Do I Have a Bad Heater Core?

bad heater coreIt’s the first chilly morning of fall!  You wake up and look outside and maybe see a bit of frost on your windshield and think, I’ll warm up my car for a few minutes before I leave the house so I don’t have to go scrape my windshield!  The reality is you haven’t seen your car’s ice scraper for about 6 months and have no idea where it is.

When you jump in your car to leave, the front is gone from the outside of your windshield but the inside is all steamed up.  When you try to wipe the steam off the inside it feels a little slimy and you’re not quite sure why your windshield keeps fogging up with the defroster on.

A foggy windshield with the defroster on is one of the most common signs of a bad heater core, but there are a few others:

Signs of a Bad Heater Core:

  1. Fog on the inside of your windshield with the defrost on
  2. Coolant leaking onto your floorboards
  3. Coolant dripping from the AC condensate hose


A bad heater core is one of the most annoying coolant leaks to fix because your heater core is hidden under your dashboard and behind your firewall so it’s almost impossible to visually inspect while it’s still installed in your car.  The other problem is that it’s often very difficult to remove to inspect usually requiring the removal of a significant portion of your ventilation system or your entire dashboard.

Since you can’t easily inspect your heater core for a leak, try looking for the symptoms listed above.  A very small heater core leak usually shows up at a foggy windshield. When your engine is operating at normal temperature the coolant in your heater core is near 200 degrees and under a lot of pressure.  This pressurized coolant can force its way through a tiny crack in your heater core where the hot coolant will turn into steam in your ventilation air. When your defroster is on this coolant steam will get blown onto your cold windshield and condense.  One way to prove you’ve got a heater core leak rather than just humidity condensing is to turn on your AC with your defroster. If you still are getting fog on your windshield then you’ve got a heater core leak

dashboard removalIf you’ve got a larger leak in your heater core then the coolant will drip down into your ventilation system.  Depending on the design of your system, the coolant will find it’s way to the lowest point in your system which will either be out of your AC condensate drain onto your driveway or out of the footwell ventilation ducts onto your passenger’s feat!  The heater core is the only component with coolant running through it in the passenger compartment so if you’ve got coolant inside your car, you’ve got a bad heater core.

A hard-part repair is always the best way to fix a bad heater core, but in many cases, the cost can be prohibitive or just not make sense for your vehicle.  If you find yourself in this position, consider using BlueDevil Pour-N-Go Head Gasket Sealer to stop your heater core leak.  BlueDevil Pour-N-Go Head Gasket Sealer is an easy DIY fix you can do in your driveway without any tools and can keep your old heater core for leaking guaranteed!

You can pick up BlueDevil Pour-N-Go Head Gasket Sealer at one of your local auto parts stores like:

  • AutoZone
  • Advance Auto Parts
  • CarQuest Auto Parts
  • NAPA Auto Parts
  • O’Reilly Auto Parts
  • Pep Boys
  • Canadian Tire
  • Walmart

Pictures Provided By:

heater_core.jpg – By Stason4ic – Licensed By Getty Images – Original Link
dashboard_removal.jpg – By Aleksandr Kondratov – Licensed By Getty Images – Original Link


4 responses to “Do I Have a Bad Heater Core?

  1. Hi, I’ve a 1993 Volvo 940 turbo wagon. The heater core is fogging the windshield, so I can safely assume the core is leaking coolant. There’s the scent of coolant in the fogging, too. This is also happening in a 1974 BMW 3.0 cs 6 cylinder engine, with windshield fogging issue. What is your suggestion as to which product I should use for these 2 vehicles.
    I had just used the yellow label head gasket sealer in a 1985 bmw 635csi which was losing coolant into cylinder one about 1/2 pint every 10 days. It has been about 3 weeks since I did the job, and it seems to be holding up well.
    Looking forward to your reply.

    Thank you.

    1. Wendell-

      Thank you for asking about your Volvo 940 and BMW. Based on your description, the BlueDevil Head Gasket Sealer would work best for sealing a leaking heater core. If you are noticing a loss of water/coolant, you would be a good candidate for the product. Feel free to contact our technical support line at 888-863-0426 with any other questions.

      Thank you!


  2. I pour blue devil in top radiator hose and run for 50min…..after half hour i check the floormat in a passenger side and found coolant doesnt work?? whats wrong? anyadvise..!

    1. Tom-

      It sounds like you were trying to treat a possible heater core leak? The product does not instantaneously seal, so you would still lose a little water/coolant during that 50 minute run. After the 50 minutes is up, shut the vehicle off and allow it to cool down for a couple of hours. Once it cools, top the system back off to the proper level. You can dry the passenger side floorboard and then take it for a test drive to see if it is still building up moisture. Feel free to contact our technical support line at 888-863-0426 with any other questions.

      Thank you!


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