Engine Oil Cooler


It is best if the temperature of the engine oil does not rise above 100C. If the temperature rises above 125C however, engine oil lubrication properties will deteriorate suddenly. Therefore, to maintain lubrication properties, some engines are equipped with an oil cooler. Normally, all of the oil flows into the oil cooler and then flows to each engine part after it is cooled. At lower temperatures, oil has a higher viscosity and tends to create a higher oil pressure. When the pressure difference between the inlet side and the outlet side of the oil cooler rises above the specified value, the relief valve opens and the oil flowing from the oil pump bypasses the oil cooler and flows on to the other parts of the engine, thus preventing damage from occurring.

Oil Consumption

Even when the engine is not leaking oil on the outside, some engine oil is consumed as it enters the combustion chamber and is burned up. The routes by which oil is lost, are listed below. The gap between the cylinder and the piston The gap between the valve guide bushing and valve stem Loss due to oil contained in blow-by Gas

Cooling System Description

1. Cooling system

When the engine heats up, the cooling system transfers that heat to the surrounding air lowering the temperature of the engine. Conversely, when the engine is cold, the cooling system makes it easier for the engine to warm up. In this way, the cooling system serves to maintain an appropriate engine temperature. There are air-cooled and water-cooled types, however the water-cooled type cooling system is mainly used for automobile engines.

(1) Water-cooled type cooling system

In the water-cooled type cooling system, coolant is circulated to the water jacket, absorbing the heat generated by the engine and thus maintaining an appropriate engine temperature. The absorbed heat is released through the radiator and the cooled coolant is recirculated to cool the engine. Also the heat of the coolant can be used by the heater. The two types of cooling systems, differentiated by the position of the thermostat are listed below

<1> Thermostat in water pump inlet side

<2> Thermostat in water pump outlet side Cooling system can also differ in whether or not a bypass valve that controls the bypass circuit is present. In recent years, almost all engine cooling systems are equipped with a thermostat that has a bypass valve.

Flow of Engine Coolant

1. Thermostat in water inlet type (With bypass valve)

This type features a thermostat mounted in the water pump inlet. The thermostat is equipped with a bypass valve and regulates the coolant that passes through the main route and bypass route by opening and closing the thermostat according to changes in coolant temperature.

(1) When the coolant is coldWhen the coolant temperature is low, the thermostat closes and the bypass valve opens. The coolant then circulates through the bypass circuit without passing through the radiator. This helps the water temperature rise so that the appropriate engine temperature can be obtained more quickly.

(2) When the coolant is hotWhen the coolant temperature is high, the thermostat opens and the bypass valve closes. All heated coolant flows to the radiator where it is cooled and passes through the thermostat to return to the water pump. In this way the appropriate engine temperature is maintained Compared to engines with no bypass valve, when the coolant temperature is high, it is not circulated to the bypass valve, so the cooling effect is higher. This also serves to sensitively operate the thermostat so that coolant temperature change is lessened, allowing the engine can run at a stable temperature.


An engine equipped with a thermostat with bypass valve should not be run with the thermostat removed. The bypass circuit is wider in engines with a bypass valve. If the engine is run while the bypass valve (thermostat) is removed, more coolant will flow through the bypass circuit, making it easier for the engine to overheat.


There are two types of thermostats, one that includes a bypass valve, and the other without a bypass valve. The cylinder in the thermostat is moved by the thermal expansion of the wax in the cylinder. This causes the main valve to open, regulating the amount of coolant flowing to the radiator, so that it is maintained at a proper temperature. The bypass valve operates along with the main valve. (When the main valve opens, the bypass valve closes.) After the coolant is drained, air from the engine cannot be released easily and the coolant cannot enter easily because the thermostat is closed when refilling. Therefore, air is released from the jiggle valve, simplifying the coolant refilling process. When the engine is running, the jiggle valve is kept closed by the water pressure from the water pump.

Water Pump

The water pump is driven by a V-belt (V-ribbed belt) and circulates coolant through the cooling system and the heater. The rotor and water pump body use a mechanical seal to prevent coolant leakage. If this seal is defective and coolant leaks out, coolant leakage or stem is discharged through a drain hole in the pump body so that coolant does not leak onto the bearing. Therefore, when there is coolant leaking or leakage traces from the drain hole, it is likely that a defective mechanical seal or bearing is the cause.


Ordinarily, the water pump cannot be repaired by disassembling it and requires a full assembly replacement. There are some models, however, where the pump can be disassembled and repaired.

There are water pumps that are driven by the teethed side of the V-ribbed belt or water pumps that are driven by the backside of the belt.

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