Cooling Systems

by Bob Cerullo
Illustrations by Russell Von Sauers

POPULAR  SCIENCE
 (Reprinted with permission from POPULAR SCIENCE, July 1983)

Tips from a pro: trouble-shooting High-pressure, high-temperature cooling systems

 What you can do

I didn’t have to look hard at the Olds V8 brought into my diagnostic center to see that a great deal of work had already been done to solve a stubborn overheating problem. The water pump, radiator cap and core, thermostat, hoses, and cooling-fan clutch had been replaced. But even with all those new components, the temperature gauge reading would begin to soar after an hour’s highway cruising.

There were no external leaks, and the engine was properly tuned. Everything checked out okay, but the coolant was disappearing. It had to be going somewhere.

I had a hunch. I placed the probe end of an exhaust-gas analyzer near the uncapped radiator-filler neck, with the engine warm and idling. Normally, this device is used to test carburetor adjustment and emissions controls by monitoring the hydrocarbon and carbon monoxide levels at the exhaust pipe. In just a few seconds, the needle in the hydrocarbon gauge of the emissions analyzer started to bump and dance wildly.

I was looking at a serious problem. The analyzer was indicating the presence of exhaust gases in the radiator—a sure sign of a leaking cylinder-head gasket, a cracked head, or, even worse, a cracked engine block. I pulled the dipstick and examined the oil. It had a milky-white appearance, confirming the analyzer’s diagnosis. It was time to pull the head.

Cooling-system leaks aren’t usually this difficult to locate, not does diagnosis require expensive equipment. Most coolant leaks are the result of a ruptured hose, a punctured radiator, or failure of a water-pump seal. So I’ll discuss how the cooling system works, how to diagnose some of the more unusual high-pressure cooling-system problems, and how to head off costly cooling-system repairs.

Components

The radiator cap is a pressure-relief valve designed to maintain a pressure of roughly 15psi. The system is pressurized to raise the boiling temperature of the coolant. For each pound increase in pressure, the boiling temperature goes up by three degrees F.

The boiling point is raised further with ethylene glycol-base antifreeze in a 50-50 concentration. A concentration greater than 60 percent is not only uneconomical but reduces cooling-system efficiency, since water can absorb more heat per gallon than any other practical coolant. In fact, at the maximum concentration, the coolant will absorb only 85 percent as much heat as water. The pressurized system and a 50-50 concentration of antifreeze combine to bring the boiling point of the coolant to 265 degrees F.

Even with all this protection, systems sometimes overheat. And increased temperature causes increased pressure. This can force some coolant out through the overflow tube in the radiator-filler neck to a reservoir. As the system cools and the pressure is reduced, a vacuum is produced. This draws the vented coolant back into the system.

A small but important part of the cooling system is the thermostat. Its job is to get the engine warmed up as quickly as possible by restricting the flow of coolant until a preset temperature is reached.

Although it’s important to keep the engine from overheating, it is equally important to prevent over-cooling. Over-cooling encourages the formation of acids and sludge in the oil. It also tends to reduce combustion-chamber temperatures by quenching the flame front. This results in higher emissions and greatly reduced fuel efficiency, because much of the fuel remains unburned. That’s why it is never a good idea to substitute a thermostat with a lower opening temperature than that of the one you are replacing.

After the thermostat opens, the coolant circulates between engine and radiator, maintaining a temperature between 190 and 210 degrees F. It picks up a normal increase of 15 degrees as it passes through the water jacket surrounding the combustion chambers and cylinders.

The most easily recognized part of the cooling system is the radiator. Its job is to transfer heat from the coolant to the outside air. Most radiators are made of brass, copper, or aluminum because of the corrosion resistance and heat-transfer ability of these metals. The heat-transfer capacity is based on the number of fins per inch and the thickness, number, width, and height of the coolant-carrying tubes.

