Clogged Heater Core Symptoms: Causes and How To Unclog?

Is your baby ride struggling to send in warm air on those cold mornings or in the winter seasons?

It could be clogged heater core symptoms. The heater’s core function is to send warm air into the vehicle interior through the blower motor.  The moment you turn on the heat, the heater core pulls hot coolant from the engine, and the blower motor sends air across the heater core tubes into the car interior via the air vents.

The warm air in the interior can be used to defog or defrost the windshield and warm up the driver and the passengers.

In this article, we’ll discuss air in heater core symptoms, what a heater core is, the causes of a clogged heater core, and how to test and fix it. Grab a seat and have a 5-minute read.

What Is A Heater Core?

air in heater core symptoms

A Heater core is an essential component of a cooling system. It looks like a small unit of a car radiator, designed with tiny gridlike tubes. The heater core sends warm air into the car interior via vents or heater ducts. It also aids the defroster and heater core to function.

It is essential to know the heater’s core location for easy diagnosis. The heater core is located behind the dashboard, either on the passenger side or on the center of the dash.

The heater core is coated with housing or casing and is found under all the components behind the dash.

Clogged Heater Core Symptoms

A clogged or defective heater core will display some symptoms to alert the driver and to fix it promptly. Here are some notable signs:

Foggy car interior: A common sign of a bad heater core is a foggy car. Do your car interior fogs up for no reason?  I don’t mean a little moisture on your car windows and windshield. I’m referring to a warm condensation, having all the windows covered with moisture.

A notable cause of this significant moisture is a blown heater core. It could be that the heater core blew up while cruising down the road. If this component blows up, it’ll cause warm coolant to leak into the car cabin. When the warm and temperature-regulated coolant leaks out, it will evaporate as steam and cause severe fog in the cabin and car interior.

Sweet smell: Do you perceive an odd smell from the cabin? It’s pretty possible for a heater to leak without fogging up the car windows and windscreen. But it will give a freshly melon smell from the vents.

The sweet smell is coming from the car coolant, which signifies a coolant leak into the cabin. If you perceive this lovely smell outside the car, look around the underneath of the vehicle and see how much coolant loss is under it.

Too much coolant loss: If you notice too much coolant loss and you can’t figure out where the coolant is going, you may have a leaking heater core. Unfortunately, if you can’t track down where the coolant is leaking, it is likely happening on the heater core and dripping into the cabin.

If this occurs when the system is cold, instead of creating fog, it’ll form a puddle in the cabin.

Once you suspect a defective heater core, look around the passenger side floor. If the floor mat is wet with a pleasant smell, consider inspecting the heater core.

Cold passenger compartment: As much as it signifies a defective heater core, it can indicate other system component issues. It could be a symptom of a bad blower motor or other faulty heater components that may be easier to fix than the heater core itself.

In any case, if you experience it with a lack of heat, it is pretty clear that the heater core is the culprit – and should be fixed.

If a puncture or hole exists on the heater core, the warm air in the heater will evaporate before getting to the heater ducts in the car interior. The evaporated air will keep the passenger compartment cool.

Depending on the severity of the puncture, it’ll send lukewarm, mildly warm, or even cold air into the car interior.

Hot engine, But cold cabin: Is your engine overheating, and you have a cold cabin? If yes, you need to diagnose the state of the heater core.

Do not forget that other system components can contribute to engine overheating. If the heater ducts stop sending in warm air and the engine is running hot, check if there’s a coolant leak within the cooling system.

Overheating can cause catastrophic damage to the engine components. That said, do not allow overheating [no matter how small to linger]. Track down the culprit and fix it as soon as you can.

Always pay attention to your instrument cluster while you journey. If you see your temperature gauge creeping up towards the danger zone, park the vehicle in a safe place. Find out the fault and fix it before hitting the road. If you can’t fix it, call your mechanic or tow the vehicle to a mechanic garage.

Now, you may be asking, ‘does the heater core affect ac since it can cause overheating?’ No, the heater core cannot prevent the ac from blowing cold air.

Clogged Heater Core Symptoms

What Causes Heater Core To Clog?

There are various causes of a clogged heater core. While they may differ in origin, it all leads to one troubling condition, which should be rectified to get the heater core working optimally again.

