When your temperature gauge reaches "H' it may too late to prevent a major breakdown. Knowing the symptoms of an overheated car and how they occur may be the difference between being inconvenienced and incapacitated.
Other than a low oil level or low oil pressure light, there is not a more significant part of a car's instrumentation than a rising temperature gauge or a glowing "Hot" light. These lights are really the only confirmation a driver has that his car is really overheating. It is the identification of the symptoms of an overheating car that enable the motorist to avert a badly damaged engine. Overheating is always a traumatic event for a car's engine, which makes the early identification of the symptom an important addition to the informed motorist's tool kit.
The car's thermostat is a valve that controls coolant flow from the engine block to the radiator. When the engine is cold the thermostat remains closed so that the coolant can reach operating temperature quicker and also provide heat to the passenger's compartment. The thermostat has a spring on it that moves depending on coolant temperature causing the thermostat to open. Sometimes the thermostat fails to open thus restricting coolant flow to the radiator where it would be cooled down. This condition is often the cause of overheating. The symptoms of this cause would be a rising temperature gauge and possibly the loss of heat inside the car.
A car's radiator will have thousands of gallons of coolant passing through in its lifetime. Along with the coolant comes particulate matter in the form of corrosion breaking loose from various parts of the car's cooling system. These contaminates collect in the tubes of the radiator reducing its efficiency. Extensive "plugging" in the radiator will cause the car to overheat. The symptom of this condition would be a rising temperature gauge which goes up when you accelerate.
A car's cooling system is a closed loop system. You are not supposed to lose coolant. Sufficient coolant loss will cause the engine to run hot because engine is heating less coolant to higher temperatures. The symptom of overheating induced by coolant loss would be a pool of coolant on the pavement when the leak is external. Steam under the bonnet as the lost coolant hits hot parts of the engine, or a rising temperature gauge in the case of a undetectable engine related leak. Of course, the gauge would also go up if the leaks were not detected.
Deteriorated Water Pump
Cars use a belt driven pump to push the water and coolant mixture through the cooling system. This part is called the water pump. Rarely the impeller that draws the coolant through the pump will rust away making it impossible to push any through the system. If this occurs the temperature gauge will climb and coolant will boil over in the radiator.
Most cooling fans are electrically driven. Some are driven by fan belts. If a belt breaks or the electric supply to the fan is interrupted overheating may result. Electric fans are tuned on thermostatically when needed. When the car runs at idle for extended periods or the weather is extremely hot, a failed fan will cause overheating otherwise it serves as a standby assist to the rest of the cooling system. In stress conditions an inoperable fan will cause the temperature gauge to rise.