Rick's Free Auto Repair Advice

Symptoms of a bad heater core

Sgns of a bad heater core?

The symptoms of a bad heater core may be hard to detect at first but one thing is sure; it won’t fix itself and the symptoms will get progressively worse. Here’s a checklist of what to look for as a sign you’ve got a bad heater core.

How a heater core fails

Your car’s heater core either has hot engine coolant running through it any time your engine is running, or only when you move the temperature dial to HEAT. On vehicles where coolant runs through the heater core whenever the engine is running, the constant flow can slowly wear bad heater core showing corrosion out the inside of the small tubes that make up the heater core. However, that wear accelerates if you’ve neglected changing the coolant on time. That’s because coolant contains anti-corrosion additives to prevent galvanic corrosion between the different metals used in your engine and cooling system.

When the anti-corrosion additives wear out, metal transfer begins and the cooling system circulates small metallic particles. Those particles act like sand to wear out the areas where heater core tubes bend or connect to a common plate. So you’re literally sand blasting your heater core from the inside-out.

heater control valve for heater core

Cable operated heater control valve

Vehicle that use a heater control valve to regulate the flow rate through the heater core experience a different kind of failure. When the heater control valve is closed, there’s no circulation through the heater core and the coolant sits in place. That’s not a problem if the coolant is in good shape. But if it’s old and worn-out, it’ll corrode the heater core from the inside out.

Bad heater core symptoms

Heat isn’t hot. You turn the temperature dial to its highest setting and you get only luke-warm or cold air. This failure is caused by plugged tubing in the heater core that restricts the flow of heat and thus the transfer of heat from the hot engine coolant to the air blowing past the heater core. Sometimes, shops are able to flush the heater core and restore its function. But this procedure must be performed at the early stages. If you ignore the low heat condition, the sediment can harden and prove impossible to remove.

Coolant reservoir is always low. You refill the coolant reservoir under the

bad heater core may result in low coolant level in coolant reservoir tank

Check coolant level in recervoir

hood and within a few weeks, it’s low on coolant again. The system might be leaking from a radiator or heater hose, but you can usually spot those leaks by looking for drips and wet areas. Or, you may have a head gasket leak that’s allowing coolant into the engine oil or the combustion chamber.

If you’ve checked the hoses and head gasket but are still losing coolant, check out a heater core leak.

Coolant drips from your vehicle but the puddle is back near the firewall. When a heater core springs a leak, the leaked coolant drains down the same tube as the AC condensation. So you’ll see a puddle of coolant on the ground near the passenger foot area.

Fogged windshield. If your windshield fogs up and the fog doesn’t clear with the defrosters on, the fog may actually be misting coolant. Wipe your finger across the fog. If your finger feels oily, your heater core is leaking.

Sweet fruity smell when the blower is on. Engine coolant has a sweet fruity smell. If that type of smell gets stronger when you operate the blower, chances are you’ve got a heater core leak.

For more information on what’s involved in flushing a heater core or heater core replacement cost, see this post.

Unsure if your coolant is still good? Read this post to learn how to test for failed anti-corrosion additives.

Wondering if you can fix a leaking heater core with a stop leak product? Read this post for the answer.

©, 2017 Rick Muscoplat

Posted on by Rick Muscoplat

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