Why Does My Car Overheat When I Turn on the Heater?by Richard Rowe
An engine's cooling system performs a fine balancing act under even the best of circumstances. While rerouting fluid through the heater core won't generally impede system function, it will exacerbate any existing problems. If activating your heater causes your engine to overheat, it's time to take a close look at the system to find minor problems on the verge of escalating into major ones.
Heating System Basics
Your car's heater is kind of like a miniature radiator. The difference between the primary radiator and the heater core is that air going through the primary radiator comes in through the front grille and exits into the engine bay, while air passing through the core comes in through intake ducts in the cab and comes out the vents. On most cars, the heater core is a bypass system -- it draws fluid directly from the engine without waiting for it to pass through the thermostat. This allows the heater to come online before the engine gets up to full operating temperature.
Your engine's cooling system builds up an enormous amount of pressure. Under normal circumstances, this pressure will prove sufficient to shove coolant through an almost completely clogged radiator. But opening the heater core valve will give the pressurized fluid another route to flow through, and it'll always follow the path of least resistance. The net result is a searing hot jet of air flowing from the heater vents, and engine overheating as a result of little to no coolant flowing through the radiator.
Even under the best of circumstances, a thermostat valve provides a restriction to coolant flow through the system, and pressure holds it closed. This slight restriction means that engineers must perform a fine balancing act when designing the system. Too small a valve and water can't flow through; too large a valve and pressure in the system will keep it shut. Many times, thermostats won't fail in either the completely open or closed position but will instead get stuck halfway. A stuck thermostat will have the same net effect as a clogged radiator, allowing pressure to release through the heater core instead of shoving through the stuck valve.
Low Coolant Levels
Don't think of low coolant levels as an absence of coolant so much as they are an excess of air. Air, being lighter than water, will always try to rise to the highest point in the system, and that's your upper radiator hose. Once water levels drop past a certain point, the upper hose will fill with air and the system will fail. Opening the heater core valve will suck another gallon or so of water out of the system, dropping fluid levels in the motor and pushing your cooling system just over the edge so it fails to function.
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