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Heater Core: What You Need to Know About Its Problems and How to Fix Them

Heater Core

Not everyone knows what a heater core is or how to tell when it's not working properly anymore. The heater core is the system which transfers heat from the coolant running through your radiator to the interior of your car when you turn on the heater to warm yourself up. It's also one of the few systems in your car that can leak inside the vehicle where you and your passengers are. That means when something goes wrong with the heater core it could be a potentially messy issue. It may seem like a small system to break down, but it can lead to much bigger problems if it’s not addressed quickly.

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How Do I Know if My Heater Core is Clogged?


There are a few ways for you to tell if you're having a problem with your heater core. Although some of these symptoms could be indicative of a different problem, if you're experiencing more than one of them, and are having trouble heating your car, then you might want to consider that your heater core is the problem.


  • The most obvious sign of a clogged or broken heater core is that the heat simply doesn't work when you turn it on. The blower fan will still function as normal but even long past the time when you know heat should be blowing out the air is warm or even cool.
  • A slimy kind of film or coating on the interior of your windows and windshield. It may also seem like they’re getting foggy as you drive. This is another indication that you have a heater core problem as the coating is not normal condensation but coolant.
  • If you notice coolant on the floor of your car, it likely leaked in through the vents because of a problem with your heater core. 
  • Coolant typically has a very sweet smell, so if you notice a persistent sweet or fruity odor when you're inside the car driving then that probably means you have a leak in the system somewhere or a clog and it's backing up in the air vents where you can smell it. 


Heater Core Leaks Signs


There is a film that can form on your windows or even a kind of thick fog like you might see on a mirror when you're having a hot shower that indicates you have a heater core leak. This isn't actually steam that's forming on your windows; it's coolant that has been aerosolized by the blower fan and is blowing out on your windows. The fluid coats your windows and makes it difficult to see, and it could also be difficult to clean off. 


You may also find that there's a leak coming from inside your dashboard that can pool on the floor of your car on the mats. You might also see it on the console in the panel in front of you as well. This will not necessarily be coming from the vents because it does leak from inside, but further down at any joints or cracks. If you see coolant leaking from those spots, then you know you have a heater core clogged or leak of some kind. 

What Is a Heater Core Flush?


If the heater in your car isn't working and when you turn it on, and the fan just seems to be blowing cool air that it's possible you have a clog in the heater core itself. You can potentially fix this by performing a heater core flush. A heater core flush as the name suggests is when you flush the heater core system out to remove any debris or clogs that are causing it to work improperly. This can either be done with compressed air and water or just water alone, but compressed air may help loosen up some of the bigger clogs if it’s a serious problem. 


Heater core flush is the kind of repair that you could do at home if you're comfortable disconnecting and reconnecting hoses in your engine and attempting to clean out the lines on your own. You can find walkthroughs and even videos that show you how to do it online but it's definitely a little more complex than something like adding more washer fluid to the reservoir. If you've never done any car repairs on your own before, and you don't feel comfortable with this level of work then it's probably best just to take your car to a mechanic to get it done. A mechanic might charge you somewhere between $100 and $150 to do a coolant flush like this.


If you are feeling confident enough to pull this off with the guidance you can find online, we can show you a basic step-by-step that could potentially save you time and money getting it fixed elsewhere.


How to Perform a Heater Core Flush


A heater core flush is not all that complicated. If you’re feeling up to it, this is what you'll need.


  • A large bucket to collect all the coolant from the lines. You don’t want to let coolant drain or spill out onto your driveway or garage floor as not only is it harder to clean up, it’s bad for the environment. Try to dispose of as much of it as you can in a responsible way. Old, dirty, and burnt fluid can be dropped at most garages and mechanic’s shops to be safely disposed of.
  • Some clean plastic tubing 
  • Some rags, gloves and safety glasses
  • Your garden hose
  • Pliers or a screwdriver to remove the clamps that are holding the hoses in place
  • An air compressor 
  • A ¾” barb fitting adapter


Once you have the necessary parts, the steps are fairly simple. Make sure your car is rested and cool before doing any of this, so you don’t burn yourself.


