Rick's Free Auto Repair Advice

Car radiator

What does a car radiator do?

A car radiator removes heat from an engine. An internal combustion engine burns a gas or diesel fuel to generate heat energy and then transforms that heat energy into mechanical energy. The average engine is only about 25-30% efficient. In other words, approximately 70-75% of the heat generated during combustion isn’t used to make power.

The coolant circulating throughout the engine absorbs heat from around the cylinders and cylinder heat. The engine’s water pump then forces the hot coolant to the radiator where the heat is removed. If the flow rate is too fast, the coolant won’t pick up enough heat or lose enough heat while traveling through the radiator. If the coolant flow is too slow, the coolant will pick up too much heat while in the engine, possible causing it to boil.

coolant flow through a cross flow car radiator

Hot high pressure coolant enters the inlet tank and flows across the tubes to the outlet tank. Airflow across the tubes cools the coolant and lowers both the temperature and pressure. Since the radiator cap is on the outlet tank, high RPMs won’t cause a pressure release

The flow rate is determined by engine RPM driving the water pump, the water pump design, the size and condition of the thermostat, the size of the radiator and the size and condition of the radiator fans. A failure in any of those components will affect engine cooling.

Hyundai water pump

Typical belt driven water pump

How coolant flows through the engine


Coolant is sucked into the engine from the bottom of a down-flow radiator or the side opposite from the upper radiator hose on a cross-flow style radiator. The cooled coolant if forced around the cylinders and cylinder head(s) to pick up heat from those areas. Some hot coolant is diverted to the heater core to warm the passenger compartment. The hot coolant exits from the top of the engine and thermostat area and moves through the upper radiation hose to the top tank of a down-flow radiator or the intake side of a cross-flow radiator. Flow rate is determined by the thermostat.

Heat is removed with fans and ram-air

As the hot coolant flows through the radiator fins, outside air is forced through the fins based on vehicle speed or pulled through the radiator fins with radiator fans. The rate of cooling is directly related to vehicle speed and radiator fan speed.

On a down-flow radiator, maximum cooling is achieved near the bottom of the radiator. On a cross-flow radiator, maximum cooling is achieved on the side opposite the upper radiator hose intake.

Electric radiator fans can be single, double or variable speed, depending on radiator fans to cool car radiatorthe year, make and model. Mechanical fans are driven by an accessory drive belt connected to the crankshaft. Most mechanical fans employ a temperature or electrically controlled viscous clutch. At high vehicle speeds where ram-air velocity is greater than mechanical fan RPM, the viscous clutch disengages the fan from the drive belt, allowing it to free-wheel. If the mechanical fan was not allowed to free-wheel at high vehicle speeds, the ram-air would slow the mechanical fan, causing an engine drag and lower MPG.

On late model vehicles with multiple electric radiator fans, one fan is often used to supplement the engine cooling fan when the AC system is operating.

Automatic transmission coolers

Vehicles equipped with an automatic transmission often incorporate an automatic transmission cooler inside the radiator. Since automatic transmission fluid temperatures are higher than engine coolant temperatures, the radiator does double duty by cooling both engine coolant and transmission fluid.

The transmission cooler is located at the bottom of a down-flow radiator and the suction side of a cross-flow radiator.

What goes wrong with a car radiator?


Corrosion and plugging is the most common failure and it’s always caused by owner neglect. Antifreeze/coolant contains anti-corrosive additives that have a limited lifespan. When car owners exceed the useful life of the coolant, dissimilar metals in the cooling system begin a galvanic action that creates metal migration from one component to another. Some metal component thin and develop pinholes, while others accumulate excess deposits that

radiator damage from corrosion

Corrosion has rotten out the tubes and fins

clog and impeded the flow of coolant. In addition, the loss of anti-corrosion inhibitors causes internal coolant system metals to corrode and settle in the small tubes of the heater core and radiator core.

heater tube leak

Worn out coolant can rot out metal heater tubes as well

Once clogged, it is almost impossible to clear out the small passages.

radiator leak

Seal leakage between the radiator core and the end tanks. On an aluminum radiator with plastic tanks, this leak is not repairable.

Even when cleaning is possible, the metal has been compromised and failure is likely to occur in the near future.

That same type of corrosion can cause the transmission cooler to develop leaks which can cause catastrophic transmission damage. Cooling system pressure can force engine coolant into the transmission, causing a severe lack of lubrication.


What’s the cost of radiator replacement?

Depending on the year, make and model, radiators typically cost around $400 for the part and require approximately one hour to install. Radiator replacement also requires a cooling system flush and refill with fresh coolant.

©, 2017 Rick Muscoplat


Posted on by Rick Muscoplat

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