The water temperature requirement of diesel generator is clear, but some operators like to adjust the temperature very low, some are close to the Lower limit of water temperature, some are less than the lower limit. They believe that if  the water temperature is low, the water pump will not appear cavitation phenomenon, cooling water (liquid) will not be interrupted, there is a safety factor in use. As long as the water temperature does not exceed 95℃ cavitation will not occur, cooling water (liquid) will not be interrupted. On the contrary, if the water temperature is too low, it is extremely harmful to the work of diesel generator. 

The temperature is low, the combustion condition of diesel oil in cylinder is deteriorated, the fuel atomization is bad, the combustion period increases after ignition, the engine work is easy and rough, the damage of crankshaft bearing, piston ring and other parts is aggravated, the power is reduced, and the economy is reduced.

After combustion, the water gas is easily condensed on the cylinder wall, resulting in metal corrosion. 

The diesel fuel to burn may dilute the oil and make the lubrication worse.

The fuel combustion is not complete and forms colloid, which makes the piston ring stuck in the piston ring groove, the valve is stuck, and the pressure in the cylinder decreases at the end of compression.
The water temperature is too low, the oil temperature is too low, the oil thickening, the fluidity becomes worse, the oil pump oil quantity is less, the generator set oil supply is insufficient, plus the crankshaft bearing clearance becomes smaller, the lubrication is poor.