What causes a car to overheat?

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Here's a list of 7 items to check. It's pretty much standard for all engines.
Check the oil level add as needed. Check the water (and antifreez) add as needed Check the rediator cap. re-place if it's old or missing the seal gasket Check the thermostat. replace if old or not working See that the belt to the water pump is adjusted properly Check to see if the water pump is working. Check the radiator. Make sure it isn't clogged
It's a good idea to visually check the oil, water, belts and hoses everytime you fill up. It takes less than 2 minuites and could save you large amounts of money and headaches.  

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make shure that the seal on the cap are not cracked  

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1 insufficient coolant in the system. 2 water pump drivebelt defective or not adjusted properly. 3 radiator core blocked or radiator grille dirty and restricted. 4 thermostat faulty. 5 fan blades broken or cracked. 6 radiator cap not maintaining proper pressure, have the cap presure tested.  

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Check fuses for the electronic fan.  

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One other thing to add o this good information. If after changing coolant or any time you have worked on the system, you must bleed all air out of the cooling system. Trapped air can cause an air pocket which will cause a hot spot and overheating. Some cars have bleeder screws and some don't. Check your owner's manual. If there is no bleeder valves and you suspect trapped air, park the car uphill or jack up the front, start the engine with the radiator cap removed and run until it reaches operating temp. This will allow trapped air to escape. Also make sure when replacing the thermostat and radiator cap you install the correct ones.
 

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Lot of good answers here. I have found evaporating cooling fluid to be a problem when everything else is working correctly. Common cause for this is too much water, not enough antifreeze. Evaporation is called "boil over". Adding antifreeze to water increases its boiling point. Since water boils at 212 degrees, and (my car) optimal operating temperature is 190 degrees - there is a thin line from running normal to "boiling over". Solution - the mix in the radiator should be 50% antifreeze and 50% water. To get the correct ratio antifreeze to water, buy a coolant test kit or buy your coolant in pre-mixed bottle.
 

a different situation

what if the electric fans are not coming on?  

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In that case, (Fans not running), it may be a blown fuse, bad fan motor, or a bad thermal sensor.

Answer. If fans are not coming on, check coolant temp switch. I ahd same problem with my car. I had my cooling fan direct. Had a bad coolant temp switch