Is your car over-heating? Do you have to add antifreeze? Do you know what could be causing your car to run hot? Where do you start, and how does the cooling system in a car really work? Let's talk about them below.

Reader Question: My car is overheating, what causes this to happen?

Dear concerned car owner,

To properly diagnose your overheating complaint we need rule out a few basic things.

First: Is there enough coolant/antifreeze in the radiator? Don't just look inside the plastic overflow bottle, but remove the radiator cap (when the engine is cold) and look inside the radiator. You should be able to physically see the fluid level if it is at its proper level. Most cars and trucks will hold 1 1/2-2 gallons of coolant and water mixture. If you have to add more than a pint of fluid you should have the cooling system pressure tested for a leak. If you see any obvious fluid loss on the ground or in the engine compartment, you should also have the system tested for leaks.

Second: If no coolant leak or low fluid level is present, then determine when the overheating complaint occurs.

If the engine overheats while at a stop or idle only:


Most front wheel drive cars use an electric cooling fan motor located in front or behind the radiator. The function of the cooling fan is to improve airflow across the radiator at stops and low speeds. The fan is controlled by sensors that regulate the engine temperature and additional load that might be placed on the engine.

The air conditioning compressor will require the cooling fan to operate at idle as long as the compressor is on. A quick way to check the cooling fan operation is to turn on the air conditioner. The cooling fan should come on with the air conditioner compressor. Some cars will have two electric fans, one is for the radiator and the other is the air conditioner condenser fan. Usually the radiator fan is closer to the middle of the radiator. The radiator fan is responsible for engine cooling, and the condenser fan is responsible for increasing air conditioning efficiency at idle and low speed.

If your vehicle does not have an electric cooling fan on the radiator it will have a belt driven fan blade and fan clutch. This fan should be pulling a large amount of warm to hot air across the radiator onto the engine. What you want to determine with either fan situation is that there is ample airflow across the radiator at idle. The radiator is the primary heat exchange for the engine, and airflow is crucial.

What if the engine overheats while at high speeds on the freeway?

Again, airflow and coolant circulation are crucial. At 55 MPH we can assume you have ample airflow across the radiator, so proper antifreeze circulation is the thing to inspect. I compare overheating at 55MPH to jogging with a sock in your mouth. The faster and longer you jog, the more air you are going to require, and with a sock in your mouth you are going to have to breath extra hard to maintain the proper amount of air to keep you going. At 55MPH the water pump is pumping a large amount of hot antifreeze throughout the cooling system.

If there is a restriction in the system like a kinked radiator hose, a restricted radiator, or a stuck thermostat, it will produce the same affect as the sock in the mouth scenario. Rust and water calcification can accumulate in the radiator and drastically reduce the flow of coolant at high speeds. Removing the radiator from the vehicle for disassembly and cleaning or radiator replacement are the only two real cures for a clogged radiator.

Using a can of "radiator flush" additive might help as preventive maintenance, but will probably just be a waste of time and money trying to correct a restricted radiator. You can read my past article about radiator flushing in the archive link below.

Of course there are more technical issues that could produce an overheating complaint, but the check list described above will identify the most common. Anything you can do to help the mechanic diagnose the problem will probably result in less diagnostic charges to you, and might help insure a proper diagnosis of the problem. I talk about other problems like this in my eBook What Your Mechanic Doesn't Want You to Know,