The need for the engine to maintain a film of lubricating oil on pistons, cylinder walls, and the load-bearing surfaces of other moving parts is discussed in chapter 8 of this manual. The oil film must be maintained if adequate lubrication is to be provided. The formation of such an oil film depends, to a large degree, on the viscosity of the oil. If the engine cooling system did not keep the engine temperature below a specified level, viscosity would decrease. This condition would result in inadequate lubrication, and metal-to-metal contact would occur with resulting excessive wear of the load-bearing parts. Also, the heat absorbed by the lubricating oil (from the combustion process and from fluid friction in bearings) must be removed so that oxidation of the oil and formation of sludge can be kept to a minimum.

Prevention of overheating is generally thought of as the primary function of an engine cooling system. However, it is possible that a cooling system might remove too much heat. If an engine is operated at lower than normal temperatures, condensation takes place in the crankcase, causing acids and sludge to form in the lubricating oil. Also, cylinder temperatures must be main-tained high enough to minimize the condensation of corrosive gases on the cylinder walls.

Excessively low operating temperatures tend to increase ignition lag, a condition that causes detonation. Thus, the cooling system of an engine must maintain the operating temperatures within a specified range. The range of operating temperatures for a given engine can be found in the applicable manufacturers technical manual.