Radiator in a car: how it works, symptoms, problems, testing

May 01, 2022
The radiator is part of the liquid engine cooling system in a car. The cooling system maintains the operating temperature of the engine by removing the excess heat generated by combustion.
Cooling system diagram Cooling system diagram. Click for a larger image
It's filled with liquid coolant (antifreeze) that circulates between the engine and the radiator.

When the engine reaches the operating temperature, the thermostat valve opens and the hot coolant flows to the top of the radiator through the upper radiator hose. The radiator is made out of two tanks connected by many flattened tubes and fins (core) and acts as a heat exchanger. The heat of the coolant is removed by air that passes through the radiator. The cooled down coolant flows to the bottom of the radiator. The water pump pulls the coolant from the bottom of the radiator through the lower radiator hose back into the engine.

Radiator problems

Crack in the radiator Coolant leaking from a crack in the radiator
The most common problem with radiators is when they leak. Leaks can originate from the area around the seal between the radiator core and the top or bottom tank or from a crack, as in this photo.
Often the radiator core can be damaged by some object or corrosion. Symptoms of a leaking radiator include coolant drops under the front end of the vehicle, a wet lower portion of the radiator, low coolant level, rising engine temperature and a lack of heat from the vents when the car is stopped at a red light.

It's important to repair cooling system leaks as soon as possible because a lack of coolant can cause overheating of the engine; a severe overheating can even damage the engine.

Corroded radiator Radiator fins separated due to corrosion
The radiator may also need to be replaced if it can no longer provide sufficient cooling. This could happen if the tubes inside the radiator are partially restricted or the radiator core fins are badly corroded as in this photo.

If the vehicle is often overloaded (e.g. when towing), it's a good idea to use a heavy duty radiator as a replacement.

Another reason to replace a radiator is when the transmission fluid cooler built inside the radiator leaks.

How the cooling system is tested

The most common way to find leaks within the cooling system is pressure testing. Whenever there is a problem with the radiator, the whole cooling system must be properly checked to find the root cause of the radiator failure.
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Mechanics check for the presence of hydrocarbons (HC) in the coolant, which may indicate a blown head gasket. A sticking thermostat can cause the engine to run hotter, which may also cause the radiator failure. Often, the thermostat is replaced together with the radiator as it's an inexpensive part. The radiator fan that is not functioning can also cause overheating and radiator failure. The fans also need to be checked. A failed radiator pressure cap can cause excessive pressure in the cooling system; it also needs to be checked or replaced.

Radiator replacement

Radiator Radiator
Replacing the radiator in an average car in a repair shop will cost 1.5-3 hours of labor plus the parts and extra coolant. For example, the Mitchell 1 Estimating Guide calls for 2.5 hours of labor to replace the radiator in the 2007 Ford Explorer (plus 0.6 hr. for Aux A/C). Often, the thermostat is replaced at the same time.

After replacing the radiator the air must be bled from the cooling system. Once the repair is completed, it's also important to verify that the radiator fans come on when the engine is fully warmed up and the heating system inside the car provides good heat from the vents at any engine RPMs. Lack of heat at idle can be caused by air pockets inside the cooling system.

Can a leaking radiator be repaired instead of replacing? In some cases, the aluminum core can be repaired with soldering or other methods. It takes quite a bit of time and not always works, but if you want to try, there are plenty of Youtube videos on this subject.

Another option is to see if there is a local radiator repair shop that can repair a leak. If it's noticeable cheaper than replacing the rad with a new part, it's also worth considering.