I happened across a great article by Steve Litt on cooling systems the other day and saved the link. This seems to be a well researched and written page and should help sort just about any cooling system woe. Sorry for pasting a book, but it is great information which many of us should apply.
For those who will look at this later and see the link broken:
How Cooling Systems Work
By Steve Litt
The cooling system has one and only one purpose -- to remove excess heat from your engine. As your engine burns gasoline, a little less than a third of the released energy goes into mechanical energy to run your car. The rest is converted to heat. Some of that heat is blown straight out the tailpipe, while the rest heats the engine itself. Without a cooling system, the engine would be destroyed by heat within 3 to 30 minutes of startup.
The cooling system works by moving coolant (water plus antifreeze) through the engine, and moving that heated coolant through the radiator, where its heat is transferred to the surrounding air. The cooling system must have enough cooling capacity to cool a car ascending a long, steep mountain road, where the driver might have the gas pedal 2/3 of the way to the floor.
But it must be regulated in such a way that at a steady 40 MPH on a flat road in subzero weather, the engine's temperature is allowed to quickly rise to the manufacturer's recommended temperature (usually about 200 Farenheit, give or take 20 degrees). That recommended temperature should also be maintained when the car goes up a 10 mile 7% grade in 100 degree weather. Such regulation is accomplished by the car's thermostat -- a heat sensitive valve that allows coolant to flow through the radiator at high temperatures, but cuts off that flow at low temperatures.
The water pump sucks cooled coolant from the radiator and pushes it into the engine. The coolant flows through the engine, absorbing the engine's heat. If the thermostat is open, that coolant then flows into the radiator for cooling. As it flows through the radiator, it heats the tubes and fins on the radiator, and that heat is transferred to the air flowing through the radiator. At low speeds that air flow is maintained by the fan, and at high speeds it's maintained by the relative velocity of the vehicle in relation to the outside air.
Meanwhile, a parallel path brings hot coolant from the engine through the heater in the passenger compartment, and back into the water pump. That path is controlled by the heater valve, which in turn is controled by the lever or electronic climate control on the dashboard. The parallel path is not restricted by the thermostat, so passengers get heat even when the thermostat is closed. However, some cars have a mechanism which shuts off coolant through the heater during an overheat, I guess on the theory that you want to maximize flow through the radiator by shutting off the heater. While such a theory might be credible when the cause of overheating is low coolant, it prevents the alert driver from turning on the heater full blast and thereby letting the heater act as a second radiator. Perhaps such a shutoff is a safety feature so there's no way overly pressurized coolant can rupture the heater and spray on the passengers. So if you've had symptom where "the car overheats and then the heater blows cold air", the heater probably has been shut off due to overheat.
The entire system is sealed with one exception. The radiator cap contains a spring which maintains a constant pressure by venting coolant (to the reservoir tank) when pressure rises above its specified value -- typically around 15 PSI. It's normal for some coolant to vent in this way, which is why the reservoir is more full when the car is hot than when it's cold. The radiator cap also allows the vacuum created when the system cools to "suck back" coolant from the reservoir. But in the case of an extreme overheat, vented coolant overflows the reservoir, thereby creating a low-coolant situation and making the overheat even worse.
Looking at the diagram, you see that oil, gasoline, combustion gasses and coolant all flow inside the engine. These materials are kept separate by the head gasket(s). A breached or broken head gasket, or a bent head, allows any or several of these materials to mix. Coolant into the cylinders produces huge clouds of white exhaust (steam) out the tailpipe. Coolant into the oil produces a yellow/white foam or gunk on the oil cap, as well as degrading the oil, possibly past the point of lubricating usefulness. Combustion gasses leaking from the cylinder to the coolant might produce no obvious symptom, but it's an extremely dangerous condition, because it can cause an overheat by any one or more of four different mechanisms:
1. By forcing excessive coolant out the reservoir, thereby creating a low-coolant situation
2. By forming a gas bubble around the thermostat's sensor, thereby preventing the thermostat from opening
3. By heating the coolant to such a degree that the radiator cannot dispense all the heat
4. By breaking down the coolant's corrosion protection, thereby damaging the water pump or radiator, ultimately causing overheating
It's possible for a broken head gasket to allow combustion gasses into the coolant, without allowing coolant into the cylinders or coolant into the oil or oil into the coolant. In such a situation, the broken head gasket could silently cause overheats. The definitive test for this type of head gasket problem is to test for combustion gasses at both the radiator fill pipe and at the reservoir.