In a departure from convention, some companies have been making radiators with nylon end tanks and either an aluminum or copper-brass core. The tanks are attached to the core by header tabs bent over the edges of the tank—tabs similar to those used to seal the lids on pails of roofing tar.

The water pump is the heart of the cooling system. It will pump as much as 4,000 gallons per hour in a V8 engine to maintain the delicate balance between an over-cooled and an over-heated engine. When it’s cold outside, it must also push hot coolant through the heater core to provide a comfortably warm passenger compartment.

Diagnosis

Leaks are the most common form of cooling-system problem. The best time to catch one is just after you shut down the engine. While the engine is running, coolant temperature and pressure are kept below maximum by radiator cooling. But just after shutdown, the higher temperature and pressure are just the ticket for revealing hard-to-locate leaks.

The cooling-system-pressure tester is a special tool for finding elusive leaks. It looks like a small bicycle pump with a pressure gauge on the side, and it’s installed on the radiator neck in place of the cap. You just pump up the system to the maximum specified pressure on the cap, then look for leaks (see illustration).

A third method for finding external leakage is to apply low-pressure air to the cooling system through a drain cock or vent fitting in either the engine or radiator. (Note: Don’t attempt this procedure without a pressure regulator and gauge or you might apply more than the recommended system pressure.)

The condition of the coolant can tell you a lot about your cooling system’s health. A reddish foam in the radiator is a tip-off to a transmission-cooler leak that’s allowing transmission fluid to enter the radiator.

Oil floating on top of the coolant is a sure sign of internal leakage. If oil is entering the radiator, coolant is probably leaking into the engine. If this is the case, you’ll also probably spot steam escaping from your exhaust pipe.

If you suspect that exhaust gases may be entering the coolant but don’t have access to an emissions analyzer as I did with the Olds, here’s a simple test you can do: Start by removing the fan belts, then remove the thermostat and reinstall the thermostat housing without the upper hose. Next, fill the thermostat housing with water until the water spills out. Start the engine and check the water in the thermostat housing for bubbles. Remember, you’ve removed the belts, so the water pump isn’t turning; there’s no flow of coolant. In a short time the engine will overheat. However, you’ll have sufficient time to accelerate the engine six or eight times while watching the housing’s outlet for bubbles, indicating combustion leakage into the cooling system.

A heavily rusted cooling system—one in which the coolant looks more like chocolate soda than radiator coolant—may be caused by the rapid deterioration of rust inhibitors. This often occurs when exhaust gases enter the cooling system.

Reduced cooling-system efficiency may be as simple as a loose belt. Fan-belt checks are fairly easy to make, but be sure you twist the belt to view the underside, where cracks develop first.

What about hoses? Squeezing them may seem like a relatively accurate way to judge their condition, but since hoses wear from the inside out, it may not be your best bet. It’s best to simply replace the hoses every few years. Certainly by the time you have 35,000 miles on your car all the hoses should have been replaced.

A bad thermostat, like a bad hose, is difficult to spot. If your engine is operating cold, the thermostat is probably stuck open and will have to be replaced. To check your thermostat, fill a container with water. Insert a feeler gauge into the thermostat’s opening. Suspend the thermostat in the container by one end of the feeler gauge. Position a thermometer in the water bath and heat the container. The water bath is heated until the thermostat slips off the feeler gauge. The temperature recorded when the thermostat falls is the opening temperature. It should be within five degrees F above or below the temperature stamped on the thermostat’s body.

Bearing and seal failures account for most water-pump problems. Bearings are easily diagnosed by applying hand pressure to the end of the pump shaft: There should be little or no side play in healthy bearings. Seal failures are even easier to spot. Simply look for any leakage at the pump’s vent hole when the engine is running or stopped.

