With the arrival of the summer driving season, many enthusiasts start to think about their engine’s cooling system. But the truth is, this system is just as important on a cold winter day as it is during the summer–and problems can occur regardless of the time of year. There has been much written about upgrading the cooling system, but little if any effort put forth to explain the theory and basic diagnostic procedures. In most instances, factory parts are more than adequate to get the job done–so follow along on this common sense approach to keeping your cool.
IDENTIFYING THE PLAYERS
The components of a basic cooling system include the water pump, radiator, hoses, thermostat, and the cooling media itself. Diagnosing a cooling system issue first requires you to understand that the radiator’s job is to remove heat from the coolant, and the liquid’s task is to cool the engine. Thus, the only impact the radiator has on the engine is its effectiveness to remove heat from the liquid that has passed through the engine block and cylinder head. The radiator is the most visible part of the cooling system and is also the most misunderstood. If your engine is getting too hot you need to determine if it is due to the radiator not providing enough of a temperature drop to the heated liquid, or if the issue is in another part of the system such as the water pump, thermostat, hoses, or even the coolant itself.
The water pump in most if not all production engines is a centrifugal design. All centrifugal water pumps have an inlet, outlet, and impeller along with a cavity for the impeller to reside in. Most water pumps are made from die-cast aluminum, but years back cast iron was the material of choice. A vent or weep hole is usually cast into the snout that supports the shaft bearing. The purpose of this hole is not to allow coolant to weep out or vent the system as its name may first imply, but to allow easy installation of the bearing and seal. If this hole did not exist, the bearing and seal could not be pressed into the casting since there would be no way for the air to escape. The pump’s impeller is designed to work efficiently at an engine’s common range of speed. The downfall of a centrifugal pump is that it is not efficient at every rpm–its output will not increase beyond a certain flow rate if the rpm is raised, and it may actually drop. When this occurs it is identified as cavitation. The operating speed of the pump is a function of the crankshaft and water pump pulley ratio.
The thermostat is an important part of the cooling system and should always be used on a street car. The first use of a thermostat was in 1914 by Cadillac on their V-8 engine. The basic purpose of a thermostat is to regulate the flow of coolant through the radiator. Other reasons for having a thermostat in the cooling system are to reduce engine warm-up time, maintain optimum running temperature, and help with passenger compartment heating. The thermostat consists of a poppet control valve that is opened by a thermal expansion device and is closed by spring pressure. Movement of the valve is directly related to the temperature of the coolant surrounding it. The thermostat is almost always installed between the coolant outlet from the cylinder head jacket and the inlet to the radiator. At normal operating temperature, the control valve is open and coolant circulates through both the engine jacket and the radiator. But when cold, the valve is closed and circulation through the radiator is restricted. The engine is allowed to warm up much more quickly, since a smaller quantity of coolant is being heated. The temperature rating that is stamped on the thermostat (for example 180 degrees) is the crack-open value. That describes the coolant temp that the thermostat just starts to open. It then requires approximately another 10 to 20 degrees to become fully open. So, a 180-degree thermostat will just start flowing coolant to the radiator at 180 degrees, but will not be fully open until 190 to 200. Many enthusiasts and mechanics falsely believe that the temperature rating means that the engine should always run at that value.
The unsung hero of the modern vehicle cooling system must surely be John Karmazin of GM’s Harrison Radiator Division, who invented the radiator pressure cap. It was first used on 1939 Buicks, and has been taken for granted ever since.
The purpose of the pressure cap is to raise the boiling point of the coolant (three degrees for every one psi above atmosphere), while also helping to move the coolant from the hot spots in the cylinder head water jacket. This is essential for good cylinder head cooling. If the coolant is not pushed from the hot spots in the cylinder head, a vapor bubble will form and block coolant flow, causing localized overheating of the area around the exhaust valve and combustion chamber. The radiator pressure cap is a combination of a filler cap and pressure control valve. It is installed at the highest point in the cooling system and seals against a seat in the filler neck of the radiator. Some 1960s Ford engines had the pressure cap located on a surge tank above the radiator. With the engine running and the cap in position, the cooling system pressurizes. This occurs automatically because the coolant expands as it gets hotter. The cap consists of a spring-loaded plate valve with a rubber facing that is preloaded onto a fixed seat. Depending on whether the cooling system is an open or closed design, the fixed seat is either the filler neck or the cap itself. If the pressure in the system rises above the control valve limit, the spring loading on it is overcome so that the valve is lifted upwards off its seat and vents the system to either atmosphere (open system) or to an expansion/overflow tank (closed system).
The coolant itself is paramount to the system performance, and is commonly misunderstood. During combustion, heat is transferred in all directions to the metal of the combustion chamber, cylinder wall, and piston. Heat then flows through the metal walls with a minimum of resistance. The most common automotive coolants are a mix of ethylene glycol (EG) and water. By itself EG does not possess very good thermal transfer properties, but it does boil at a much higher temperature than pure water. Mixing EG with water in a 50/50 solution creates an acceptable coolant. Many enthusiasts use pure water in the cooling system. Contrary to what many believe, water by itself is a terrible cooling media since it causes rust and corrosion that not only makes parts fail, but insulates the water jackets and limits thermal transfer. This keeps the heat in the cylinder head and block instead of letting it into the coolant. Also, it boils at a much lower temperature than a 50/50 mix of EG and water. Plain water with a 15-psi radiator cap will boil at 257 degrees at sea level, while a 50/50 mix with EG will raise the boiling point to 264 degrees. The higher the boiling point, the more heat that can be pulled from the cylinder head. Once the coolant boils it becomes a vapor and no longer has the ability to cool the metal castings of the engine, and the temperature inside the engine (not necessarily on the gauge) spikes and causes premature failure and detonation. It needs to be recognized that the liquid temperature that a dashboard gauge reads is only determining the amount of heat that transferred into the coolant. When pure water is used the dash gauge may read lower but the metal surface temperature of the cylinder head is much higher. The result will be metal fatigue and the need to retard the ignition timing to eliminate ping and knock. The same will occur if a concentration of EG stronger than 50 percent is employed.
