A Cooling System Is Only as Good as Its Weakest Link

Photography by Carl Heideman

We’ve all been there, blissfully cruising along in our classics only to come to a halt thanks to a steaming, hissing radiator. Once the car stops rolling, the next step is usually to pop the hood and blame something beneath it. 

In reality, the blame lies with us owners. A cooling system is fairly simple in terms of cause and effect, but allowing just one link in the chain to falter can lead to problems. 

When it’s time to troubleshoot or improve a cooling system, many people misunderstand the basic thermodynamic principles involved and waste time, money or some combination of both. We’re here to help.

Hot Topic

Before you can think about keeping your engine cool, you need to know why it gets hot in the first place. One is directly correlated to the other.

Our cars run on the Otto thermodynamic cycle, described and developed in the 19th century by German inventor Nikolaus Otto. The Otto cycle is perhaps best known for describing the action of a four-stroke engine. Lesser known, but germane to this discussion, is where the heat of combustion goes.

The good news is that engines turn heat into power. The bad news is that only about one-third of that heat turns into power at the crank. Another third of the heat goes straight out the exhaust pipe. The last third goes into the cooling system. 

Engineers and mechanics have been working for more than a century to make engines more efficient than this, but the truth is that they’ve only made slight progress. Some of the most efficient engines out there today don’t even send 40 percent of their heat and power to the crankshaft.

Now let’s think a little further: How much power and corresponding heat does a typical car generate? We’re going to use a basically stock 93-horsepower MGB as our working example. 

Whatever your car’s power output, it’s not making peak horsepower all of the time, so it doesn’t always need maximum cooling capacity. In the case of our 93-horsepower MGB, engine output starts near zero and builds to its maximum power level as the engine speeds increase. 

We put data-acquisition equipment on the MGB and found that during aggressive street driving, the car spends about 30 percent of its time at or near idle thanks to stoplights, traffic and coasting to a halt. The car spends about 60 percent of its time at cruise, with only the remaining 10 percent of its time under acceleration or load. 

While it seems obvious to assume we have a 93-horsepower MGB all the time, the truth is that we only have that 93 horsepower for short bursts of time—at high engine speeds and under hard acceleration. The rest of the time we have a pretty low-horsepower engine.

Some more figures to know: It only takes about 5 horsepower to keep the engine idling. When the car is cruising on a level surface, it only needs about 15 to 20 horsepower to keep rolling—that’s why your foot doesn’t have to push the accelerator pedal very far.

The accelerator pedal could also be called the “horsepower pedal” or “heat pedal,” as it limits the peak potential power (and heat) the engine makes. This actually makes our MGB more efficient, as it only uses the power it needs for the current operation. Instead of wasting 93 horsepower of fuel, wear and heat when idling, it only uses 5 horsepower. And instead of wasting 93 horsepower just to cruise along, it only uses 15 to 20. 

When we mash the pedal to accelerate, we’re still not making our full 93 horsepower—at least not right away. We’re making power that is somewhat proportional to the engine’s speed, as shown in our horsepower curve. For example, at 2500 rpm under load, we’re making 50 horsepower. Moving up the tachometer, at 3500 rpm we’re making 75 horsepower, while our MGB’s peak 93 horsepower comes at 5000 rpm. 

Now, let’s take our 93-horsepower MGB and port the cylinder head, increase the compression ratio and install a more aggressive camshaft. These upgrades will increase the car’s horsepower to 115. 

Will it need a bigger radiator? Probably not. If the car is driven in the same manner as before, it will still only need about 5 horsepower to idle and about 15 to 20 to cruise. The engine will produce more power and heat, but only under brief periods of acceleration. Most radiators can absorb this incremental increase in heat. 

Let’s say someone convinced us to switch to an electric fan since the engine fans usually rob some power. After the change, we’re suddenly running very hot at idle. Is it because of the power upgrade? No, the Otto thermodynamic cycle tells us we can’t be making any more power and heat at idle. The problem must lie with the fan. If it’s not pulling its weight—or, more appropriately, pulling enough air through the radiator—then the car will run hot at idle. 

