What are the Symptoms of Clogged Heater Core? | How to Test a Heater Core?

The heater core serves as a small radiator that heats the cabin of your car by utilizing hot coolant from the engine. When the heater core is clogged or bad, it produces various signs. One common indication of a failed heater core is the presence of liquid in the car’s front foot wells, accompanied by steam emanating from the air vents or excessive fogging of the interior windows. This article explains the clogged heater core symptoms, causes, and how to test it.

What is a Heater Core?

The heater core is a component of your vehicle’s heating and cooling system that is used to warm the interior of the vehicle.

Symptoms of a bad Heater Core

The heater uses a coolant. The cold coolant gets heat from the engine and converts it into a hot coolant.

As the hot coolant flows through the small tubes of the heater core, heat is transferred from the coolant to the fins of the core. Fins attached to the core tube increase surface area and help the fan blow air over the heated fins of the heater core, warming up the air. This heated air is directed into the cabin of the vehicle, providing warmth and comfort to the occupants.

Symptoms of a bad Heater Core

The main function of the heater core is to produce heat that is utilized to warm up the interior of the vehicle and prevent your defroster from getting, well, frosty.

When the heater core is bad or clogged, it produces one or more of the below-given signs:

  • Insufficient Heat
  • Engine Overheating
  • Fog or Film on the windshield
  • Frequent Loss of Coolant
  • Constant Smell of Coolant Inside the Car Cabin

1) Insufficient Heat

A bad heater core can exhibit several symptoms that indicate its malfunction. Insufficient heat in your vehicle cabin is one of the clear symptoms of a bad heater core.

heater core not producing heat

If you turn on the heater and set it to a maximum, but no hot air comes out, it is a clear indication that the heater core is not functioning properly.

2) Engine Overheating

A malfunctioning heater core can contribute to engine overheating. If the coolant lines in the heater core become damaged, it can lead to low coolant levels, which in turn can cause the engine to overheat.

Symptoms of Engine Overheating

Therefore, if you observe your engine running hot, it is advisable to inspect the coolant level first and investigate the heater core as a potential source of the problem.

Read More: Engine Overheating Causes and Symptoms

3) Fog or Film on the windshield

The fog on the windshield is another clear sign of the bad heater core. When the heater core is leaked, the coolant vapors may escape through the vents and condense on the window, leaving a film or fog that smears when wiped. This foggy residue with a distinct sweet odor is an indication of damaged coolant lines in the heater core.

4) Frequent Loss of Coolant

Issues with the coolant can also be indicative of heater core problems since the heater core is responsible for circulating coolant.

Coolant Leaks

If you regularly experience coolant loss or notice coolant leaking on the floor of your car, it may suggest leaks or obstructions in the coolant lines connected to the heater core.

Read More: Coolant Loss Causes and Symptoms

5) Constant Smell of Coolant Inside the Car Cabin

The coolant in your vehicle emits a distinct sweet odor. If you haven’t had the chance to experience the smell of coolant, you can simply remove the cap of your vehicle’s coolant tank and take a whiff to understand what I mean.

Smell of Coolant

When the coolant begins to leak into the passenger cabin, the smell will persist continuously. In such conditions, a coolant smell will be present in your vehicle’s cabin even if your engine turns off.

As your engine warms up, the smell becomes even more prominent. This is because the coolant starts to evaporate, intensifying the odor.

Read More: Why Car Overheats when the AC is ON

Causes of a Clogged Heater Core

The following are the reasons of a bad or clogged heater core:

  • Scale or Rust Accumulation
  • Bad Thermostat
  • Use of Stop-leak Products
  • Mixing of Coolant and Oil

1) Scale or Rust Accumulation

The dirt or corrosion may stop your cooling system from functioning efficiently. The heater core, like other components within the system, is particularly susceptible to scale and rust accumulation.

Prolonged use of coolant in the radiator can contribute to the formation of rust within the system, leading to poor performance of the heater core. 