Before you start to condemn parts, some thought needs to be given to the problem. No matter what year your car was built, Detroit never produced a vehicle that was prone to overheating or running hot. Even if you modified the engine and stepped up the horsepower, the stock system in full working condition should be more than adequate. If the engine is making 150 to 200 horses more than stock, only then will you need to consider a larger radiator or other aftermarket components. So if your relatively stock engine all of a sudden starts to run hot, it is the result of a component failure–not a poor design.
If the engine runs too cold as read on the temp gauge, then the most common problem is a stuck-open thermostat that will not allow for the coolant to reach operating temperature. A less common cause would be a low coolant level. This may trick the gauge into thinking that the engine is cold since the sending unit is not bathed in liquid.
The majority of cooling system issues is with a perception of the coolant being too hot–not an actual overheating condition or boil-over of the coolant.
When the engine gets hot is going to be critical in diagnosing the problem. A typical vehicle only requires 25 to 40 horsepower to cruise down the road or drive around town, even if the engine under the hood has the potential to make 600 ponies. Thus, the cooling system only needs to absorb the heat load based upon the power being produced. The higher the power, the more heat. If the engine gets hot at low speeds but runs cooler on highway trips, then the coolant flow through the radiator and the air flow into it need to be examined. If the engine gets hot while producing peak power, a high-flow water pump, larger radiator, or both is usually required.
It is common to find the wrong size pulleys on an older engine because at some time they were changed. If the water pump pulley is too large in relation to the crankshaft, the amount of coolant flow through the engine and radiator will be greatly diminished and will result in an elevated temperature at low speed. The front of the radiator needs to be examined for debris and for disintegration of the fins from road salt and age. The fins are responsible for heat transfer. The core also needs to be examined by removing the radiator cap and draining some coolant. If the tubes are partially blocked with corrosion, then the core will either need to be replaced or a new radiator will need to be installed. Years ago a radiator shop would remove the tanks and perform a procedure called “rodding” to remove the corrosion from the tubes. It will be almost impossible to find someone to do that today.
A thermostat can also act funny, especially at higher coolant flow rates. A good diagnostic procedure is to change the thermostat as a first step if a visual inspection shows nothing wrong. In like fashion, the radiator cap (though there are tools to check them) is usually changed as a valid diagnostic step. Never run a low pressure cap no matter what the auto parts store tells you. For most engines a 14- to 17-psi cap is desired.
If you recently purchased the car then check the amount of EG in the coolant mix using a hydrometer. If the concentration is wrong then the operating temperature of the engine will be skewed. A higher concentration of EG will usually show a falsely cooler gauge reading while too much water will show a higher temperature.
Another common area for elevated temperature is a missing or broken fan shroud–or in some vehicles such as a Camaro or Firebird, a damaged under-car air dam. As a rule, if the vehicle does not have much of a grill opening then an air dam is used to induce airflow over the radiator core.
If the temperature fluctuates from hot, to cold, and back to hot, then the system is air bound and needs to be bled.
The radiator should be filled to about one-half inch to one inch below the top of the neck, and all the tubes should be covered. Keep a good supply of premixed coolant nearby. As the air is displaced, the liquid level will drop. Start the engine and run it at a fast idle of around 1,500 rpm. The heater should be turned on to full hot, but the fan speed is not critical. The high idle speed is required to increase the flow rate and push the air out. It is necessary to turn on the heater to bleed the heater core.
As the engine warms up, the thermostat will start to crack open. When this happens, the coolant level will drop and need to be topped off to keep the radiator full and air out.
As the coolant heats it will expand and overflow. To eliminate this I recommend the Lisle Radiator Fill Kit that is seen in the pictures. As the engine is run at high idle the coolant level will eventually stabilize, but you will need to watch the fluid for any signs of air bubbles. These may be large and obvious or inconsistent small bubbles. When you feel confident that the system is empty of bubbles (be patient–it may take 10 to 20 minutes), have a helper raise the idle speed slightly and then put the radiator cap on. The idle can then be returned to normal.
From my years of working with engines many cooling problems are rooted in a lack of consideration given to the removal of air from the system.
Premature component failure such as the head gaskets, heater core, radiator, and water pump can be traced to a lack of coolant service. When traditional antifreeze is made an additive package is installed that consists of corrosion inhibitors and other protections. Through the heating and cooling cycle that is normal to engine operation, these additives are consumed. The coolant still looks fine and has not had its freezing point altered, but is now ineffective. A hydrometer just checks the freeze point, not the chemical composition. For this reason it is best to change the coolant with a 50/50 mix of EG and distilled water every three years.
When it comes to keeping your cool, a good service schedule of new belts and hoses, a new thermostat and radiator cap, and fresh coolant is all that really needs to be done. Spend the rest of the time driving and enjoying your cool car.
Classic Restoration Enterprises, Inc.
39 Transport Lane
Pine Island, NY 10969
Evans Cooling Systems
496 Fricks Lock Road
Pottstown, PA 19465