Problems with a Car's Cooling System

Radiator

Blocked Radiator
Debris in the cooling channels. Sealant from engine rebuilds or gaskets from the water pump itself can be a cause of this.

Leaking Radiator
A leaking water radiator/cooling system will cause the level to drop and the coolant to boil more easily.

Too Small Radiator
If your radiator is too small, the water temperature will rise. You may only notice this during hard driving/high load when more heat is generated, causing the water to become hotter and require more cooling.

Airflow Not Passing Through the Radiator
Air takes the easiest path. If the air is not directed through the radiator's cooling fins, there will be no heat exchange, resulting in reduced or absent cooling. This leads to increased water temperature.

Damaged Cooling Fins
This is one of many causes that can lead to high water temperature. Damaged cooling fins produce the same result as airflow not being able to pass through the radiator.

Water Pump

Damaged Impeller
Debris in the cooling system is especially common during rebuilds or repairs. Always make sure to keep it clean during service. The debris can easily damage the water pump's impeller. Do not use sealant if the water pump has its own gasket made of paper, metal, or O-ring (unless this is explicitly recommended by the manufacturer).

Incorrect Coolant
This is not as common today, but incorrect coolant/glycol/additives can damage the water pump's seal as well as other seals in the cooling system.

Mechanical Water Pump
Damaged Impeller / Vane Wheel
A damaged impeller directly affects the flow and thus the cooling capacity negatively. There is a high risk of overheating. If you have a damaged vane wheel, it is most likely due to debris in the system, but it can also be due to carelessness during the installation of the water pump. Where the damage is located on an impeller often indicates what the cause is.

Damaged Gasket
A damaged gasket can lead to leakage blocking cooling channels, but it can also indicate a damaged impeller on the water pump.

Worn Bearing
When the bearing in the water pump is worn, it will eventually start to make noise. A bad bearing is first noticed when the water pump's pulley wobbles or when the water level drops due to leakage through the weep hole.

Weep Hole
A mechanical water pump often has a weep hole. This hole is located at the bottom of the water pump between the innermost seal and the water pump bearing. This is to prevent water from reaching the bearing and prematurely wearing it out, as water would quickly wash away the bearing's grease, leading to failure.

Excessive Sealant
Sealant can end up in the cooling system as debris and can block cooling elements.

Electric Water Pump
No flow / Poor flow
If flow in an electric water pump is not achieved despite the pump spinning, it is due to air in the system. Air can easily get trapped in a water pump, and an air pocket can be difficult to pump out as an electric water pump typically does not flow as much as a mechanical one at low RPM. The solution may be to change the position of the water pump entirely or just temporarily during installation/bleeding of the system.

Air in the System
Air in the system can occur just as easily whether you have a mechanical or electric water pump. An electric water pump is not as sensitive to air pockets.

Thermostat

Thermostat stuck in the open position / no thermostat installed.
If the engine struggles to reach operating temperature, it is often due to the thermostat being stuck open (or no thermostat being installed) and continuously allowing coolant to flow through the radiator. This means the engine does not reach temperature quickly or at all.

Thermostat does not open / does not open fully
Debris can easily clog the thermostat, but it can also seize for other reasons. When this happens, the engine can overheat as no coolant is flowing through the radiator.

Check the thermostat
Check its function by comparing the temperature of the upper and lower radiator hoses as the engine warms up. One radiator hose should heat up in line with the engine, while the other should remain cooler. When the operating temperature is reached, the thermostat will open and allow coolant to flow through the radiator, equalizing the temperature of the two hoses.

Incorrect thermostat temperature / incorrect type
An incorrect opening temperature can mean the engine runs too hot or too cold. In racing, you often have a thermostat that opens at a lower temperature to cool the engine sooner and avoid overheating.

Irregular Function
An air pocket can become trapped at the thermostat housing and cause various symptoms. This is especially true if you have a system with lower flow, such as when you have installed an electric water pump, or if you have a system with a low-mounted expansion tank.

Expansion Tank / Boiling System

The tank is mounted too low
The expansion tank should be the highest point in a cooling system. With a tank positioned too low, air pockets can become trapped in other areas of the cooling system, leading to localized boiling and overheating. Ensure that your expansion tank is mounted as the highest point. Ideally, the lower connection of the tank should be mounted higher than other points in the cooling system.

Leakage
Leakage from the expansion tank presents classic symptoms such as decreasing coolant levels and boiling. However, the leak is easy to locate as an expansion tank is often readily accessible.

Too Low Level
Air pockets are automatically created when the level in the cooling system is too low. This causes the engine to run hotter than it should, increasing the risk of boiling. A low level can indicate that there is a leak in the cooling system.

Radiator Cap

Spring in the Cap Sticking
If the spring in the radiator cap is sticking, it may mean that a boiling system does not release pressure as it should, and in the worst case, a connection elsewhere may burst. Most likely, the cap/spring will only open at a higher pressure than it was originally designed for.

Incorrect Pressure in the Cap Spring
If you have incorrect pressure on your radiator cap, for example, too low pressure results in a lower boiling point, so a heavily loaded engine can cause the coolant to boil. A cap with high pressure places greater demands on the other connections in the cooling system, which must then withstand at least the same high pressure.

Seal Not Properly Sealing
A radiator cap seals with a gasket that can also move when opening and closing (if the pressure in the radiator cap is exceeded). Thus, the gasket can leak, which can lead to the system having a lower boiling point and boiling more easily, allowing water to leak out from the gasket without needing to overcome the spring pressure in the cap to open.

