Causes of car overheating when idling or is at a stop

When a car overheats only when it is not in motion, it is a sign of something wrong with the cooling system.


We have fixed a number of faults where the car runs fine at normal or high speeds but as soon as the car comes to a stop, it begins to overheat. At times, the cause of this condition is not very obvious and can be difficult to trace.


Fan speed - It is possible for this to happen because cooling system fans are designed to run at variable speeds. The implementation varies in different cars. Some cars use 2-speed fans operation, some use 3-speed fan operation, some use pulse width modulation technique for varying fan speed. Let's take the 3-speed fan as an example. If a relay, fuse or wire that controls the low speed operation is faulty, fan will work well when there is more cooling demand but will not work well when there is the need for low fan speed. This mostly happens during idling. Again, it depends on driving condition because it is not only the fan that aids in cooling the engine. The convection of air in the environment helps to cool the engine too. When the car comes to a stop after a period of being driven, the effect of cooling from ambient air is greatly reduced and the fan needs to switch to a high speed to maintain same engine temperature. If the high speed circuit is faulty and fan keeps operating at low speed or does not turn at all, the car will overheat at idle or stop.

Broken fan blade - When a blade of the cooling fan is broken, cooling efficiency reduces. The impact depends on the size of the fan, number and size of blades that is broken. A fan that has any part broken should be replaced.

Stuck thermostat - A thermostat that doesn't open or close completely might allow the engine to overheat at idle. This is because at higher engine speeds, the water pump speed is also higher and therefore the coolant is pumped at a higher pressure. This higher pressure might be enough to mask the effect of the obstruction due to the partially closed thermostat. As soon as the engine speed is reduced, that obstruction is enough to starve the engine of sufficient coolant circulation and the car begins to overheat during engine idling.

Dirty radiator - A dirty radiator will reduce cooling efficiency as the radiator will be less in contact with the coolant to conduct heat away from the coolant.

Blocked radiator - A radiator can be blocked with dirts, scale, rust sediments or other foreign things. This blockage will limit coolant circulation of coolant to the engine which can in turn negatively affect cooling.

Fan reverse spin - A fan that is spinning in the opposite direction to the direction it is supposed to spin will reduce cooling efficiency. Cooling fans that seat behind the radiator are suppose to pull air towards the engine and fans that are placed in front of the radiator are suppose to push air towards the engine. A reverse electrical connection of the fan can change the direction of spin which reduces the efficacy of cooling.

Faulty water pump - A water pump that is not performing well might not show sign of overheat at high engine speed until the engine speed is low. Examples of such situation are: - When parts of the water pump are broken. - When the impeller is spinning freely on the shaft.

Air lock - When there is air lock or cavitation issues in the cooling system, the cooling efficiency will drop and the effect might be pronounce at low speed or during idling.

Fan fault - At highway speeds, the air in the environment hitting the radiator at high speed might be sufficient to cool the engine especially when the weather is cold but the the speed is low or engine is idling, there would not be enough air passing through the radiator to remove heat from the coolant and cool the engine.

Blocked exhaust- A blocked exhaust prevents hot exhaust gasses from exiting the engine. When the engine is idling, the rate at which these gasses leave will be further lower. If the cooling system is not very efficient, there will be signs of overheating when the engine is idling.

Low coolant level - A low coolant level can cause overheating when idling. When the car begins to move, additional air convection can mask the problem.

Contaminated coolant - A coolant causes decrease in cooling effectiveness. Let's see two examples:

-When the coolant is mixed with either engine oil or transmission fluid or both due to faulty heat exchangers, bad head gasket or cracked cylinder head, the cooling effectiveness will reduce at idle.

-When there is detergent in the coolant and it foams, it will easily cause cavitation at the water pump and the result is possible overheating.

Fan belt - Worn or incorrect fan belts that slip badly causing loss of drive of the fan might drive the fan when the engine speed is increased. This masks the fault when the engine is not idling.

Faulty 2-fan operation - some cars uses two fans for cooling. When more cooling is required, for instance in traffic situation, both fans may be required to work. If one is faulty, the car will show symptom of overheating at high speed.

Coolant and antifreeze proportion - Coolant solution is majorly a mixture of water and ethylene glycol and other additives for protection. When the proportion of water is higher, you get better cooling efficiency but less protection of the cooling system. When ethylene glycol has higher proportion, you get more protection and less cooling efficiency. In many cases especially with premixed coolants, it is a 50/50 mixture of water and ethylene glycol. When the proportion of ethylene glycol compared with that of water is too high, the cooling efficiency will greatly reduce. This can make the car to overheat during high power demands or during idling.

Pressure cap - When the radiator or coolant reservoir cap is bad, it can cause overheating.

Hoses - Hoses that have kinked or that are causing leakage can cause overheat. Kinked Hoses causes restriction to coolant flow. The impact is significant during idling just as in the case of thermostat that is stuck and partially closed.

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