Understanding how the system works can make for quick detective work when problems arise. Now let’s get a little deeper into the subject and discuss the basics of our cooling system.

A Collection of Parts

A cooling system is made of a radiator, water pump, thermostat, coolant, some hoses to tie everything together and usually a fan or two. Each of these components must be sized and matched to each other and their applications. If there’s a failure in the system, you could find yourself on the side of the road.

The factory installed a shroud inside the Triumph TR6’s engine compartment to direct all the incoming air through the radiator. Air that’s allowed to bypass the radiator doesn’t help cool the engine.


Radiators come in two basic styles. Older vehicles use down-flow radiators, where the coolant flows from the top of the radiator to the bottom. Newer vehicles tend to use cross-flow radiators, where the coolant enters on one side and exits out the other. These radiators are often wider than they are tall, and they’re usually more efficient than their down-flow counterparts. 

The most common radiator materials used in older cars are copper and brass. The cores are made of copper due to its excellent conductivity, while the tanks are made of brass since copper would quickly harden and break. Unfortunately, brass and copper radiators weigh quite a bit. 

Aluminum radiators weigh less, but they are technically not as efficient—aluminum has a lower heat transfer rate than copper. However, a well-engineered aluminum radiator will often outperform an OEM brass and copper unit.

A third style of radiator has found its way into a large part of today’s production cars, including some of our classics from the late ’70s and early ’80s like the VW Rabbit and Mazda RX-7. Japanese manufacturers pioneered compact, high-efficiency radiators. They usually have aluminum cores fitted with efficient, tightly spaced fins and tubes mated to plastic tanks.

Radiator Cap:

No matter what the radiator construction, the cap does more than just plug the top of the unit. It also pressurizes the cooling system in order to raise the boiling point of the coolant. For every pound of pressure the radiator cap holds, the boiling point is raised 3 degrees Fahrenheit. 

Our newer classics have caps in the 10 to 15 psi range, thus raising the boiling point 30 to 45 degrees. Older cars often have lower pressurized systems—4 to 6 psi is common. Really old cars have open systems—no pressure at all—which don’t benefit from an increased boiling point.

Water Pump:

Water pumps are often driven by the crankshaft via a belt and pulley. Most water pumps are cast from iron or aluminum and will have cast or stamped impellers. When they fail, they either leak at their seals, wobble at their bearings or both. 


The coolant found in the radiator and cooling system is usually a 50/50 mix of water and ethylene-glycol, the latter commonly known as antifreeze or engine coolant. Coolant technology has greatly improved in recent years, mainly in corrosion resistance and environmental friendliness. Antifreeze is a misnomer, as this all-important liquid both lowers the freezing point and raises the boiling point of the coolant. 

Always maintain a 50/50 mix of water and coolant. Too little coolant can lead to freezing and overheating, as well as corrosion problems. Too much usually leads to overheating. 

Coolant boosters like Red Line WaterWetter, DEI Radiator Relief and Royal Purple Purple Ice also exist, and these products lower coolant temperatures by reducing surface tension. Some are also designed to reduce corrosion and lubricate the water pump. (Since many racing regulations prohibit cars from running coolant—it can cause a very slippery mess should any land on the track—a coolant booster plus water is a popular cooling system recipe.)

Don’t use these products to try to cure other problems. Remember, you need to treat the cause, not the symptom. If your cooling system passes inspection, then these products usually make it work a little better. If your cooling system has a leak, a plugged radiator or a similar issue, then you have other problems to fix.


Thermostats regulate the flow of coolant through the engine and radiator. A cylinder filled with expanding wax pellets causes a piston found inside the thermostat to open at a predetermined point—commonly 160, 180 or 195 degrees Fahrenheit. 

A thermostat stays closed at warm-up to bring the engine up to its ideal operating temperature as quickly as possible. Once the engine is warmed up, the thermostat opens to regulate the flow of water out of the engine and through the radiator. The thermostat ensures that the warm coolant cools off sufficiently in the radiator before it heads back into the engine. When the thermostat is closed, water recirculates within the engine through a bypass line in the thermostat housing.

