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Vol. 2, No. 14
June 12, 2004

Car Overheating?
by The Automotive Mileposts Staff

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Image: Troubleshooting/Tech Tips Series

Every year, at about the time the weather starts warming up, it seems we start to receive inquiries about how to prevent a car from overheating. And each year, we have people tell us what they have done to attempt to correct the problem. Some are satisfied that the problem is corrected, while others are still suffering with a car that loses its cool. And some of the things people do to implement a fix surprise us, because they are actually creating more problems for themselves. While the immediate concern, overheating, seems to be resolved, there are often more serious hidden issues developing.

Some of the "fixes" to correct overheating people have told us about include installing a lower temperature thermostat, removing the thermostat completely, running straight water in the system, adding after market overflow tanks to prevent coolant loss from boil over, and adding electric fans to improve air flow across the radiator.

Depending on when your car was designed and built, its designed in ability to cool itself can vary. Cars of the fifties and earlier were prone to overheating, especially in hot weather and when the engine is under a heavy load. Back then, drivers were aware of the limitations of cooling systems, and knew what to do to reduce the likelihood of overheating. It was not unusual to see cars sitting at the side of the road, with their hoods up, its occupants outside fanning themselves and waiting for the engine to cool down. This was a limitation of the design, and that's just the way it was.

When larger, more powerful engines became popular, which was also about the time that factory air conditioning was available on a mass production basis, engineers improved automobile cooling systems to handle the additional heat. They weren't always entirely successful, and under some conditions a properly working cooling system could still overheat. It didn't take long for the engineering department to respond to demands from dealers for improved cooling systems, and within a few years overheated cars were becoming a rarity, even in hot desert climates.

Most of the cars built since the mid sixties are able to keep their cool, even in today's heavier traffic, without a problem. Many built in the early sixties and late fifties are also fine in the harsher environment cars are subjected to today. We spend a lot more time idling today than we did decades ago, which taxes the cooling system to a greater degree. Still, an old car with a properly functioning cooling system should be able to handle most conditions today without too many problems.

It's important to remember that when these cars were new, they were exposed to the same high temperature idling that they are today. Perhaps they didn't have to idle as long, but they were designed to keep their cool in Palm Springs in the summer, when temperatures are well over the century mark most of the time. And there's no reason why those same cars today shouldn't be able to operate under severe conditions.

All too often, when an overheating problem presents itself, owners attempt a band aid fix, rather than trying to find the source of the problem. Fixes such as installing a lower temperature thermostat or removing the thermostat entirely are not wise ones, and are not recommended. When the engineers designed these cars, they did so with the object of it being able to operate in the frigid winter temperatures up north, as well as the sizzling summer temps down south and in the desert. They planned for high humidity, extended idling, pulling trailers, etc. A reserve was built in to the design to handle these situations.

The cooling system capacity, the size of the radiator, the number of cooling fins, the design of the fan, the size of the pulleys, the water pump flow, thermostat temperature range, etc., were all considered and designed to work together. When any of these things are changed, the characteristics of the cooling system as a whole are changed, and it can no longer operate as expected.

Here's a list of repairs that should not be attempted, and why:

1. Installing a lower temperature thermostat. This is a bad choice because lower engine temperatures allow sludge to build up in the engine. As part of the combustion process, moisture is created. This coupled with lower temperatures and short trips that don't allow the engine time to burn off all the moisture can cause sludge to build up. Sludge can clog oil pumps, oil filters, oil passageways in the engine, and do considerable damage. The by-products of the combustion process create acids that can be corrosive to the engine. You need heat and time to filter out and burn off these by-products. A lower temperature thermostat than specified might not let the engine get hot enough to remove these by-products.

2. Removing the thermostat. This is a really bad choice. In addition to the issues mentioned above, this also allows the coolant to flow too quickly through the system. The thermostat acts as a dam, so to speak. It allows the coolant in the engine to warm up to a certain temperature before it begins to open, and when it starts opening, it does so gradually, and when fully open acts as a regulator to control how quickly the water pump moves the coolant through the radiator and engine block. If the coolant moves too fast, it doesn't absorb as much heat in the engine, nor does it dissipate as much heat in the radiator. The longer the coolant takes to circulate through these areas, the more heat it is able to take on and give off. While the car may appear to run cooler after removing the thermostat, most owners find under severe operating conditions the car gets hotter than ever.

