Got Heat? What Your Car's Radiator Output Says About Problems Under the Hood

by Rob Hopp, Cars for Keeps Owner

If you hold your fingers above the defrost vents with the temperature set on high, as a general rule, you shouldn't be able to leave your fingers there for very long without them getting pretty uncomfortable - if the heating system is working well.

When we perform routine inspections (aka "Pit Stops"), we usually check the heat output at the center vent; a good heater thermostat will put out 150° temps.

We have seen them run as high as 160°f. At 130°f degrees you may start getting a few complaints, though it’s still pretty livable, at 120°f people are getting unhappy, the coats are staying on and below 110°f NO ONE is happy.

More importantly, a malfunctioning heater often points to deeper problems below the hood - problems which, if left unattended, could result in interior coolant leakage, engine inefficiency or an overheated engine.

Is it the radiator, the heater core or the thermostat?

These are the three components which influence your radiator's heating performance the most. Small, easy to fix problems in any of these can result in low heater output, and more serious problems with any of them can lead to mechanical failure or damage to other parts of your vehicle.

Radiator - The radiator is the component which cools engine coolant to keep the engine cool. When vehicles' heaters are on, the heat extracted (by air cooling) from the coolant (heater core) is pushed through the dashboard heater vents. Clogs, cracks or leaks in the radiator impede coolant fluid flow, resulting in low heat out put and, in many cases, engine overheating.

Heater core - The heater core is essentially a heat exchanger which transfers heat from the engine to the radiator. Hot coolant fluid is passed through the core's winding tubes from the engine, which heats the fluid, to the radiator, where fans blow hot air off the heater core tubes and through the vents. Heater cores are made up of small tubes which sometimes get clogged, causing radiator failure. A clogged heater core also means the engine is not being properly cooled, which can lead to engine damage if left unattended. Heater cores may also become leaky, resulting in lower heat output and improper engine cooling.

Thermostat - The thermostat is what activates the flow of coolant fluid through the engine. It controls a small, heat-activated valve in the heater core which blocks or opens coolant flow through the engine. To minimize engine wear and make the most efficient use of engine energy, thermostats ensure that coolant doesn't flow through the engine until it reaches its maximum operating temperature (about 200 degrees F). When thermostats aren't working correctly, they could be stuck open - causing inefficiency - or they could be stuck closed, causing the engine to become hot and blocking hot air from coming out of the car's heating vents.

If you stop by with your car, mini-van, light truck or SUV, we'll test the output for free!
While the symptoms can seem similar, subtle differences in your description or in our testing tell us which component is to blame. Most clients aren't aware that something as simple as low coolant (antifreeze) can cause some pretty weird symptoms.