Understanding a cooling system thermostat

The purpose of a thermostat is to reduce engine warm-up time, maintain optimum 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 then when at normal operating temperature. When an engine is cold the fuel vaporizes poorly and this leads to dilution of the engine oil. Also sludge forms in the crankcase due to a build up of condensation. Thus, the goal is to get an engine to operating temperature quickly. A thermostat makes this possible.

Thermostat function

The thermostat sits in the liquid coolant and is used to completely block or with some engines limit coolant flow to the radiator when cold. Since there is little or no flow to the radiator the coolant in the block warms-up faster due to a lack of heat rejection.

A thermostat includes a flow orifice, 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: the aneroid that is 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 (larger diesel engines) and radiator bypass circuits (both gas and diesel).

The aneroid thermostat has not been used for quite sometime 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 cooling system.

The hydrostatic thermostat is what most will be familiar with and has been in use for the past 60 years.

The flow path 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; but since the rubber acts like an incompressible

hydraulic fluid it moves the thrust push rod. 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 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.

Important facts

Every thermostat is marked with a temperature rating on it. Contrary to what most believe it is not the coolant temperature the thermostat is fully open or the engine will run at. It is the temperature that it begins to open. This is when flow starts. It takes approximately 20 degrees F more temperature for the thermostat to be fully open.  A thermostat rated at 180 degrees F will crack open at that coolant temperature but will not be fully open until approximately 200 degrees 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.

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 very quickly.

A wax motor that is weak will allow the thermostat to modulate when it should not and the engine temperature as read on the gauge will see broad swings. It will respond as if the cooling system is air bound. This thermostat should be replaced since it is ready to fail.

A weak spring may allow the thermostat to loose control of the engine temperature. Depending on its location in the cooling system (suction or pressure side), as engine rpm is increased the thermostat can open prematurely resulting in a drop in coolant temperature or 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 temperature 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 and due to poor bleeding during refill ruined a good thermostat. This confounded the farmer with an overheat condition right after a repair.

A thermostat is considered a consumable and should be replaced when major cooling system service/work is performed.