum engine temperature.

Read Also: Low Coolant Light Comes On and Off – Reasons and Solutions

Different TyHoses

What do you need to circulate the coolant to and from the engine or radiators? It would be the rubber hoses. There are enormous hoses all over the engine bay that might confuse you. But each of these has a specific job to do.

Upper & Lower Hoses

Two major hoses in the system are the upper and lower hoses. You’ll notice a wide hose attached to the engine block near the top of the radiator known as the upper hose. Again the lower hose is connected at the bottom of the radiator.

Coolant flows from the engine into the lower hose and gets transported to the radiator. After being cooled, the upper hose carries the coolant back to the engine.

These hoses are mostly made of reinforced rubber or other polymeric materials that can withstand the high temperatures and pressure of the entire cooling system.

Bypass Hose

It’s a comparatively small hose creating a bridge from the lower hose back to the engine. This hose only works when the thermostat is closed.

When the engine temperature is not high enough, coolant recirculates through the engine using the bypass hose. As a result, the engine gets enough time to warm up and the thermostat valve opens to reactivate the cooling system.

Electric Cooling Fan

You can see these fans behind the radiator grill. These are controlled by a temperature sensor and the car’s computer. When the engine detects a specific temperature, the computer (ECM) sends a signal to the fan relay activating the fan.

The fan pulls air through the radiator fins and spreads it to the surrounding environment. As a result, it helps to dissipate heat and cool the coolant as it flows through the radiator.

Coolant Expansion Tank

It’s a coolant container below the radiator also known as a “radiator overflow reservoir”. The expansion tank is connected to the radiator by a hose.

As the engine heats up, the coolant expands and gets overpressurized. Hence, it flows out of the radiator and goes to the overflow radiator reservoir to minimize the threat of overheating or any further damage.

With time when the engine cools down, the coolant contracts and flows back into the radiator from the coolant expansion tank. It helps to maintain a stable and consistent coolant level in the cooling system.

In some vehicles, the overflow radiator reservoir also serves as a place to add coolant to the system when needed.

Read Also: Can I Mix 50/50 Coolant With Concentrate

Coolant Flowing Pathway Through An Engine

Now that we know about the internal components and their mechanism principle, it’s time to assemble them and get a complete view of the cooling system.

Starting Point: Engine Block

When internal combustion takes place, a lot of heat is generated along with the movement of crankshafts attached to the engine. 

This crankshaft activates the water pump, which pulls the coolant from the radiator and passes it through the engine head and cylinders.

The coolant flowing through the chambers of an engine block receives heat and its temperature increases. 

Hence, the thermostat becomes activated and its valve slowly opens to pass out the heated coolant.

Ending Point: To The Radiator

The water pump thrusts out the heated coolant from the engine toward the radiator using the lower hose. The radiator dissipates heat from the coolant through a process called convection.

Hot coolant enters the radiator from the engine and passes through lots of small tubes in series called “cores.” 

As the coolant flows through the cores, it contacts the metal walls of the tubes. This allows heat to shift from the hot coolant to the cooler metal of the tubes.

Heats from the core are dissipated to the surrounding environment in a natural way or sometimes using electrical fans. As a result, the coolant comes to its original reduced temperature and is ready for the next cycle.

In some vehicles, coolant goes to the heater core to transfer heat inside the cabin. Afterward, it returns to the radiator to start the loop again.

Read Also: Accidentally Put Concentrated Coolant

Factors Affecting Coolant Flowing Efficiency

As the engine cooling system is composed of lots of components, if something goes wrong the whole system fails to lead to overheating. Here are common factors that can result in a failing cooling system and must be treated immediately.

Coolant Volume And Quality

The coolant level in your car must remain at the recommended level for a stable flow and heat transfer. Low coolant levels can cause air bubbles to form, which can reduce the effectiveness of the system.

Moreover, the concentration of the coolant (mixture of water and antifreeze) must be maintained in a proper ratio. The concentration is too low, reducing the ability to absorb heat, and thus, the engine may overheat.

Faulty Water Pump

A damaged or faulty water pump fails to circulate the coolant in the given amount of time. As a result, the engine begins to overheat and the coolant flowing speed gets reduced.

Filthy And Clogged Radiator

Different driving conditions may cause dirt and debris to accumulate on the radiator. Hence, the radiator core becomes clogged and doesn’t allow heat to dissipate in the environment. Besides, the flow of coolant gets hampered as the mini tubes become blocked by debris.

Leakage And Blockage In Hoses

Hoses are the main paths through which the coolant circulates throughout the cooling system. 

A faulty hose or blocked hose naturally reduces the flow time and volume of the coolant. Besides, leakages cause air bubbles or a reduction of the coolant in the expansion tank.

Faulty Thermostat

A damaged thermostat can cause the valve to not shut or open in the required time. If the temperature sensor is broken, the valve may stay shut causing the coolant to circulate around the engine only. 

As a result, the engine would get overheated in no time which may result in massive internal damage.

How To Ensure Steady Coolant Flow

For your car engine, it is very crucial to keep the engine operating temperature optimum. That is why you must keep track of the cooling system and especially ensure the flow of coolant is uninterrupted.

  • Keep the radiator clean dirt free
  • Top up recommended coolant regularly
  • Keep an eye on the water pump and thermostat operation quality
  • Flush the system on a regular basis
  • Check if the fans are working nominally

Read Also: Can You Mix Red and Green Coolant? (Know Before Mixing)


These FAQs from users and experts regarding the coolant flow would be helpful for you to know even more.

How to tell if coolant is flowing smoothly in the cooling system?

Check the temperature gauge in the instrumental panel, and the coolant level in the radiator, and observe the flow of coolant to ensure proper coolant flow. Moreover, you can use an infrared thermometer to check for consistent temperatures.

When should you change the engine coolant?

Over time the rust inhibitor and other additives of the antifreeze lose their effectiveness. Besides, the coolant level reduces as well to some extent. Therefore to avoid engine overheating you must change the coolant after driving 30000-35000 miles.

Does coolant run through the intake manifold?

Coolant does not run through the intake manifold directly. But in some cases, it may flow through passages located within the intake manifold to help regulate the temperature of the engine bay.

How does coolant run out?

Coolant volume in the engine cooling system can reduce with time because of leaks, evaporation, or normal consumption. It’s essential to monitor the coolant level and top-up as needed to maintain a proper cooling system and avoid engine overheating.

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