A common trick, especially on race cars, is to remove the thermostat. This trick is rarely beneficial and is not recommended since a properly operating thermostat does nothing but help performance. We highly recommend that you keep your thermostat. However, if you must remove it, it’s important to replace it with a restrictor, a component that resembles a large washer. The restrictor slows the flow of coolant and helps to control the foaming that occurs when a thermostat housing is left unrestricted.


The radiator doesn’t do much good unless there is air flowing through it. To help, most cars have a fan or two.

Fans are powered by either the engine or an electric motor, and each type has its advantages and disadvantages. Engine-driven fans generally move more air through the radiator, but they can be guilty of robbing small amounts of power and running inefficiently at low engine speeds, like idle. 

Electric fans can be mounted forward or aft the radiator. Front-mounted fans are called pusher fans, while rear-mounted fans are called pullers. Puller fans are better since they block less airflow than most pushers, but they’re often tougher to mount because of engine clearance issues. Generally, electric fans are more helpful for cooling an engine at lower speeds, while engine-driven fans work better at higher speeds.


A few sundries complete the cooling system. Many systems have shrouds around the fans to better direct airflow. Cars with engine-driven fans often have a fan clutch to prevent parasitic loss at higher engine speeds. Cars with electric fans usually have a thermostatic switch so the fan will only turn on when it’s needed. 

Expansion tanks are common additions. They catch the small amount of normal overflow that results when coolant expands as it’s heated. These tanks allow the radiator to recover this overflow when it cools down. Of course, street cars usually have a heater and at least one heater valve to regulate flow.

Troubleshooting the System

Now that we’ve covered the basics of how a cooling system works, let’s start the troubleshooting process. There are two tools that come in handy when troubleshooting. 

The first tool is a digital infrared thermometer, and you can buy a nice one for about $30. The other useful tool is a pressure tester—prices start at $75, but some auto parts stores have loaners available. You can probably get by without the pressure tester, but you’ll never regret buying the thermometer thanks to its wide range of uses.

The first thing most people blame when there’s a cooling problem is the thermostat, followed quickly by the radiator. While these are good places to inspect, there are some other basic components to check out first.

Step one is to determine if the engine is really overheating. Don’t just trust the gauge, which can often be inaccurate. Confirm its accuracy by checking various locations in the drivetrain with your infrared thermometer. Good spots to check are the top and bottom of the radiator, the cylinder head, the block and the thermostat opening. As a rule of thumb, if an engine is not boiling over—meaning it’s not puking onto the ground—it’s not overheating. 

The digital infrared thermometer is your best friend for troubleshooting cooling system issues. Here we’re checking the temperature of the engine block as it warms up. You can buy a digital infrared thermometer for about $30.

Many people worry that their car is overheating if the coolant temperature goes above 190 or 200 degrees. While this can be discomforting, it takes about 250 degrees to start damaging an engine. Most classic cars normally run at around 180 degrees, but temperatures 10 or 20 degrees higher do not necessarily mean there is a problem. In fact, engines are technically more efficient at higher temperatures—that’s why many modern cars run above 200 degrees. As long as the car is not boiling over, a few extra degrees could actually be helping. 

When it comes to coolant temperatures, the most important thing to keep in mind is consistency. If the car normally runs around 200 and is not boiling over, it’s probably nothing to worry about. However, if the car usually runs around 180 but is hitting 210 on hot days and at idle, there might be an issue.

Once you’ve determined that there is, in fact, a problem, ensure that the cooling system is absolutely full with a 50/50 mix of water and coolant. If the system is not full, air pockets can cause flow problems, hold steam or both. When filling a cooling system, make sure the heater valve is open so you’re filling the entire system. Then squeeze the upper hose to feel that there is coolant inside of it. 

Now check the coolant. It should be green, not brown, and a hydrometer will tell you its general condition. If the coolant fails the test, dispose of it responsibly (animals find coolant tasty yet lethal) and replace it. 

The next step might sound obvious, but it’s important: Check for leaks in the system, no matter how small. Even a pinhole leak causes two different problems: First, the system won’t stay full. Second, the leak will not allow the system to build pressure, reducing the boiling point significantly. 