Here are common causes of a lousy heater core;

Scale/Rust buildup: Corrosion and rust prevent the cooling system from providing peak efficiency. In any case, heater exchangers suffer more where there are scale/rust build-ups within the system unit – the heater core is no exception. Rust buildup comes as a result of using prolonged coolant in the radiator.

Mixing of coolant/oil: When oil and coolant mix, it’ll cause sludge, otherwise known as rust, in the cooling system.

As the coolant travels through the coolant hoses and pipes, it circulates the sludge to various parts of the car’s cooling system components, including the heater core. As the circulation continues, it will start clogging system components. This means hindering or slowing coolant passage in the heater core.

Here are common reasons for oil in coolant and coolant in the oil.

Using stop-leak products: There are various stop-leak products in the market that promise to fix coolant leaks without lifting a finger. Of course, these products fulfill their promises, but they have adverse effects.

Sometimes, these products clog the cooling system components. And the heater core has no superpower over it. This happens mostly when you use more than the recommended amount. It can clog the radiator as well.

Bad thermostat: Defective thermostat may not clog the heater core, but it will prevent it from releasing warm air.

When you start up an engine, the thermostat remains closed until the engine heats up to an average operating temperature. The moment the engine heats up, the thermostat opens to allow cold coolant from the radiator to circulate to the engine and the heater core as well.

Suppose you have no thermostat, or it remains stuck open; it’ll not allow the engine to get to a standard running temperature. It will cause the heater to send in lukewarm or cold air in place of warm air.

How Do You Test A Heater Core?

Indeed, one of the causes of a clogged heater core or why the heater is sending heat is because you have a defective thermostat. So, when a heater fails to produce the desired amount of heat, the first thing most technicians do is replace the thermostat before troubleshooting for any other cause.

However, as alluded to above, other factors can cause a lack of heat from the heater ducts. The following few paragraphs explain how to test a heater core.

Step 1. Start and heat the engine: Fill the radiator with the required amount of coolant. Start and run the engine to the average operating temperature. Hold and see if the upper radiator hose is extremely hot to hold.

  • If the hose is cold or warm, change the thermostat to a new one.
  • Check if the hose has enough pressure. If it doesn’t, replace the radiator cap.
  • If everything seems okay, head on to the next step.

Step 2. Feel the heater hoses: Start the engine and allow it to run for a couple of minutes. While the engine is running, set the heat setting to maximum heat and feel the hoses. Both hoses should be very hot to hold, not warm or cold.

If the hoses are hot as they should be, inspect the heater control valve to ascertain if it’s working fine. If one of the hoses is hot and the other is warm, disconnect the hoses and flush the heater core thoroughly.

Step 3. Inspect airflow blend door: If the heater hoses are hot and yet the heater is not sending in warm air, consider inspecting the airflow blend door. Consult your service booklet for direction or seek professional advice.

Note: If heat comes and disappears from the heater ducts, check your coolant level. It is likely a coolant issue. A low coolant level is responsible for causing heat fluctuation from the heater ducts, especially at higher engine speeds.

Pressure Testing

Pressure Testing is a quick and easy means of testing a cooling system for leaks using a hand-operated testing kit.

To use this kit, shut off the engine and allow it to cool down. Remove the radiator pressure cap and attach the kit to the radiator opening. Operate the plunge on the testing kit. By doing so, it’ll pressure the entire cooling system, including the heater core.

However, do not go beyond the manufacturer’s specified pressure. Most manufacturers warn not to exceed 14 PSI (100kpa). Higher pressure can damage the heater core, radiator, water pump, or hoses.

If the pressure kit maintains a certain pressure without dropping, it means there’s no leak in the system components. But if the pressure drops, it signifies a leak in the system. Inspect all the cooling systems for leaks.

Consider centering your inspection on the following areas; cylinder head, radiator, radiator hoses, heater core hoses, heater core, cylinder head, and engine block core plugs.

Pressure testing should only be carried out when there is a suspected leak in the system or when the leak is apparent but you haven’t figured out where it is dropping from. Pressure testing can also be used to diagnose radiator pressure caps.

How To Unclog A Heater Core

A clogged heater core can inconvenience your driving experience. It will block warm air from coming when you need it on cold mornings. Unclogging a heater core is as simple as ABC. However, it requires a how-to-do guide and unclogging kit.

Luckily, the following few paragraphs have provided sufficient information to guide you.

Step 1. Locate the heater core hoses: The first step is to find the easiest way to locate the supply and return hoses. It is typically located on the firewall. On every vehicle, there are always two hoses that go into the firewall – that’s the heater core hose.

