When an engine overheats, it is an indication of a problem with the car’s cooling system. The cooling system maintains the optimal operating temperature of the engine. When the engine is running, it would naturally emit heat, and your car hood would feel warm, but the engine shouldn’t exceed its normal operating temperature. An overheating engine is not good for its health and optimal performance.

An overheating engine can cause engine blocks to crack and head gaskets to explode. Too cold is not good for the engine’s operation either. The reason is that the engine needs to warm up for optimal operation (particularly to prevent stalling), and it compensates for the cold temperatures by having the fuel injectors send more fuel through the system in order to create a high gas to air ratio.

The implication is that you end up burning more fuel than you would when the engine is running warm while also increasing the chance of fuel residue or carbon buildup in various parts of the engine as excess fuel reaches the exhaust.

So, while you don’t want to run your engine cold, you also want the engine to run at optimal temperatures instead of overheating. The cooling system saves the day by continuously passing coolant through channels in the engine components to prevent the engine from overheating. Take note that some engines are water-cooled (a mixture of water and antifreeze) while others are cooled by air flowing over finned cylinder casings.

The coolant passes through these channels to absorb heat from various parts of the engine and then transport it to the radiator for cooling. Repeat. In liquid-cooled engines, sitting between the engine and the radiator is the thermostat and its job is to monitor the temperature of the coolant returning to the engine from the radiator and regulate the amount of coolant that gets recirculated back into the engine.

In other words, this valve called thermostat is a vital component of your car’s engine as it is responsible for the health and optimal performance of the engine by preventing the engine from running too hot, while also helping it to warm up as soon as possible when it is too cold.

Now, if your car is overheating and the heater is blowing cold air, then there is a fault with one or more components of the cooling system. The fault could be from the heater core, the blower motor, the thermostat, or a low coolant level. Let’s take a quick look at these components I just mentioned.

1. Bad Heater Core

The heater core functions as the heat exchanger between the coolant and the cabin. It has inlet and outlet pipes that aid its function in the cooling system. Over time, the heater core might become clogged or start to leak. This will prevent the hot coolant from reaching the heater core for the heat transfer to take place.

With heat not reaching the heater core, none can be transferred into the cabin, leaving you with a heater blowing cold air. Also, a bad heater core will leave you with coolant vapor traveling through the defrost vents and causing your windshield to fog up. If you notice the floor of your car is wet, you have just confirmed your heater core is leaking, as the leak will cause coolant accumulation on the floor of your car.

2. Low Coolant Level

If you always have to deal with low coolant level issues, it could also be because there is a leak in the heater core, and a low coolant level will translate to an overheating engine. If your engine overheats for far too long, the temperature gauge on your dashboard will go up to remind you of the dangers of running an overheating engine.

3. Faulty Thermostat

A thermostat is the valve I mentioned earlier that sits between the engine and the radiator. The valve is designed to open and close to regulate the passage of coolant. Unfortunately, a bad or faulty thermostat valve won’t open or close as it should, ultimately defeating its engine temperature regulatory and monitoring purpose. So, a bad thermostat will ultimately contribute to the engine overheating or the temperature fluctuating unduly.

To Recap

If your heater is blowing cold air and the engine is overheating, then your cooling system is not working as it should. A faulty heater core is most times accompanied by a low coolant level from an undetected leak. Blocked hoses could also contribute to this problem substantially, as it will prevent the coolant from circulating. An overheating engine could also be a result of a problem with the engine itself.

If you notice your car is overheating, turn off the air conditioner and leave your car in neutral or park to allow an increased circulation of water and air to cool the engine. You should also open the bonnet to improve engine ventilation.

Do not open the radiator to check the coolant level when the engine is overheating to avoid burns from coolant sprays. Ultimately, this is a good time to let a qualified automobile technician help you out although it definitely helps to know what you might be dealing with if you’re experiencing engine overheating and the heater blowing cold air.