2) Bad Thermostat

A faulty thermostat may not directly result in clogging the heater core, but it can block the release of warm air from the heating system.

bad Thermostat

When the vehicle engine is turned on, the thermostat doesn’t open until your engine reaches its normal working temperature. Once the engine reaches its operating temperature, the thermostat opens, allowing the circulation of cold coolant from the radiator to the engine and the heater core. 

However, if your thermostat is defective or stuck in the open position, it will prevent the engine from reaching the optimal operating temperature. As a consequence, the heater will only blow cold or lukewarm air instead of hot air.

3) Use of Stop-leak Products

In the market, there are several stop-leak products that claim to fix a coolant leak effortlessly. While these products may deliver on their assurances, but these products can also have negative consequences.

In some cases, a stop-leak product can lead to clogs in the cooling system parts, including the heater core. This is especially true when more than the suggested amount of the product is used. These products can even affect the radiator.

4) Mixing of Coolant and Oil

If your vehicle’s coolant and oil are mixed, it creates a sludge or rust-like substance in the cooling system.

coolant and oil mix, causes of bad heater core

When the coolant flows through the pipes and hoses, it carries this sludge to different components of the cooling system, including the heater core. Over time, this circulation can cause clogs and hinder the route of coolant through the heater core and other components.

Read More: Low Oil Pressure Symptoms and Causes

How To Test a Heater Core?

To test your heater core, follow the below-given steps:

Step 1: Start the Engine: Turn on your engine and wait for a few minutes until it reaches its normal operating temperature. Ensure the radiator is filled with the coolant up to the recommended level. Test the upper radiator hose by carefully touching it.

  • If the radiator hoses feel warm or cold, it indicates a problem with the thermostat. Replace it with a new one.
  • Inspect the pressure of the hose. If it lacks pressure, you need to change the radiator cap.
  • If all the above-discussed parts are fine, proceed to the next step.

Step 2: Check the heater hoses: Turn on your engine and wait for a few minutes until it reaches its normal operating temperature. Set the heat setting to maximum and feel both heater hoses. They must be too hot to hold, not cold or warm.

  • If both hoses are appropriately hot, check your heater control valve to ensure it is functioning correctly.
  • If one hose is hot while the other is warm, remove the hoses and thoroughly drain the heater core.

Step 3: Examine the airflow blend door: If your heater hoses are hot, but the warm air is still not being generated, you should check your vehicle’s airflow blend door. Refer to your service manual for guidance, or contact a mechanic.

FAQ Section

How to tell if your heater core is bad or clogged?

A bad or clogged heater core produces different signs such as insufficient heat in the passenger cabin, engine overheating, fog on the windshield, constant smell of coolant inside the car, or frequent coolant loss. If you observe these symptoms, it means your heater core is clogged or faulty.

Can I drive with a clogged heater core?

Yes, you can drive, but it is not recommended. A clogged heater core can block the circulation of the coolant, causing engine overheating. It is recommended to fix the problems promptly to prevent potential damage to the engine.

Can a clogged heater core cause overheating?

Yes, clogged heater cores may potentially contribute to engine overheating. When a heater core becomes clogged, it can impede the circulation of coolant, leading to inadequate cooling of the engine and increased operating temperatures.

Can a clogged heater core cause no AC?

It is important to note that the air conditioning (A/C) system and the heater core are typically separate systems in a vehicle. The heater core depends on hot engine coolant to produce warm air, while the air conditioning system utilizes refrigerant. Therefore, a clogged heater core would not directly impact the A/C system or cause issues with refrigerant circulation.

Can I flush the heater core?

To address a clogged heater core, one common method is to flush it with water. Flushing the heater core with water helps remove debris and coolant buildup. Some people also recommend using an air compressor to assist in dislodging stubborn clogs. By pressurizing the air compressor, it can help break up and remove blockages in the heater core, allowing the dirt and coolant to flow out through the inlet hose and into a container.

How do I know if my radiator has water circulating?

A simple method to assess coolant circulation is by checking the temperature of the lower and upper radiator hoses. The upper radiator hose should feel hot to the touch, typically about 190 to 200 °F (88 to 93 °C). Monitoring the temperature of these hoses can provide an indication of proper coolant flow through the system.

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