General Cooling FAQ

General Cooling Analysis

Cooling System Diagnosis

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General cooling analysis: top

Engine water-cooling is accomplished by first transferring heat from the engine to the coolant by way of conduction, transferring this heat to the radiator by way of coolant flow, conductively transferring the heat from the coolant to the radiator and, finally, transferring heat out of the system from the radiator by way of a fan induced airflow.

Heat distributed from the combustion chamber into the engine is limited by way of the relatively high thermal resistance of the combustion media coupled with the limited contact surface area of the combustion chamber and exposed cylinder. Under cold running conditions, a mechanical thermostat limits fluid flow, limiting heat transfer from the engine to the coolant, and from the coolant to the radiator.

The transfer of heat from the engine to the coolant is enhanced by coolant flow in that heat transfer is not only dependent on the thermal resistance, but also dependent on the temperature difference between the two medium.

Cooling system components, their functions and effect in failure

Our low mass temperature sensor provides a convenient means for both radiator and air temperature measurements and is available on this site.

The engine block and head gasket:

Iron oxide is an insulator of heat and will prevent its transfer out of the engine. Although this prevention of heat transfer will cause increased detonation due to higher combustion chamber temperatures, higher engine temperatures will not be indicated, as the more isothermal chamber will actually decrease coolant temperatures.

A leaking head gasket has the opposite effect. Heat flow into the coolant is increased substantially due to the fact that there is a direct flow path into the fluid. Since the combustion gas is compressible, flow will increase with engine load. Because heat flow out of the system will tend to track engine speed, a leaking head gasket will cause over heating under both low and high speed driving conditions. This condition will cause both the inlet side and outlet side of the radiator to be hotter than normal. Look for flakey white deposits on one of the spark plugs. This is usually the first sign of a leaking head gasket.

The water pump and thermostat:

The water pump produces fluid flow from the engine to the radiator. Older vane-type pumps flow at a rate proportional to engine RPM while more modern turbine-type pumps flow at a rate exponentially proportional to RPM. While water pumps eventually fail, their failure is nearly always byway of bearing and subsequent seal failure and not by a loss of pumping efficiency. Unless a pump is leaking or selected improperly for its application, there is no reason to consider a failure as a cause for cooling problems.

The thermostat, a thermo-mechanical device employed in limiting fluid flow, is a common source of failure. Failure is typically caused byway of iron oxide deposits, which impede mechanical movement. Although the thermostat may stick either open or closed, it is most commonly stuck partially or completely closed, limiting heat transfer from the engine to a conduction means. Over heating will occur quickly and under both idle and high speed driving. This condition will cause the inlet side of the radiator to be hotter than normal and outlet side of the radiator to be colder than normal.

The radiator and cooling fan:

The radiator transfers heat from the coolant into the atmosphere by way of heat transfer through the radiator fins attached to the core. The core can become either coated or blocked by iron oxide. Either condition will cause overheating at high speeds. Low speed temperatures are less effected. A radiator with a coated core will measure slightly less than normal temperatures top and bottom, while a blocked core will measure similarly to a system with a closed thermostat where the inlet side of the radiator is hotter than normal and outlet side of the radiator is colder than normal.

There are three types of fans used in automotive cooling applications: a thermostatically modulated mechanical fan, an electric motor driven fan, and a direct driven mechanical fan. Each is addressed separately below.

A thermostatically modulated mechanical fan is the most difficult to analyze as it will rotate at some speed even under failure. Fortunately, most water pumps are driven at or near engine speed. To analyze the clutch, mark one of the blades with white chalk. Connect a timing light and let the engine warm. As the engine approaches the correct running temperature, the fan speed should change dramatically. Check to see that the fan speed is running at or near engine speed when the engine is hot. The air temperature pulled through the fan should read at least thirty degrees lower than the fin temperature.

An electric motor driven fan is easier to analyze. First check to see that the fan turns freely. Next, run the engine until normal temperature is attained. The fan should run and the temperature of the exiting air should read at least thirty degrees lower than the fin temperature.

There is no failure mode for a direct driven fan.

A fan failure will be indicated by high inlet and outlet radiator temperatures.

Other causes of overheating:

Low coolant level:

A lowered level of coolant can cause hot spots within the engine, reduced coolant flow or, if the level is below the water pump, can reduce  flow to zero. Symptoms depend on the severity of the problem. Hot spots tend to cause detonation. A coolant level below the highest level of flow will require a threshold of work to displace the coolant before flow begins. Until this threshold is surpassed, the radiator will remain cold as there is no means for heat flow or conduction. Heating problems will occur at lower RPMs. A level below the water pump will cause the same symptoms, but at all RPMs as no pumping will occur.

Incorrect ignition timing:

An overly advanced ignition timing will cause a significant amount of the combustion charge to be utilized destructively. More heat is generated and overheating can occur. Because ignition timing varies with engine speed, overheating can occur at high or low speeds or both. A severely delayed timing will allow the combustion medium to contact a larger cylinder surface area, allowing more heat to exit the combustion chamber. heated exhaust ports and manifolds will also transmit heat into the coolant.

Symptoms of advanced ignition timing include detonation or pinging, reduced high RPM performance, and a quieter than normal exhaust note. Symptoms of delayed ignition timing include reduced low RPM performance, and a louder than normal exhaust note. In both cases, the inlet and outlet side of the radiator will be hotter than normal.

Lean Air / Fuel mixture:

A lean air / fuel mixture generates more engine heat. As with ignition timing, A/F mixture varies with engine speed, overheating can occur at high or low speeds or both. Symptoms include a lack of smoothness in engine performance, white-coated spark plugs and overheating in the RPM range where a lean A/F mixture occurs.

 Cooling System Diagnosis: Top

In order to correctly diagnose a cooling problem the circumstances of overheating must be determined:

My car over heats at idle or low speed

My car over heats at high speed 

My car over heats at both low speed and high speed

My car over heats at idle or low speed  Diagnosis

The most likely faults for this condition are:

Low coolant level

A defective or low capacity cooling fan

A lean A/F mixture

Incorrect ignition timing

Other possible causes:

A bad thermostat

My car over heats at high speed  Diagnosis

The most likely faults for this condition are:

A radiator core coated or blocked  by rust

A lean A/F mixture

Incorrect ignition timing

Other possible causes:

A bad thermostat

My car over heats at both low and high speed  Diagnosis

The most likely faults for this condition are:

A bad thermostat

A leaking head gasket

Low coolant level

A lean A/F mixture

Incorrect ignition timing

Other possible causes:

A radiator core coated or blocked  by rust