Rick's Free Auto Repair Advice

Heater core failure

What are the symptoms of heater core failure?

A heater core is a small radiator heat exchanger that sits under your dash and heats cabin air. If you neglect proper cooling system maintenance you can experience heater core failure. A heater core can fail in several ways:

• The tiny passages become plugged with corrosive materials that prevent the flow of coolant and the transfer of heat

• The tubes corrode and leak coolant into the heater box heater core leakassembly under your dash. When that happens you’ll can see an oily fog on your windshield when you set your HVAC system to defrost. Or, the leak may dribble coolant into the bottom of the heater box, where it eventually drains out the AC condensation tube. Or, the coolant may drain on the floor on the passenger side of the cabin.

If you see a mist coming from the vents, dampness on the floor, or are constantly low on coolant, suspect a leak in the heater core. Coolant will feel oily to the touch

• Too much coolant flow can wear out the thin tubes heater core flow restrictorin the heater core, causing them to leak. Excessive flow is caused by the removal of a factory installed flow restrictor when servicing the heater hoses. Factory restrictors must NEVER be removed from the lines.

• Excessive pressure on the heater core tubes during heater hose replacement can break the solder seals where the metal tubes attach to the heater core. That causes the heater core to leak.

• When a heater core leaks, you most likely will smell a sweet vapor coming from the air vents. This is vaporized engine coolant. It is harmful to your health and you should fix the problem right away.

What causes heater core failure?

The #1 cause of heater core failure is cooling system neglect. All car makers specify a recommended coolant change interval. Changing coolant on time is mandatory to protect all the metal and plastic components in your car’s cooling/heating system. Neglecting this service or performing a cooling system flush with the wrong coolant can cause damage years down the road.

All coolants contain 5%-10% anti-corrosive additives. However, since the additives differ in how well they protect aluminum versus cast iron, and how well those additives coexist with plastic and rubber seals and gaskets, using the correct coolant is CRITICAL to the life of your heater core and radiator.

Most new cars are filled with long life coolant that lasts up to 5-yrs or 100,000 miles. However, if you replace that long life coolant with traditional green coolant, you risk damage to cooling system components. Traditional green coolants have a life of just 24,000 miles or 2-years. But lab tests show that its anti-corrosive properties start to degrade as soon as 18-months after installation.

Which coolant to use?

Aftermarket coolant manufacturers advertise “all makes, all models coolant.” Not a single car maker approves of these coolants. In fact, since the aftermarket companies try to cover their basses by adding materials that protect aluminum, cast iron, magnesium, steel and plastics, they wind up with a formula that can actually accelerate corrosion damage on some of those parts. If other words, some anti-corrosion additives that protect aluminum can accelerate corrosion when they come in contact with cat iron parts, and vice versa. Avoid the all makes, all models formulas and buy the EXACT coolant recommended by your car maker and change it according to the maintenance schedule. For more information on coolants, see this post

How to prevent heater core failure

• Change coolant according to the car maker’s maintenance schedule.

• Use the exact coolant specified in your owner’s manual, heater core replacement costEVEN IF IT MEANS YOU HAVE TO BUY IT FROM THE DEALER! Paying $20 more for factory coolant costs a lot less in the long run compared to replacing a heater core to the tune of $1,500.

• When replacing heater hoses, locate and reuse the factory installed flow restrictor. NEVER RUN your heater core without the flow restrictor.

• Use care when replacing heater hoses. Avoid twist the rubber hoses to remove from the heater core tube. Excessive force can damage the solder joints, causing a leak.

Fixing a heater core leak

Most professional mechanics scoff at the idea of using a best head gasket sealercooling system sealer to stop a heater core leak. In most cases, they’re justified. Many times those sealers cause far more harm than good. BUT, that’s usually because the DIYers don’t follow the dosing directions. When it comes to sealing a leak in a heater core, LESS IS MORE!. You want to use the least amount of sealer product, which is just the opposite of what most DIYers do.

When you add too much sealer, you clog the entire heater core. That may stop the leak, but it also stops the flow of coolant. So you think you’ve solved the problem until you try to get cabin heat the next winter. That’s when you’ll discover that the heater core is completely plugged and must be replaced.

©, Rick Muscoplat

Posted on by Rick Muscoplat

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