Causes of car engine overcooling

Just as engines overheat, it is very possible for them to also be overcooled by their cooling systems or the inrush of environmental air.

Quite some some of people have experienced engine overcooling and wonder how it is possible. Some others simply think that their engines are performing well believing that the cooler, the better which is not actually true.


What it means for an engine to be overcooled

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

The causes of the engine not attaining optimal temperature can be grouped into 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 speeds. 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 mechanical fan that allows the fan to lock in operation mode will cause overcooling since the fan's speed will always the the same as the engine's speed. 

A faulty ECU that sends wrong signal to the fan or electrically controlled thermostat can cause the thermostat to stay opened 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 their plug can cause the system to run in fail safe mode which keeps the fan running 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 leads 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 that creates sufficient clearance between the piston ring and the cylinder wall.

3. Engine oil becomes contaminated as the heat of the engine is not adequate to burnt off moisture and fuel in the oil.

4. The overall efficiency of the engine is reduced as engines are most efficient at their operating temperatures.

5. The engine oil does not thin out to the viscosity that it should be at operating temperature. This also increases fuel consumption as the engine works harder to overcome the weight of the oil. The oil might also not thin out enough to lubricate tight spaces in the engine.

6. Carbon and sludge build up is another effect of engine that does not attain operating temperature.

7. An engine that is overcooled has higher emissions level.


How to fix an engine that is running too cold

1. Replace a faulty thermostat with a good one.

2. Fix cooling fan system to work in the factory operating mode.

3. Replace a faulty temperature sensor with a good one.

4. Replace a faulty ECU with a good one.

5. Fix bad wires, connectors and plugs.

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