By November 22, 20053 Comments Read More →

My Car is Over Heating – What Causes This to Happen?

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

Love your site,
John Harrison

Thanks John,

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.

3 Comments on "My Car is Over Heating – What Causes This to Happen?"

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  1. richard says:

    How to troubleshoot an engine overheat:
    1.Cleaning the entire cooling system go to any nearest car radiator cleaning shop maybe the system is blocked.
    2.Check the radiator fan if it is working properly meaning when youre temp gauge reaches half way check the fan speed it should be strong.
    2.Check the thermostat condition by removing it and putting in on a boiling water see if it opens (you will see a movement on the body itself)
    3.Checking the water pump condition by removing the radiator hose to the engine thermostat body run the engine and check the water pressure coming out on the engine from the thermostat body it should be strong pressure releasing the coolant from the engine(you should have a hose with water supply inserted on the coolant tank incase if the cooling system is empty.
    4.Checking the timing belt if it is worn out replaced it.
    5.Checking the head gasket by looking at the oil dipstick if the oil color change like milky it means you have a bad gasket that needs to be replaced

    I am not a mechanic but I had an expirience with this kind of issue and hope that it will help you thru this message.


    Richard fro Philippines.

  2. lex says:

    i have the same problem with my toyota fx 97 model, when it is idle the temp is getting hot..when it is running the temp gauge is in the middle depending to the climate..What should i do?i already fixed my fan..i already have my auxiliary fan,i changed my head gasket,overhauled my radiator..pls help me..i dont want to spend a lot of money for trial and error..tnx

    • By Austin Davis says:

      It still sounds like you either don’t have enough airflow across the radiator and there is an electric fan problem still OR there is not enough coolant in the system to maintain proper cooling system pressure. Make sure there is not an air pocket inside the cooling system.

      Check or replace the radiator cap

      Pressure test cooling system for leaks

      When the engine is at normal operating temp, the upper radiator hose should be firm and hard to squeeze together with your bare hands, unless there is a leak somewhere.

      I would also be suspect of the radiator, which is almost impossible to “flush out”. Rust and sediment build up at the bottom which restrict the flow of coolant. If its the original radiator and you have ruled out the other issues above, I would replace it with a new one.

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