Will a clogged heater core cause overheating?

close-up of car heater knob

What is the Heater Core?

With fall coming in fast and winter already knocking on the door, you want to make sure your home heating system is in working order right? What about your vehicle’s heating system? It won’t hurt to have it checked out too, cause if your vehicle has a clogged heater core, you are going to have one cold ride to work. 

What is the heater core and why is it so important for helping your vehicle stay warm inside? Let’s review how the heater core works:   

A heater core is like a small radiator, with the key difference being it diverts the coolant in the motor from the cooling system and heats the interior of the vehicle. As the engine runs and warms up the coolant (aka antifreeze), the water pump sends it through the cooling system. It is passed through the main radiator to keep the engine from reaching a boiling point and overheating.

When the heater inside the car is turned on, a valve opens, allowing that heated coolant to flow to and through the heater core. There, a door opens, and the air is directed through the heated heater core. As the coolant reaches 200 degrees, it delivers heat to the vehicle’s interior. Depending on where you have the controls placed, it warms the feet or defrosts the windshield. If there is a clogged heater core, that heat won’t reach your feet or windshield. 

What are the symptoms of a clogged heater core?

If there isn’t warm air coming into your vehicle’s cabin, that would be a possible symptom you have a clogged heater core. However, that isn’t the only indicator you have a clogged heater core. Other indicators would be: 

  • Airflow from the vents is weak or there is no airflow
  • Airflow is cold coming from the vents
  • The floorboard is damp
  • There is the smell of coolant (antifreeze) inside the cabin

Where there is an obvious difference in airflow, it can be the heater core fins are bent or there is a clogged heater core restricting the airflow. The smell of coolant or a wet floorboard usually means the heater core is leaking or there is a hole either in the heater core or a hose connected to the heater core. At this point, the only remedy is to replace the heater core. 

How Do I Know When to Replace a Heater Core?

What happens when a heater core goes out? When the heater core is bad, the coolant isn’t running through the engine as it should, keeping it cool, so a lot of expensive and extensive damage to the engine can happen. So, if you notice any of the following, this could indicate your vehicle has a clogged heater core, or the radiator could have a problem:  

1. Coolant Fluid Leaking Into Floorboard

When there is fluid dripping down from behind the dashboard, that is the worst! Your floorboard is wet, and it smells inside your vehicle.

2. Heating System Not Working 

A broken or clogged heater core will keep the engine heat from transferring to the vehicle heating system to warm the interior. You can diagnose this problem yourself by driving for 20 minutes, then turning the heating system on inside the vehicle.  If the interior doesn’t start getting heat immediately, then you probably have a clogged heater core. 

3. Engine Losing Coolant

A leaking heater core or a  partially blocked heater core will affect the engine’s cooling system. A leaking heater core means the engine isn’t getting the coolant it needs either. Check the radiator for coolant level and if it is low, then there is a leak somewhere in the engine or heating system.

4. Condensation and Fogged Windows 

If there is a clogged heater core, it can’t get hot, and then only cold air will blow through the vents.  If the windows get fogged and steamed up and you can’t get the defroster to clear them, it is a typical indicator of a clogged heater core. If the windows have an excessive amount of condensation, feel the carpeting, and if it is damp the vehicle’s heater core is probably leaking. 

5. Overheated Engine 

While your vehicle’s heating system needs coolant to warm the interior, the engine needs coolant to keep it from overheating. If there is a clogged heater core, it will keep the coolant from flowing through the engine, causing it to overheat.        

How do you unclog a heater core?

There are several maintenance steps your vehicle needs, including flushing the radiator, transmission, and the heater core. The radiator and transmission flushing should be done by a professional mechanic.  The heater core is something you can do yourself, even with minimal mechanical inclination: 

  • Locate the Heater Core: This is found on the vehicle’s firewall and will have two hoses – an inlet and an outlet.  Trace those to the engine.
  • Disconnect the Heater Hoses: Place a bucket under the car then disconnect those hoses from the vehicle’s firewall. There are usually clamps holding them in place that can be removed by squeezing with pliers or loosening screws. Once you disconnect these hoses, you should have water pouring into the bucket. If not, your vehicle either doesn’t have enough coolant and water or you have a clogged heater core. 
  • Apply the Pressure: Connect an air compressor to the outlet house, seal it up with duct tape, and turn the compressor on. Let this “pressurize” the heater core for 10 minutes, and then turn the compressor off. Now, let the system drain into the bucket. 
  • The Water Hose: After everything is drained thoroughly, attach a water hose in the same manner as the air compressor. Turn the water on and it let run through the system 10 to 20 minutes, or maybe longer if your system is dirty. You’ll know when the water starts coming out clear. Now, using the air compressor, blow off the excess water.
  • Reconnect the Hoses: With the flush and pressurizing completed, reconnect the heater hoses using the same clamps and one additional clamp on each for security. 
  • Refill the Coolant: Now add new coolant back into the heater core system through the radiator – a 50:50 ratio of antifreeze and water is recommended. You will need to add some, start the car, and let the fluid get into the system, then add some more. This will ‘burp’ the system, getting all the air out. 

Will a clogged heater core leak?

Yes, and of all the components a vehicle has under the hood and behind the dash, this is the one that will leak inside your car the most.

Can the heater core affect air conditioning?

Not usually – the air conditioning and the heating are two different systems. Your vehicle’s heater relies on the coolant being heated by the engine. The air conditioning, however, uses its own refrigerant. 

mechanic inspecting vehicle interior

In Closing

During the winter, while the heater core may seem small, it is a powerful component to your comfort and the operation of your vehicle.  Taking your car in now for “winterizing” is being proactive before winter arrives. This includes flushing the transmission, radiator, and making sure you don’t have a clogged heater core. If you need this kind of help you can reach out to us by calling 847-895-9131.