Now for the fan. Air is forced across the radiator core at low speeds by a cooling fan. At high speeds air is pushed through the radiator by the forward motion of the car. For this reason, viscous fan drives were developed to reduce the speed of the fan at higher engine speeds. This both reduces fan noise and conserves energy. The clutch allows the fan to free-wheel when the engine is cool. A fan clutch must engage when the engine is hot. You can tell if it’s doing its job by listening to the fan noise on a cold engine. As the engine warms up, you should hear a distinct roar when the fan clutch engages. If you don’t hear anything, you may have a defective fan clutch.

Maintenance

Every cooling system should be pressure-tested at least once a year, using a cooling-system-pressure tester. After the first two years the cooling system should be flushed annually and a fresh fill of antifreeze installed. An effective way to do this is to install a special garden-hose adapter, available at any auto-parts store, in one of the heater hoses. With the heater controls in the “on” position, attach the garden hose to the adapter and remove the radiator cap. Run the water until the flow from the filler neck is clear.

Inspect the interior of the radiator’s top tank through the filler neck for signs of scaling or excessive rusting. If the system has been neglected for a long period of time, you may want to remove the coolant and fill the system with a flushing solution. Run the engine for at least 30 minutes, and drain the solution. Then add a neutralizer, fill the system with water, and run the engine for five minutes. Finally, drain the neutralizer solution completely and fill the system with fresh water. Run the engine for five minutes and drain. Refill with the proper antifreeze solution. (Note: Make sure the flushing solution is compatible with your car’s radiator-core material.)

The volume of your cooling system is specified in the owner’s manual. A chart, usually located on the antifreeze container, will tell you how much is needed for cold-weather protection desired. Belts have a habit of breaking at the wrong time. They stretch, wear and crack. If a belt is loose, it may not drive the water pump properly. A belt tension gauge is the only reliable method of checking belt tension. The best maintenance is to check belts frequently and to replace them at regular intervals. Throw the old belts in the trunk. They may serve well if one of the replacement belts gives up sooner than expected.

Most new-car water pumps don’t require any maintenance or lubrication. However, the drive-belt tension should be checked carefully. Too little tension won’t turn the pump, and too much will cause premature failure of the bearings.

THE COOLING SYSTEM

Internal-combustion engines are unable to convert all the energy produced by their fuel into power. Roughly a third of the heat generated by an automobile’s engine must be dissipated to the outside air. That is the job of the cooling system. To perform this essential function, the system must be clean and tight, and it must permit the free flow of coolant under specified temperature and pressure.

The cooling system removes the heat from the engine by circulating coolant through passages surrounding the cylinders and combustion chamber. The heat is dissipated by circulating the heated coolant through the radiator, where the heat is transferred to the walls of the core tubes. The heat passes into the fins bonded to the tubes. Air passing over the fins, as well as convection, removes the heat from the system.

There are basically two types of automotive-radiator designs: cross-flow design, heated coolant from the engine goes into an end tank on the top of the core and flows down through the core tubes into the lower tank.  In the cross-flow type, hot coolant goes into a tank at one side of the core and flows across (horizontally) into another tank. One of the tanks has a filler neck with a pressure cap. A spring-loaded valve in the cap is set to open at a preset pressure. This prevents damage to the cooling system in case excessively high temperatures raise the system’s internal pressure. The excess pressure generally forces some coolant out of the system through a vent. A hose emptying into a plastic reservoir recovers the displaced coolant.

Both system types use a coolant fan, which is normally attached to the water pump shaft. The fan moves large volumes of air through the core at low road speeds when relatively little air is being forced through the core. The water pump circulates the coolant through the cooling jackets of the engine. The rate of flow is governed by three factors: the size of the pump pulley, the size of the impeller, and engine speed. The heater core is an often overlooked part of the cooling system. Essentially, it’s a smaller version of the radiator at the front of vehicle, but it’s located in front of the passenger compartment. Engine coolant is pushed into the core, and an electric fan forces air through the heater core and into the passenger cabin to warm the vehicle. (Note: If your car overheats in traffic, you can sometimes increase engine-cooling capacity by turning on your heater.)