5 Signs Your Heater Core is Leaking — State Street Auto Repair (2023)

The heater core is part of a car’s cooling system. It looks and acts like a smaller version of a radiator, circulating coolant through the little tubes and radiating heat into the cabin. It is also responsible for allowing the defroster to function properly and is linked into the air conditioning system, which is organized around similar principles.

As a car’s engine warms up while running, the antifreeze/coolant absorbs the heat and circulates around the engine and then through the radiator radiator to cool it below the boiling point. The whole system is kept at a constant temperature bythe thermostat.When you turn on the heat inside your car, air blows over the heater core, is warmed and comes into the cabin. Some vehicles have a heater valve that directs coolant through the heater core when the heat is on and bypasses the heater core when the heat is off. In other vehicles, the temperature of the air is controlled within the air blend box by how much air is directed over the heater core.

Dual zone climate control systems have a heater core that’s spilt in two, allowing the driver and front-seat passenger to choose their own cabin temperature. Some high-end luxury models and large SUVshave an extra heater core to allow rear-seat passengers to regulate their own temperatures as well.

Coolant/antifreeze contains corrosion inhibitors that coat the surfaces inside the cooling system, including the heater core. When the corrosion inhibitors are depleted, the cooling system can become corroded, filled with contaminants and may even start to leak. Coolant/antifreeze leaking out through the heater core means that the overall coolant level in the system will be low and the engine is in danger of overheating – the most common cause of mechanical breakdown. Without coolant, the warning light or temperature gauge may not even indicate a problem, because it is not able to read the temperature of the now empty water passages. So it’s important to know the 5 signs to look for a leaky heater core.

1. Your Car Smells Sweet

You may notice a sweet smell from your vents. This is the smell of your radiator fluid and is definitely a sign that coolant is leaking into your car. You might also smell this lovely smell around the outside of your car, which means it's time to look underneath your vehicle and see how much of that coolant has spilled out onto the ground.

2. Your car windows become foggy

A very common tipoff to a heater core problem is the inside of your car suddenly fogging up for no reason. It’s important to note that we’re not looking for a little bit of mist on the edge of the windshield, but for every window becoming covered with moist, warm condensation. This fog is caused by warm coolant leaking into the cabin of the vehicle and evaporating into steam as it hits the cooler air inside your car.

3. Your car is blowing cold air into the cabin

When a hole or puncture develops in the heater core, all that warm air may escape too quickly to reach you at the other end of the heater ducts. Depending on the size of the puncture, you might feel mildly warm, lukewarm, or absolutely freezing air coming from your heater.

4. Your car is devouring coolant

If you find your vehicle has suddenly been needing a higher than usual amount of coolant and you cannot figure out why, it could be a blown heater core. If the leak is hard to find, the coolant may leaking into your cabin when the system is cold, and instead of making fog, is creating a puddle. Check the passenger-side floor to see if it is wet.

5. Your car's cabin is cold, but the engine is hot

Overheating is very serious for your car. All sorts of major components of your car will wear and break down in record speed when they get too hot. If you find that your vehicle has overheated, or continues to overheat, you'll want to check on the health of your heater core, but remember that many other parts of the car could be involved in the overheating. If your heater stops putting out heat but your engine seems warm, investigate whether there is a coolant leakelsewhere, or some other issue with your car.

The heater core is a mostly maintenance free part of the car, but you should be sure to check the hoses that carry the coolant to it periodically - about every 6,000miles or six months. Changing your coolant/antifreeze as directed by your vehicle manufacturer will help ensure your coolant has enough corrosion inhibitors working to protect the system. Also, quickly repairing any leaks and inspecting hoses for signs of internal breakdown will help keep things from becoming an emergency. State Street Auto Repair can help you determine when cooling system services should be performed.

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