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Lack of heat

A common wintertime complaint

The passenger compartments of motor vehicles are heated by using the heat generated by the engine. When the outside temperatures get down to the freezing zone, a lack of heat in your car becomes an evident and urgent concern. Usually this problem is easily corrected, even by the owner.

First, let’s review how the climate control system works for heating the vehicle interior.

• 1. The engine, when running, produces a lot of excess heat
• 2. This heat must be managed and dissipated
• 3. A mixture of water and ethylene-glycol is circulated by a belt-driven water pump through the engine, heater core and radiator
• 4. The coolant is pumped, usually directly, through the heater core which is located in the car inside the heater plenum. Some vehicles use a control valve to stop flow through the heater when it is not needed. For our purposes, consider the heater core and hoses as part of the engine since engine coolant is circulated continually through the core.
• 5. Engine coolant temperature is controlled by a heat sensitive thermostat.                                                 The thermostat blocks coolant flow through the radiator until a certain temperature is reached. That temperature is typically 192 degrees Fahrenheit. Correct engine operating temperature is critical to engine and heater performance

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If heat output is poor and the engine temperature gauge* stays low, you might as well just replace the thermostat. Why? Because thermostats are, by far, the leading cause of poor heat. They are usually easy to replace and the cost of parts is low. Since you are still in guessing territory, a new thermostat may not fix the problem, but if not, don’t feel bad. You didn’t spend much time or money and you now have a new thermostat, a part that fails so often that it should be considered a maintenance item. If you opt to have a mechanic do the work, you can avoid diagnostic charges by specifying that only the thermostat be replaced. The mechanic should be okay with this because she, (or he), is off the hook for diagnosis responsibility. If the new thermostat does not produce better heat, then you, the customer, are responsible for that decision. If you then have the mechanic continue on to fix the problem, you have already completed a big part of diagnosis. If you want to do the diagnosis and repair yourself, then continue reading.

So, its winter and you are cold in your car. Is the heater working right, and if not, why not?

Over the years, I have found these simple tests to be helpful in finding the cause of low heater output.

• 1. With the engine warmed up to normal temperature, the temperature gauge on the instrument cluster at or just below midrange, the heater temperature control full hot, the fan control in the middle setting, and the air flow set to vent or floor. Place your hand close to the air duct. The air coming out of the heater should be hot enough that it quickly becomes uncomfortable. An actual temperature of 130 degrees Fahrenheit is usually sufficient. I see most vehicles at from 145 to 160 degrees. If the airflow is not uncomfortably hot or is less than 130 degrees, check the temperature of the coolant flowing in and out of the heater core.

2. This test is done at the heater hoses near the heater core, but on the engine side of the firewall. As the engine warms up, these hoses should each be at nearly the same temperature as the coolant in the engine. With a hot engine, they should be too hot to hold for more than a few seconds. If the engine is hot, but these hoses are only warm, then you are back to changing the thermostat. If one hose is hot and the other warm, then the problem is probably some blockage in the heater core itself. For the heater to work properly it needs hot coolant and plenty of it. 

• 3. If the engine and the heater core are plenty hot but the air at the heater vents is only lukewarm, more testing is needed. On vehicles with A/C the airflow is directed through the air conditioner evaporator first, then through the heater core. Outlet temperature is controlled by a temperature blend door. As the name implies, this door can give passengers full cold or full hot air or blend the two for any desired temperature in between. By passing the incoming air through the A/C evaporator first the air will be dried, (only if the control is set to defrost or another position where the air conditioner is on), when the cold evaporator condenses moisture out. From there the chilled air is passed through the heater to reheat the air for passenger comfort. This dried air helps to clear fogged windows. By now you can probably see that if the blend door is sticking, or the controls are faulty, some cool air could be blended in when it is not wanted. Blend door control systems can be simple or complicated. The simplest ones use a cable for a direct input from the heater control to a lever connected to the door. More complex systems use an electronic control and an electronic actuator that can monitor door position. Even more complicated are those with automatic controls and dual heat. In any case, sometimes it is possible to peek in at the plenum, find the blend door parts and watch what happens when different temperatures are selected. Using a flashlight and feeling around with your hand may get you a long way to solving the problem.
• 4. For vehicles with vacuum controls, a common problem is a damaged vacuum supply line. Look under the hood for a thin, wire-sized, plastic line going into the heater box or firewall. You should find a rubber connection somewhere along this line. Disconnect and check for vacuum while the engine is running
• 5. There are a few other possibilities for lack of heat. A bad head gasket can allow combustion gasses to enter the cooling system and push coolant out of the heater core. You may have cold air leaking in from the outside from a missing or damaged grommet or seal on the firewall or passenger door. Finally, it is possible that the outside temperature is soooo cold that the heater has a difficult time keeping up. Try warmer clothes.

I hope this rough overview of the heating system helps you get on the right path toward fixing your car. My next task will be to put together a glossary where I will explain the words in italics.

* Not all vehicles have an engine temperature gauge, but instead have a warning light that comes on when an overheat condition is detected.