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August 2011 Willie's Workbench

Posted in Features on August 1, 2011
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Have you ever heard someone say, “Take the thermostat out and the engine will run cooler,” or “You don’t really need a thermostat; it’s just something else to go wrong”?

Both of these statements are totally incorrect. Let’s go through a checklist of the thermostat’s purposes and discuss the hows and whys of its operation. A thermostat:
->Provides an adequate temperature for heater operation in cold weather
->Promotes quicker engine operating temperatures
->Provides for an even engine operating temperature
->Increases durability and engine life
->Increases fuel mileage
->Lowers emissions output

The modern engine is designed to operate in a relatively warm temperature range: 180 to 220 degrees Fahrenheit. Late-models with computer-controlled emissions equipment are designed to function best at the high end of this scale. In reality, the thermostat is really nothing more than a simple water control valve that’s activated by changes in water temperature; the thermostat operates on a bellows-type arrangement that is connected directly to a disc that opens and closes.

What’s important is that all thermostats that are marketed for a certain temperature aren’t all the same. Let’s say that you’ve picked one for 180 degrees. It should start to open at around 160 degrees and be completely opened at the 180-degree mark. Another brand may not start opening until it reaches 180 degrees and then be fully opened at 200 degrees. With a quality product, you will find this information printed on the box or on the information sheet inside. If there is any doubt, you can check for correct operation by placing the thermostat in a pan of water and put it on the stove. Turn on the heat and insert a thermometer in the water. Watch the thermostat as the water temperature rises. When it starts to open, note the temperature then and when it is fully opened. Yep, water boils at sea level at 212 degrees Fahrenheit, so you’re not going to get to 220 degrees.

The thermostat actually does two jobs that are directly related to each other. Some people will say that it restricts the flow so that coolant stays in the radiator longer to provide more heat exchange. That’s not exactly true. One of the reasons we use a thermostat—or, in race engines, a restrictor—is to set up a pressure differential to decrease “cavitation,” which occurs when small air-pressure bubbles develop and then “pop.” They actually explode with enough pressure to pit metal. Some engines, such as the 7.3L Ford diesel, are very prone to this. Second, think about how small the water passageways in the engine are. That means that there is, in reality, very little water in actual contact with the engine’s metal surfaces, so it can get really hot quite fast. If it stays in one spot too long, it can exceed its vaporization temperature and actually flash to steam, which then has no cooling effect and leads to dangerous and damaging hot spots.

Moving the water from the engine block into the radiator is quite important. That is why we see so many performance aftermarket water pumps available. The faster the hot water gets out of the block and into the radiator, the better chance it has of keeping the engine cool. Then again, proper radiator sizing is also important because if the radiator does not have enough square inches of surface for proper heat dispersement, you’re fighting a losing battle.

As we said, most engines are designed to run between 180 and 220 degrees. This provides for the proper thermal expansion to maintain the proper internal clearances. Drivability and fuel mileage are enhanced through better fuel vaporization. A too-cold running engine will actually lose horsepower as combustion temperature is lost. Remember, an engine is nothing more than a heat pump; the hotter the expanding gases are, the more cylinder pressure will develop.

Another problem with an engine that runs too cold is non-vaporized fuel leaking past the piston rings and polluting the oil. Low water temperature also means low oil temperature and resulting moisture condensation, which can lead to a multitude of lubricating problems—one of which is the buildup of various acids and sludge.

For a street vehicle or off-road vehicle, a thermostat is not only a useful temperature control device, but it’s a necessary one. So keep that thermostat where it belongs—in your engine.

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