Causes of car engine overcooling

A cars engine is the source of mechanical power that drives the car. As chemical energy in the fuel in converted to mechanical energy, a lot of heat energy is generated during combustion. Friction due to moving parts in the engine also causes heat. 

The cooling system is designed to transfer heat from the engine and dissipate it into the environment.

The engine is overcooled when the temperature is below the normal operating temperature of the engine as a result of cooling action of the cooling system.

Quite some number of people have experienced engine overcooling and wonder how it is possible.

Some others simply think that their engines are performing well by believing that the cooler, the better. This is not actually true.


What it means for an engine to be overcooled

This is a situation where the engine runs sufficiently but does not reach normal operating temperature.

The causes of the engine not attaining optimal temperature can be grouped by the operating mode of the car as described as follows.


1. Engine overcooling when the car is running at highway speed

A stuck opened thermostat is the cause of this condition when the vehicle is operating at highway speed. This is because even without fan assisted cooling, the air that is rushing into the radiator's fins as a result of the car's speed is sufficient to cool, even in excess.

A stuck opened thermostat allows continuous circulation of coolant in the cooling system. This implies that heat is taken away all the time from the engine to the radiator and the heat is removed from the radiator by the inrush air. This high current of air is what makes this condition to happen when the car is driven at high speed. The thermostat is responsible for controlling the temperature at the lower limit by closing when the temperature is dropping too much. It also opens when the temperature is rising.

With a thermostat that is stuck opened, the engine can still reach operating temperature at idle or in the city but will take a longer time. This is because the cooling is not assisted by natural high current air.

This type is common in cold areas, cold mornings or cold rainy periods.

Note that a car can run without cooling fan if the outside air is cold and the vehicle has enough speed to maintain the current of the inrush air to the radiator.

A fan that stays on further compounds this problem as the air current and cooling will increase.

2. Engine overcooling when the car is not running at highway speed

A faulty coolant temperature sensor that sends wrong temperature signal to the engine control unit can cause the engine to not reach optimal temperature. When the sensor temperature reading is more than the actual temperature, the ECM will send command to the fan module for more cooling. This will have impact more with cars that have electrically controlled thermostat. 

Another cause is a faulty or bypassed fan control module. This makes the fan to work at maximum speed causing excessive cooling of the engine 

A faulty fan clutch of a mechanical fan that allows the fan to lock in operation mode will cause overcooling since the fan speed will always be the same as the engine speed. 

A faulty ECU that sends wrong signal to the fan or electrically controlled thermostat can cause the thermostat to stay open or cause the fan to run continuously at high speed thereby causing too much cooling of the engine.

A bad wiring to the temperature sender or faulty fan control module signal cable or bad plugs can cause the system to run in fail safe mode which can in turn cause the fan to run at high speed.


Effects of engine overcooling

1. The engine will run rich and consume more fuel because the ECM will react to the low temperature signal from temperature sensor by dumping more fuel to allow the engine attain closed loop and operating temperature. This can lead to overfueling that causes bore wash which in turn causes rapid engine wear.

2. Internal parts like piston rings will wear out easily as there won't be sufficient heat that causes expansion which allows sufficient clearance between the piston ring and the cylinder wall.