  1. Locate your car's firewall in the engine, the metal wall that separates your engine from the interior, and you'll see two fairly thick hoses running out of it. Those are the coolant lines that you're going to need to flush. One takes coolant in; one runs coolant out. 
  2. The hoses will be attached to the firewall with clamps. You’ll want to have the bucket handy to collect the coolant as it drains out when you remove the hoses. Use your pliers and the screwdriver to remove the clamps, and you may want a spray lubricant like WD40 handy as well if they’re not easy to move. Be careful not to damage the clamps or the hoses. If you do, they’ll need to be replaced.
  3. Clean up any excess spillage in the engine and on hoses or other parts with some rags.
  4. If you have an air compressor, you can attach it to the outlet hose at this point. You’ll need a coupler to complete the seal, or in a pinch some duct tape. Pressure shouldn’t exceed 20 psi maximum for a total of no more than 10 minutes to force any grit and grime out. Any more could damage the system. Again, if you don't have an air compressor, this step isn’t entirely necessary.
  5. Using the ¾” adapter and the tubing, attach the garden hose to the outlet. Just as with the compressed air you need to be aware of pressure. A full blast hose can push out about 40 psi, which may be too much for your system to handle. Try to keep it down around 10 psi to 20 psi. Run water through the core until it drains out clear in your bucket.
  6. To be extra through, repeat the process a second time with the inlet.
  7. If you have an air compressor, you can use it not to force any excess water out of the core.
  8. At this point you may want to use a radiator cleaner to flush the whole system as well. Something like Royal Purple Radiator Flush is a good quality product that costs under $15. Follow the directions on the label but expect to let this sit for about a half hour.
  9. Rinse everything with water again to clean out the radiator flush.
  10. Reattach the hoses and tighten the clamps but don’t over tighten them.
  11. Clean up any spilled coolant that may be on your engine parts. Make sure your belts in particular are clean and dry as you don’t want them to slip because they’re covered in antifreeze.
  12. Top up your radiator’s coolant reservoir with a 50/50 mix of antifreeze and distilled water as outlined in your owner's manual.
  13. Start your car and let it run for a while to allow the coolant to circulate. You’ll find you may have to add more as it runs through and also air is released from inside, causing the levels to decrease. This process can take some time.
  14. Take the car for a short test drive, long enough for the heater to get up to temperature. If the heat is circulating normally, do a second inspection under the hood to look for any fluid leaks where you’ve reattached the hoses. 
  15. If the heat is not working still, then the problem has moved beyond what a simple flush can repair, and you will likely have to take your car to a mechanic.


What Does a Heater Core Replacement Cost?


If the heater core flush doesn't work to get your heater core back to the way it should be performing, then you may need to have your heater core replaced entirely. Depending on the make and model and year of your car, the cost of a heater core replacement could range anywhere from about $500 to $1,000. Obviously this is quite a substantial range, but unfortunately we can't really narrow it down any further because it really does depend on the type of car you have, the condition that it's already in, and also where in the country you live. As you've no doubt experienced the cost of repairs can vary greatly even between two shops in the same town, and it’ll definitely change from one city to another. Clearly though this is an expensive part to replace if it comes to that so some routine maintenance can definitely save you a lot of money in the long run. 


Can I Drive Without a Heater Core?


Some people might think that it's not a big deal to drive with a malfunctioning heater core. While the biggest problem seems to be that you'll be having to deal with a cold car on a cold day it could be a symptom of a much bigger problem. For starters, if it is leaking fluid then your windows can fog up and obscure your vision. 


If you have a coolant leak somewhere in the system, you're running the risk of the engine overheating and leading to much bigger and more expensive problems in the long run. When your engine starts to overheat, you’ll notice some of the following symptoms.


  • Noises from the engine, what is often called “engine knock.” This is the earliest symptom of an overheating engine and it happens when the temperature is as low as 20 to 40 degrees above normal. If this persists, it can lead to broken pistons and piston rings.
  • Lack of power. As the temperatures increase, the engine will perform much worse. In the 40 to 80 degrees above normal range the knock will become a serious rattling sound. You’re risking piston damage as well as damage to the bearings.
  • When temperatures exceed 100 degrees above normal it means you’ve likely run out of coolant altogether. This can lead to a total engine meltdown. Potential damage incudes warped cylinders and a cracked head gasket. The repair bill can get well in excess of $2,500 for this sort of damage to the head gasket. Warped and cracked cylinder heads will be at least $500.


Even if you don't feel that you need the heater core functioning, if there is a problem with it you really should get to the root of what it is just to be sure you're eliminating any bigger issues. As with many systems in your car, if you ignore a small problem it can grow into a big problem very quickly. What could have been a fix that cost under $100 could now be one that cost over $1,000.


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