What makes it over heat

Hi Zane,

I assume you're asking in general since you've provided no information on either your car or any specific symptoms, so that's how I'm going to answer.

The car's cooling system is responsible for dealing with the heat generated by the explosions that happen within the combustion chambers. If any part of that system fails, the temperature of the coolant can rise to the point where the system can no longer cool it, and when that happens it overheats.

Here are the parts involved in the cooling system.

• Radiator. The coolant flows through it and its fins allow the heat to be dissipated by air flowing through it. The radiator can leak, resulting in loss of coolant, or it can get clogged, reducing its ability to lower the temperature of the coolant.
• Thermostat. Keeps the coolant inside the engine until it reaches operating temperature, then it opens to allow the coolant to flow through the radiator. A thermostat that fails in the closed position won't let the coolant get to the radiator, resulting in overheating.
• Water pump. Circulates the coolant between the radiator and the engine block, and incidentally through the heater core as well. A leaking water pump loses fluid like a leaking radiator, and a failed pump won't move the amount of water it's supposed to, reducing the cooling ability.
• Fan belt/timing belt. Something has to spin the water pump, and that is usually either be a fan belt or the timing belt. If the timing belt fails, you'll have a lot more problems than the water pump not working, but a loose or broken fan belt will mean little or no water circulation and thus an overheat condition.
• Hoses. Radiator hoses connect the radiator to the engine block and heater hoses connect it to the heater core. Any of the hoses can develop a leak and lose coolant, plus the radiator hoses can be bent sharply, reducing coolant flow.
• Head gasket. The head gasket's main purpose is to seal the cylinder head to the engine block, containing the engine combustion as well as sealing the coolant and oil passages that run through the block and head. If the head gasket fails, it can allow combustion gasses into the coolant, overheating it, or it can allow coolant to escape into either the combustion chamber or the oil pan; either one means you lose coolant and thus the ability to cool.
• Heater core. The heater core does the same thing as the radiator, but it routes the heated air into the passenger compartment to warm it instead of just dissipating it into the ambient air. A blocked heater core won't cause overheating, but a leaking one can again result in coolant loss.
• Fan(s). When the car is in motion, there is generally enough airflow through the radiator to cool it sufficiently. Any other time there must be a method to get air through the radiator, and that is one or more large fans. Older model engines on rear wheel drives generally can use a fan mounted on the front of the engine that's driven by a belt, but newer transverse mounted engines usually make use one or two electrical fans mounted directly on the radiator. If the fan(s) or fan belt fails, not enough air will circulate to take away the heat generated by the radiator.

Those are all the components and failure modes I can think of right off the top of my head; hopefully if I've missed anything our other contributors will chime in and keep me honest.

If you have specific questions or a specific car in mind, add a comment and we'll be happy to respond.