The Cap is Misaligned
If the spring or cap is misaligned, the gasket in the cap does not seal against the expansion tank, leading to a leak.

Heater Core

Debris in Cooling Channels
If there is debris in the cooling system, it quickly clogs the radiator and the heater core in the cabin. Both of these heat exchangers have cell packs with small tight cooling channels that can easily be blocked by debris, which in turn impairs or completely stops the flow.

Fan Too Small
A small heater fan can result in insufficient heat distribution in the cabin.

Insufficient Coolant/Glycol
This can result in the heater core not receiving enough warm coolant, causing the fan to blow cold air instead of warm air.

Swirl pot

Air Pockets in the Cooling System
If you are experiencing problems with an overheated engine or localized boiling, a so-called swirl pot can solve your problem. This is typically only mounted on modified cars and during racing where air pockets need to be evacuated quickly. This unit/container is mounted as the highest point along with the expansion tank and allows venting from two points in the cooling system.

Expansion Tank Not Mounted as the Highest Point
If the expansion tank cannot be mounted as the highest point, a swirl pot can be installed as an aid to bleed the cooling system. 

Cooling Fan

Cooling Fan Does Not Start
If the cooling fan does not start at idle or to assist during hard driving, the cooling system can overheat.

Blown Fuse
A cooling fan is off or on and puts a heavy load on the electrical system at startup, which is why a fuse can blow if the wiring is not well rated and a suitably sized fuse is not chosen. A slow-blow fuse is often used for the cooling fan, but despite this, temperature differences affecting the mechanics can cause the fuse to blow or burn out.

Temperature Sensor
A wrongly installed or defective temperature sensor can also be the reason the cooling fan does not start. You can always check if the cooling system's fan works by connecting it directly to the battery, which is an effective troubleshooting method that allows you to proceed to check the temperature sensor and other wiring/control systems.

Too Small Cooling Fan
A cooling fan that is too small and cannot move/pump enough air can lead to an overheated cooling system when an engine is working hard and generating a lot of heat. Installing the fan on a shroud that covers the entire cooling package is a solution that provides better cooling efficiency.

Temperature Sensor (Temp Sender)

Broken Temperature Sensor
A broken temperature sensor generates error codes in a modern control system but is also a cause of overheating in the cooling system.

Misplaced Temperature Sensor
A temperature sensor that is incorrectly positioned will signal the ECU and cooling fan at the wrong time. Often, two temperature sensors are used: one before and one after the thermostat. This way, the ECU always knows the temperature throughout the system and when the thermostat has opened.

Incorrectly Wired Temperature Sensor
A temperature sensor that is incorrectly wired usually shows maximum or minimum values, but it can also display incorrect values, which is harder to troubleshoot without connecting to the ECU to read those values or measuring directly at the sensor, which requires you to have data related to that sensor.

Incorrectly Set Temperature Sensor
A temperature sensor can be correctly installed but configured with incorrect values in the ECU. This also results in incorrect temperature control. Always ensure to check that your temperature sensor shows the correct temperature by comparing it to another temperature source that measures the same thing in the same location, such as the temperature in the room.

Intermittent Temperature Sensor
A faulty sensor can be intermittent, which gives irregular values. This is easy to troubleshoot if you have an engine control system with logging capabilities. Otherwise, it may be noticeable on the dashboard when the temperature gauge fluctuates unusually fast. You will also notice this if the cooling fan starts and stops despite the engine not being warm.

Coolant Hose

Damaged Coolant Hose
A damaged coolant hose can appear in many ways. It may be cracked or old. It can also be exposed to radiant heat and chemicals. Regardless of the cause, this can result in leakage. Furthermore, it can be difficult to locate as it may only occur under pressure. The solution is to pressure test the cooling system. First in a cold state and then in a warm state.

Poor Flow
A kinked coolant hose can be easier to locate and replace. However, a hose that has collapsed internally is harder to detect.

Collapsed Coolant Hose
A coolant hose may look good on the outside but be collapsed on the inside, resulting in restrictions in the cooling system and potentially clogging both the radiator and heater core.

Too Small Coolant Hoses
Too small coolant hoses can lead to reduced flow and cooling capacity.

Coolant

Air in the Cooling System

Discolored water = No coolant additive

Insufficient Coolant/Glycol
This can cause the engine to experience localized boiling as the coolant cannot be properly pumped around the system.

Leakage
Coolant leaks are one of the most common problems in a cooling system. The cooling system can leak externally and internally. This results in different symptoms.

External Leaks
A leaking cooling system means that sufficient pressure cannot build up and the cooling system will not function optimally. When coolant leaks out and the level in the system becomes low, the engine will eventually overheat. An external leak means that coolant is leaking out away from the engine. This can be visually inspected, meaning you can check for colored coolant on the outside of the engine.

Internal Leaks
When you have a leaking cooling system but cannot find an external leak, it may be leaking into the engine and combustion chamber. When it leaks at the head gasket, pressure from the combustion chamber can escape into the cooling system, which can cause discoloration and a higher coolant level. This can be measured with testing equipment by looking for exhaust gases in the coolant.
When other internal leaks occur, this follows into the combustion chamber and creates whitish smoke in the exhaust.

Refilling Coolant
A cooling system is closed, and you should not need to refill coolant. If you need to refill coolant, then you have a leak. When a cooling system has been disassembled and is filled with coolant again, it is common to need to top it off afterwards as air pockets dissipate. After this, the closed system should not need further filling.

Do Not Mix Different Types of Coolants
Different types of coolants can react and create a harmful environment that damages or impairs the function of the components and seals in the cooling system.