In any automotive car, the engine produces heat as a by-product and this heat needs to be dissipated to prevent overheating of the engine and its internal components.
Thus the cooling system consisting of the water pump, radiator, thermostat and fan switch is required.
Though it is desired to keep the engine's working temperature down, that does not mean that the ambient engine operating temperature (assisted by the cooling system) should be as low as possible.
The engine optimum temperature is said to be within a range of 90 degrees celsius (with a plus or minus 5 degree accuracy) so as to achieve the optimum viscosity of the engine oil that is flowing within it. To achieve this working temperature range, the thermostat and fan switch is the critical components in play.
The thermostat exists along the coolant flow routes, usually mounted near to the engine head. It serves as a temperature valve that opens upon the coolant reaching a certain temperature and allows the coolant to flow in and out of the engine, cooling the block and head. Before reaching this temperature, the coolant will not flow into the engine and the engine will begin to warm up to its optimum temperature.
When the thermostat is closed, the coolant is still being moved around by the water pump, into the radiator and its hoses. And by moving into the radiator, the coolant is being cooled constantly. The moment the thermostat opens upon reaching its "open" temperature setting, the cooled coolant will start to enter the engine and hot coolant will exit, cooling down the engine in the process of the coolant exchange. And as the temperature goes lower, the thermostat closes and the whole cooling process repeats again.
However, there will be times where the vehicle is stationary or under hot ambient weather whereby the cooling efficiency of the radiator is not able to provide coolant that is cool enough to close the thermostat. The cooling system accumulates heat that is not dissipated fast enough by the radiator and engine temperature rises. Upon reaching a high threshold, the fan switch will kick in, turning on the radiator fan to forcefully blow air onto the radiator to cool down the coolant quicker.
Most car manufacturers defaults their thermostat and fan switch settings to be at 87 and 95 degrees celsius respectively. This is supposedly the optimum setting to achieve the desired 90 degrees celsius engine operating temperature with as minimal load to the engine as possible.
|Thermostat with gaskets|
When either of the thermostat or fan switch is "turned on", it puts additional load on the engine.
- For the case of the thermostat, when it is open, the water pump (driven by pulley) requires more effort to push the larger volume of coolant in the system.
- And for the fan switch, when turned on, activates either the clutch fan or electrical fan at high speed. That also puts additional load on the engine ( even for electrical fan as the alternator needs to work harder to provide the additional current drawn ).
Though many are encouraged to put in a thermostat or fan switch of a lower temperature setting, it is not recommended technically. The combination may achieve the optimum engine working temperature just as well but at the expense of the thermostat opening longer than usual ( or even permanently open in some cases ), causing the water pump to be pumping the large volume of coolant all the time.
And as the coolant is flowing through the engine the whole time, the coolant exiting the engine never stays in the radiator long enough to be cooled effectively before it is pushed into the engine again. The increasing temperature eventually activates the fan switch and kicks in the radiator fan at high speed to create additional cooling. And in the worst case scenario, the radiator fan is constantly on high speed because it is unable to provide the cooling efficiency required.
The engine temperature gauge within the instrumental cluster may show the engine temperature to be between 85-95 degrees celsius, but at the expense of the load of the engine via water pump pulley and fan pulley or alternator load. That takes away precious RPMs from the engine speed and affects both fuel consumption and acceleration of the vehicle.
The sad fact about the pre-lambda sensor W124 (< 1991/1992 ) is that, there is no visual warning if the thermostat is failing or have already failed. With a failed/jammed thermostat, the engine temperature will climb steeply and eventually cause damage to the head(valves) and block(pistons). That might be the probable cause of workshops recommending lower thermostat and fan switch temperature settings to prevent overheating of the engine. ( at the expense of your fuel consumption ).
Read here on Wikipedia's entry on Internal Combustion Engine Cooling System
What is a thermostat?
What is a fan switch?