If you can’t find a leak visually, use your pressure tester to pressurize the system. This should help locate the leak. If that doesn’t work, it might be time to perform a leakdown test and see if coolant is getting into the engine via blown gaskets or a warped or cracked head.

The engine must also be in a good state of tune. Lean fuel mixtures and over-retarded or over-advanced timing are notorious for causing overheating, but any poor running condition can exacerbate cooling problems.

Coolant cools better than steam. Squeezing the upper radiator hose is an effective and easy way to make sure the system is full.

Next, check or replace the thermostat. Start the engine cold and use your infrared thermometer to monitor the temperature of the block and thermostat housing. 

As long as the thermostat is closed, the thermostat housing will stay at or slightly above air temperature. Meanwhile, the block will warm up steadily. If the thermostat is working properly, the temperature of the thermostat housing and the block will be equal when the thermostat opens. In other words, when the block hits 180 degrees, the thermostat will open and the thermostat housing will go from 90 degrees to 180. And as we already discussed, don’t be tempted to remove the thermostat. If yours doesn’t pass muster, replace it. 

Look at airflow next. Before you blame the fan, make sure that the grille and radiator slats are clean. Bugs and other road trash, especially on radiator slats, can significantly inhibit airflow. A trip to the pay-’n’-spray car wash can quickly blow everything clean. 

Next, check to see that air can get out of the engine compartment. It’s just as important for air to escape the engine compartment as it is to get in. If you or anyone else has made modifications that prevent the air from leaving, that might be your problem.

A simple pressure tester can help diagnose cooling system leaks. Don’t want to buy one? Many auto parts stores have loaners available.

Now start thinking about the fan. If the car has its original fan (or fans) and they’re properly working, then the problem probably isn’t here. Engine-driven fans on our classics are always very robust and consistently do the job—that is, until they turn about 50 years old. By this age, it’s possible that their fans have become bent or out of balance. The fan rivets might also be loose. Periodically inspect your car’s fan—you don’t want a blade flying off.

Original equipment electric fans also tend to be very robust. As long as the fan’s electric thermostat is turning it off and on in spec and the motor is spinning the fan quickly enough, the fan is most likely working as intended.

Aftermarket electric fans are another story. We’ve rarely seen an aftermarket fan as efficient as its OEM counterpart. As a result, we’ve fixed scores of cooling problems by removing usually undersized aftermarket units and putting the OEM fans back in. If you feel that you must use an aftermarket fan, make sure to buy the biggest, best-flowing fan you can—the cheap ones just don’t do the job.

It looks trick, but this electric fan led to cooling problems. First, its mounting location at the front of the radiator was blocking airflow. Second, it didn’t move nearly as much air as the stock, engine-driven piece. 

Once you’ve checked each of these points, it’s finally time to suspect the radiator. With the engine at operating temperature, take your infrared thermometer and scan various parts of the radiator. Temperatures should be consistent with slightly higher numbers on the inlet end—a variance of 10 to 20 degrees from inlet to outlet is common. You might find spots where the radiator is at ambient air temperature. This is a sure sign of clogged tubes, meaning the radiator is not working at capacity. 

At this point, getting the radiator repaired, recored or replaced altogether might be your only solution. If you’re considering a new replacement, do some research first. Some of the replacements on the market are coming from Third World sources, and their quality isn’t that great. If that’s the case, getting your radiator recored is probably a better option.

You’ll note we haven’t spent much time on the water pump. It’s very rare for water pumps to be the problem, as long as they’re not leaking. Every now and then a water pump impeller will come loose. If that happens, you’ll have a pretty radical and obvious overheating problem that you should be able to diagnose with your infrared thermometer. The engine will get hot, the thermostat will open, but the radiator won’t warm up.

Cooling Runnings

Now that we’ve gone through theory, practice and diagnostics, you’ve hopefully gained knowledge that will help your car keep its cool. If you keep in mind the Otto thermodynamic cycle and understand what’s producing the heat and when, you’ll be much more skilled at preventing issues with your cooling system. And that means miles of trouble-free driving.

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