Car Overheating

Reader Question

My car is overheating, what causes this to happen?



Hi There Ruth

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.


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 dis-assembly 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 the FIRST time.

I talk about other problems like this in my eBook What Your Mechanic Doesn’t Want You to Know

Austin Davis

Posted in: Over Heating

About the Author:

Austin Davis, consumer car repair advocate. "Hi there! I love to help people solve their car repair problems and I hope my site was helpful to you today. Thank you for stopping by."
  • Ruramai Murepa

    I have a Hilux Surf 2.7 petrol. It overheats at high speeds, but suprisingly the guage can reach almost 3/4 mark and suddenly drops down to 1/2 without stopping but reducing speed. What can be the problem. I have changed the radiator cap and serviced the radiator

  • Kori McKenna

    Hi Austin, my car is a 2002 Pontiac Sunfire. My car has been over-heating slowly over the last two or so weeks hovering above 195. When I turn my heat on the car will hover sometimes to around 260 but recently my car has reached 260 so I finished driving home and haven’t driven it since. There has not been antifreeze on the floor in my car. My car has used all the antifreeze in my backup tank and we recently changed the thermostat. What kind of other things could be the problem?

  • Austin Davis

    The clutch for A/C is just a little heavier duty, and is recommended. You DID replace the radiator? I would say this is probably a lack of coolant circulation in the engine…due to a restriction more than a fan clutch issue. The clutch really only helps at idle speeds, so if you are overheating while driving, I would lean more to a coolant restriction. Turn on the heater full blast when engine is at normal temp, that can help remove an air pocket in the cooling system, then top off radiator with coolant once cooled.

  • michael mathis

    I have a 92 mustang with the 302 I have a/c and when I turn it on my car overheats. My fan clutch leaks out of the front. My question is what’s the difference between fan clutch for a/c and non a/c. I have already changed thermostat water pump radiator and fan. Will it still overheat if I get fan clutch with non a/c?

  • Austin Davis

    I would look at the radiator and make sure there is not a restriction at the bottom, and time to replace it. Lack of air flow across the radiator and lack of coolant circulation in the radiator are usually what causes overheating under a load, like climbing a hill or pulling a trailer.

  • solomon kingz

    My car rover 75 overheat all the time. Why?
    It does not move fast when climbing the hill also.

  • Austin Davis

    At the front of the car is the radiator, and the two radiator hoses coming off it going to the engine. Then at the very front of the engine just behind the fan belt is the water pump. can you see any sign of coolant leaking at those locations under the hood?

    If you can not, then I would have your local mechanic do a “cooling system pressure test” to help you locate the leak. This is a simple, fast and inexpensive test that pumps air into the radiator to help push out the coolant so you can find the source of the leak.

  • Lele

    My antifreeze keeps leaking out. It seems to be coming out toward the front end of the car. The reservoir rarely stays full it like the car s drinking the fluids quickly. It overheats when its empty…what could be the problem?

  • Austin Davis

    Hi there, I dont quite understand your question. The floor of the car is hot, but not the engine? No overheating is happening under the hood?

  • kencool

    My car heating indicator was at half. I put my laptop to the floor at the back seats but it was very hot by the time I arrived home. could this be the overheating I hear about?