Excess Cooling Capacity
Automotive 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 two. Go up to 65 and on some cars you'll be near flooring it. 1/3 of all that gasoline is consumed heating the engine.
Your cooling system must be able to get rid of all that heat. Difficult enough, it becomes even more of a challenge if the air temperature is warm (less heat transfer from radiator to air), and brutal if your car is heavily loaded or towing something. If the heat generated by combustion significantly exceeds the cooling capacity, you'll severely overheat quickly (typically after a mile or two of climbing).
A well functioning cooling system has the capacity to maintain the engine at under 100 degrees temperature during continuous 50mph level drives on cool days. But of course the temperature needs to be 160-230 Fahrenheit, depending on the car (consult your owners manual). That means in most driving situations the cooling capacity must be partially defeated. This is accomplished by the thermostat, which acts as a deliberate bottleneck, regulating the amount of cooling to keep the temperature at a proper level. A somewhat typical thermostat would be closed until 180 Fahrenheit, after which it would open further as the temperature increases, until at 195 it's completely open. This means that in the 15 degrees between 180 and 195, the cooling capacity would go from 0 to the full capacity of the system (enough to scoot up a long 6% grade at 65 mph carrying 5 people in a well designed and maintained machine). Below is a graph showing how temperature increases with increased engine heat production (i.e., more gas):
The portion in blue represents a level of heat production so small that it can be disbursed by the direct contact of the engine with ambient air. In practice this might be achieved in the case of a 40mph wind blowing into the open hood of a car idling in the deep of a northern Minnesota winter's night, but otherwise this condition is never seen in real life. An idling engine, and certainly driving, at anything resembling normal conditions requires radiator cooling.
NOTE: Don't make the mistake of thinking the preceding diagram represents temperature versus time. While that graph would look similar at the leftmost part of the graph, that's not what's being represented. You can think of the preceding diagram as a graph of various driving conditions, each maintained for 10 minutes or more.
The violet portion represents heat production levels within the regulated cooling capacity of the cooling system. The slight temperature gain across this range is due to the fact that the thermostat opens slowly and steadily over a range of about 15 degrees Fahrenheit. This is the normal operating range of the vehicle. Once the engine is warmed up, all driving should be done in this range.
The red portion represents a heat production level beyond the cooling capacity of the cooling system. The temperature goes sky high. On a well maintained vehicle, you would expect the red portion only when the car is used beyond its design capabilities, like using a compact car to pull a trailer up a long 6% grade.
The bottom line is that on a well maintained vehicle, the bottleneck, by a huge margin, is the thermostat. Contrast that with a vehicle with a compromised cooling system not capable of cooling a hard worked engine, or in extreme cases even a lightly worked engine:
Here there's no regulation. The entire graph is basically a straight line. Moderate hard usage sends the car into the red. The operating temperature of a vehicle in this state of repair would vary widely with ambient temperature and length of time driving. Typically no "normal operating temperature" can be identified for a vehicle in this condition. Such a vehicle will almost certainly experience a catastrophic overheat the first time the driver takes a lengthy drive, or drives in hot weather, or drives up a moderate hill.
The controlling bottleneck of this vehicle is not the thermostat -- it's something else in the cooling system. The automotive technician's task is to find that bottleneck.
Steve Litt is the author of "Troubleshooting Techniques of the Successful Technologist". Steve can be reached at Steve Litt's email address.