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The purpose of a thermostat is to reduce engine warm-up time, maintain optimal running temperature, and help with heater output, if so equipped.
When an engine (gas or diesel) is cold, a number of undesirable events occur. The wear rate of the internal parts is excessive due to poor lubrication and friction. Some older diesel farm engines experience 1,100% more piston ring wear when cold than when at normal operating temperature. When an engine is cold, the fuel vaporizes poorly, and this leads to dilution of the engine oil.
How a thermostat functions
A thermostat includes a flow orifice, a spring, and a means to open the flow path against spring pressure. At normal operating temperature, the flow orifice is open and coolant circulates through both the engine and the radiator.
There are two types of thermostats found in farm engines. They include the aneroid (sometimes called a bellows) and the modern hydrostatic, or wax poppet style. This describes the means used to allow flow to occur. There are subcategories that include twin thermostat systems (on larger diesel engines) and radiator bypass circuits (both gas and diesel).
The aneroid thermostat has not been used for quite some time but can be found in older farm engines. A vapor-metal bellows energizes this design. Coolant flow depends on the difference between the vapor pressure in the bellows at any given temperature and the pressure in the system.
The hydrostatic thermostat has been in use for the past 60 years. The flow path with this thermostat is opened against spring pressure by an element charged with a wax substance having a high coefficient of thermal expansion. The element consists of a cylindrical metal body containing the wax substance (often called a pill) that surrounds a rubber insert. This, in turn, embraces a central operating thrust pushrod.
As the coolant temperature rises, the wax melts and compresses the rubber insert. Because the rubber acts like an incompressible hydraulic fluid, it moves the thrust pushrod. The thermostat is now opened against spring pressure, and coolant flow begins.
A twin thermostat system is used on some large diesel engines to help maintain proper temperature under normal load conditions. At very high thermal stress, it opens an additional flow path to the radiator.
A radiator bypass thermostat forces all coolant through the radiator when the engine is at operating temperature. This is in contrast to a design that allows a small amount of coolant to bypass the radiator when the engine is hot. This style is identified by an additional leg and disk.
Every thermostat is marked with a temperature rating. Contrary to what most believe, this is not the temperature at which the thermostat is fully open.
Instead, it is the temperature at which the thermostat begins to open and flow starts. It takes approximately an additional 20°F. for the thermostat to be fully open.
A thermostat rated at 180°F. will crack open at that temperature, but it won’t be fully open until approximately 200°F. Thermostat control of the engine temperature begins at crack open. It will modulate the flow up to the point it is fully open and the maximum amount of coolant goes to the radiator.
When to replace
A better thermostat will have either a jiggle pin or calibrated bleed hole(s) to permit deaeration during service fill and engine operation.
If the wax motor in the thermostat fails or becomes weak, the spring pressure will close the flow path, and the engine will overheat quickly. A wax motor that is weak will allow the thermostat to modulate when it should not, and the engine temperature (shown on the gauge) will see broad swings. It will respond as if the cooling system were air bound. This thermostat should be replaced.
A weak spring may allow the thermostat to lose control of the engine temperature. Depending on its location in the cooling system (suction or pressure side), the thermostat can open prematurely as engine speed increases, resulting in a drop in coolant temperature. Or, it can close slightly, elevating coolant readings. This is due to the increase in hydraulic force from a higher engine speed.
The wax motor in the thermostat can be damaged from excessive temperatures, as experienced by a ruptured cooling system hose or improper bleed procedure during system work. Many engines have had a water pump or hose changed. A thermostat can be ruined at this time due to poor bleeding during refill.
A thermostat is considered a consumable and should be replaced when major cooling system service is performed. If the coolant temperature is acting strangely, the first step is to replace the thermostat. It is the key to coolant flow in the engine.