If you can’t access them from the connection in the firewall, trace the two hoses and see if you can disconnect them from the engine.

Step 2. Disconnect the hoses: Take a close look and which tool you need to disconnect them from the connections. Make use of your underhood work light if the connection area is dark.

The needed tools could be pliers, Phillip-head, or flat-head screwdrivers. Unscrew the clips and twist the hose back and forth to break it free and pull it out.

Step 3. Flush the heater core: Get your heater core to unclog the kit ready. Grab a garden hose, connect it to a water supply, and attach a sprayer to it. Next, flush the heater core.

You’ll see dirty water rushing out from the other end of the pipe. After a minute or two, switch the hose to the other end of the line and flush it. Keep repeating this process until you see clean water coming from both pipes.

Step 4. Reconnect the heater core hoses: Raise one of the hoses, get a recommended coolant, and pour it until you see the fluid coming from the other end of the pipe. Reconnect both pipes.

Step 5. Check your work: Check the level and top up the coolant on the radiator and reservoir if needed. Start the engine, allow it to idle for a few seconds, and turn on the heat. Keep your eyes on the temperature gauge and ensure the car doesn’t overheat.

Finally, check if heat is coming from the heater ducts. If it’s blowing warm air, pat yourself on the back.

Will a radiator flush unclog the heater core?

The radiator and heater core are two different components that perform similar but different functions. Yes, they are both cooling system components, and the same coolant travels through them.

Flushing a radiator can help reduce the rust that passes through the heater core. The most effective way of unclogging a heater core is by flushing it with a garden hose. Scroll up to see the guides on how to fix a clogged heater core.


Can a clogged heater core cause overheating?

Apparently, a clogged heater core has a slight tendency to cause engine overheating. Did you notice I said a slight tendency? This means, in most cases, it won’t cause engine overheating, while in some instances, it can compromise the cooling system.

There are several other factors that you should look into when your engine overheats. However, if you have a clogged heater core and your engine temperature is running high, it’s best to flush the heater core.

Can you drive with a clogged heater core?

As much as you can still drive with a clogged heater core, do not journey with it as it can cause the engine to overheat and damage engine components. A clogged heater core can reduce coolant circulation, which will cause the engine to run at a higher temperature.

In any case, if you decide to continue driving with a clogged heater core, keep your eyes on the temperature gauge.

You can also decide to bypass the heater core but before doing this, find out the side effects of bypassing a heater core.

Will Vinegar clean a clogged heater core?

Vinegar is a liquid containing 5% acetic acid, and it’s perfectly okay to remove rust in cooling system components like radiator and heater cores.

To unclog a heater core with vinegar, pour a gallon of vinegar into the system and run the engine for 15-20 minutes with the heater on.

Can you fix a heater core without replacing it?

The heater core location makes it easier to fix than to replace. The heater core can be sealed off by pouring radiator-stop leak products into the radiator when it is cool.

There are numerous stop-leak products in the market. However, I recommend using BlueDevil Pour-N-Go or Simple Green stop leak for an excellent job.

How much does it cost to get a heater core flush?

Replacing is a tedious job and requires a mechanic to do it because of its position.

On the other hand, flushing a heater core is an easy job that anyone can do provided the instructions, and the unclogging kit is at their disposal.

However, if you don’t have the time and kits needed to flush it, getting it done by a mechanic will cost around $100 to $200.

How much is a heater core replacement?

Replacing a defective heater core can be an expensive repair, costing up to $550 to $100. The replacement parts are usually cheap and fall within the range of $70 to $200, but the labor due to the location and repair hours involved tend to be pretty high.

Final word

Now that you’re familiar with clogged heater core symptoms, causes, problems, diagnosis, and how to fix it, you can easily flush or unclog your heater core whenever it develops issues.

The diagnosis and unclogging procedures explained in this article are easy to follow. Nonetheless, if you still don’t feel comfortable doing it yourself, seek professional assistance.

Osuagwu Solomon

Osuagwu Solomon is a certified mechanic with over a decade of experience in the mechanic garage, and he has over five years of experience in the writing industry. He started writing automotive articles to share his garage experience with car enthusiasts and armature mechanics. If he is not in the garage fixing challenging mechanical problems, he is writing automotive repair guides, buyer’s guides, and car and tools comparisons.

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