3. Running straight water. Another really horrible idea. The thought process here is that water actually cools better than a mixture of coolant/antifreeze and water. This is really not true. Plain water will corrode a radiator and engine block in no time. If you enjoy replacing freeze or casting plugs, go ahead and run water through your cooling system, it won't take long to rust them out, unless you've installed brass ones. But the inside of your radiator and engine block are rusting faster than you can imagine. And in the winter months, can you say cracked block? Water freezes when the temps drop to near freezing, and unless you enjoy buying new engines, you don't want to take the chance.

Antifreeze/coolant lowers the freezing temperature of the liquid, protecting your engine block and radiator. At the same time, it raises the boiling point of the liquid, which allows a higher temperature threshold before boiling. Once coolant starts to boil, there is no protection. It also has anti-corrosive agents to prevent rust, scale, and corrosion. These agents do break down over time, and the ph level of the coolant should be checked annually to make sure there's still protection. Otherwise clean coolant can have new anti-corrosive agents added to maintain the proper ph levels, this is good for older cars that don't have a lot of miles put on them.

Always follow the manufacturer's recommendations for the proper antifreeze/coolant-water mix.

4. Adding after market overflow tanks. These are normally not necessary if everything else is up to par. If overflow tanks were needed, the manufacturer would have installed them. There is a reason coolant is being discharged from the overflow tube, you need to find out why. Perhaps the coolant level is too high, not allowing enough room for expansion when the liquid heats up. There could be an air pocket in the system somewhere as well. A restriction in the block or radiator can also cause higher than normal pressures in part of the system, which can lead to a discharge when the engine is shut down hot.

If the coolant looks clean, and the cooling system has been flushed recently, check the radiator cap. If it isn't of the correct pressure range, or defective, it can cause overflow. Never install a radiator cap that is a higher or lower pressure than specified by the manufacturer. Higher pressure does raise the boiling point of the coolant, but can also lead to leaks in parts not designed to handle that much pressure.

5. Adding electric fans. Again, if these were necessary the manufacturer would have installed them. If you need additional fans, and aren't operating the car under extreme conditions such as extended idling, or pulling a trailer or boat, you shouldn't need electric fans. Find the source of the problem, instead of trying to compensate for it in other ways. Electric fans create additional demands on old car charging systems, which might already be overloaded by after market stereo systems and other add-on devices.

The single most important item in your cooling system is the radiator. As the coolant passes through the radiator, air is pulled through the fins removing heat from the liquid. The liquid is then returned to the engine block to absorb more heat and then later return to the radiator to begin the cool down process again.

The fan pulls air through the radiator at idle and low engine speeds, and the forward motion of the car shoves air through the radiator at higher speeds. It's really a very efficient system, and works well when all components are working together properly.

If you have a clutch fan, check it for proper operation. When the engine is cool, spin the fan blade by hand. A resistance should be felt, and the fan should stop spinning rather quickly when you let go. If there is little resistance, or excessive resistance, or if the fan keeps spinning freely, the clutch is most likely bad, and should be replaced. A weak clutch will not allow the fan to pull as much air through the radiator, which will reduce the radiator's ability to remove heat from the coolant.

As cars age, scale and grime can build up in the cooling system. If the ph has been maintained properly, and the cooling system has been serviced and flushed regularly, these deposits should be minimal. But on cars that have sat for long periods of time, or been neglected in one way or another, sometimes even using a cooling system cleaner won't remove the deposits. At this point, your options are having your radiator recored or replaced.

The small passages in the radiator can become so clogged with deposits, that the coolant can't flow through it properly. When it's recored, it's returned to a like-new state, and its cooling efficiency is restored. Some radiators are in such bad shape that they should be replaced, and fortunately there are new replacements available for many makes and models.

Don't go another year watching the temperature gauge or light, worrying about overheating. Don't take the chance of destroying your head gasket or damaging other expensive components. There's no reason for you to put up with a hot car. Go back to the basics, and you'll soon discover the source of your problem. Your car was designed to keep its cool, it should be able to do that today just as well as it did when it was new.


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