Mental Model of the Cooling System
By Steve Litt

A car converts about a third of its fuel's energy to mechanical energy to move the car. About a third goes out the tailpipe unused. Most of the remaining third is released as heat. That heat must be conducted away from the car's engine, or the engine will reach temperatures fatal to the engine. At the simplest conceptual level, here's what happens:

The water pump pushes cool coolant from the radiator into the engine, where heat from the engine is transferred into the coolant, thereby cooling the engine. Assuming the thermostat is open or partially open, the hot coolant leaves the engine through the thermostat and is transported to the radiator, where its heat is transferred to the air blowing through the radiator, thereby cooling the coolant. The cooled water is then ready to once again go to the water pump.
Note that the areas in the engine through which the coolant flows are called the "water jacket". Note that some of these areas include intentional holes in the head gasket. Those holes do not constitute a head gasket breach, because there's coolant on both sides. It's only when a part of the head gasket separating coolant from combustion gasses from oil that the head gasket is considered breached.
Note that a parallel coolant flow path goes through the passenger compartment heater. This is why it's advised to turn on your heater full blast in the case of an overheat (but remember never to jeopardize personal safety doing this, and remember this is a last ditch workaround and the car should not continue to be driven under these circumstances).
The airflow through the radiator depends on the fan at low driving speeds, but at high speeds most cars force air through from the grille. Note also that the air conditioning coil assembly is in front of the radiator, so if the air conditioning is on, the air flowing through the radiator will have been warmed by the air conditioner, thereby reducing cooling efficiency. That's why huge inclines like the Grapevine usually post signs to turn off your air conditioner before beginning ascent.
In summary, the engine passes its heat to the coolant, which flows to the radiator and then passes its heat to the radiator, which passes its heat to the air being blown through it. Any interference with this heat flow dangerously reduces cooling capacity. If cooling capacity falls below the amount of heat generated by the engine, overheating results. The answers to the following questions help determine the cooling capacity of the cooling system:
[ul][*]Is the water jacket full of coolant?[*]Are deposits in the water jacket reducing heat flow away from the engine? (hard to determine)[*]Is coolant flowing swiftly and continuously while the thermostat is open?[*]Is the radiator free of deposits that reduce coolant flow and heat transfer?[*]Is the radiator filled to the top with coolant so that its entire surface transfers heat to the air?[*]Is the radiator designed to transfer the amount of heat being generated, in the weather encountered?[*]Does sufficient cool air flow through the radiator at driving speed ( >30mph)?[*]Does the fan supply sufficient cool air flow through the radiator at low speeds and stops?[*]Is the coolant free of combustion gasses and other gasses and contaminants? [/ul]
Later in this Troubleshooting Professional issue these simple heat transfer questions will be transformed into troubleshooting tactics.
Excess Cooling CapacityAutomotive cooling systems must have HUGE levels of excess cooling capacity. Next time you drive 60 mph on a flat deserted road, notice how far you push on the gas pedal. Probably a millimeter to a centimeter. Now see how much you need to push the gas pedal to ascend a 6% grade at 45mph. Probably an inch or 2. Go up to 65 and on some cars you'll be near flooring it.