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What Happens When Your Car Overheats?

It's not an unfamiliar sight. You'll see a car speeding down the road with what at first appears to be smoke coming out from under its body. Initially, you might wonder why the driver doesn't grasp that his car is on fire. But it isn't on fire, and that's not smoke from a fire but vapor coming from its overheated cooling system. Most likely, there's a leak in the cooling system somewhere, and that leak could quickly lead to an overheated engine. In turn, an overheated engine could lead to a massive expense.

What Happens When Your Car Overheats

In all types of cars, the engine is the costliest "system." Overheating can leave it beyond repair in a matter of a few ill-timed seconds. Naturally, you might wonder: What happens when your car overheats? Before we get to the final answer on that, though, let's examine why engines overheat in the first place.

What Causes a Car Engine to Overheat?

Typically, overheating is not the fault of the engine itself but rather a problem with the cooling system. The cooling system’s job is to maintain a proper operating temperature for the engine.

In a 4-cylinder engine running at 2,000 rpm (revolutions of the crankshaft per minute) an explosion is happening in each cylinder more than 15 times per second, producing a lot of heat. To deal with that heat the engine block and heads have passages through which antifreeze/coolant circulates to transfer the heat from the engine. The coolant goes from the engine through a hose to the vehicle's radiator. There, the heat from the coolant is transferred to the air passing through the radiator. The coolant, now at a lower temperature, then goes back to the engine in an endless loop.

Overheating can occur when one of the coolant passages in the engine block or head becomes clogged or starts to leak. But it is more likely to occur through some fault in the cooling system itself. For instance, there could be a leak in a hose that transfers the coolant from the engine to or from the radiator. Those hoses "wear" or deteriorate over time, so they fail. And when they do a hose that costs just a few dollars could be the cause of the destruction of an engine that costs several thousand dollars.

Another common cause of overheating is thermostat malfunction. The thermostat in an automobile cooling system doesn't resemble the thermostat in your home that turns your furnace off and on, but it does perform a somewhat similar function. It is a heat-sensitive mechanical device that opens when it reaches a specific temperature, allowing coolant to pass through it. That enables your car's engine to warm up to a prescribed temperature before the coolant passes from the engine to the radiator. If the thermostat fails in the closed position, it won't let the coolant travel to the radiator and overheating results.

A very simple and easy-to-fix situation is another cause of overheating: low coolant level. Coolant evaporates from the system over time, and if there is not enough coolant passing through the engine to carry away the heat, overheating is the result.

The radiator cap is a potential problem area, too. Should the engine's radiator cap be faulty it won't maintain pressure in the cooling system, and that could cause the engine to overheat. Modern cooling systems are closed, pressurized systems because that enables more efficient cooling without "boil-over."

Finally, an engine can overheat because it does not have sufficient lubricating oil in the crankcase and coursing through the engine. Like coolant, oil helps dissipate heat, so if there isn't enough of it, overheating — plus extreme engine wear — can result.

How Do I Know if My Car’s Engine is Overheating?

There are two distinct signs that your engine is overheating. First and foremost, a malfunction warning light (commonly known as an "idiot light") will begin to glow on your instrument panel. The warning is typically red or yellow and ominous. It is meant to get your attention because you need to take quick action.

The other sign that your engine is overheating is the plume of white or light gray vapor that comes from the leak in your cooling system. As you drive it is often difficult to see because it trails behind as your vehicle moves forward, but at a stoplight, it appears as though your car is creating its own fog.

What Should I Do if my Car is Overheating?

So what should you do if your engine overheats? First, you should be aware that the engine could be seriously damaged if you continue to drive for more than a short distance when it is overheating. Two immediate steps to take are:

1. Turn off the vehicle's air conditioning

2. Turn on the car's heater and set the fan at full-blast.

The reasons for this are simple. The air conditioning puts additional stress on the cooling system, stress that results in higher temperature. In contrast, the car's heater is actually an internal radiator through which the car's coolant passes. Turning it on can help keep the engine nearer its proper temperature range.

That said, you don't want to drive very long or very far with the engine overheating. Doing so risks catastrophic engine failure and a repair bill in the thousands of dollars.

If you are at all in doubt you should calmly and safely pull over to the side of the road and turn off the engine. But do not — REPEAT, DO NOT — immediately open the hood. The super-heated coolant might spew on you and burn your skin. Instead, wait several minutes until you are certain the engine has cooled. Then open the hood.

Various types of cars have similar cooling systems, most often with an overflow reservoir, and if that reservoir is empty you can try adding coolant and then restarting the engine. If low coolant level was the issue, this should cure the problem.

If it doesn't, it is time to call for a tow truck. Should you try to drive your vehicle for any real distance while it is overheating, you might as well get out a car loan calculator and begin to compare cars on the JDPower.com website. Buying a new car might just be a better bet than replacing an engine ruined by overheating.