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Frighteningly Simple Car Problems: Diesel Engine Runaway

Diesel engines have a lot of upsides, but like any machine, they have their quirks. One of these quirks, known as diesel engine runaway, can cancel your control over the car's acceleration and potentially take you on a terror of a joyride without your consent. Fortunately for all of us, runaway is a rare occurrence on diesel engines, but it can still happen (and it can be serious when it does), so it's important to understand what you might be dealing with.

What is a diesel engine runaway?

In plain terms, diesel engine runaway takes place when your engine suddenly surges to a high RPM, rocketing your car forward if it's in gear. In most modern engines, runaway is caused by the simple malfunction of a small part: leaky seals and bearings on turbochargers in the air intake system.

Runaway happens because diesel engines can combust motor oil as well as gasoline, as you probably know, and leaky seals on your turbos will allow a steady stream of engine oil to find its way into your churning cylinders. Thick, black, sooty clouds coming out of your exhaust are another symptom of runaway. If you've seen how even a small amount of oil burning in a combustion engine can cause the exhaust stream to start looking like smoke, imagine how burning only oil might look.

How can I prevent it?

Regular maintenance of seals and bearings is the best way to keep yourself from ever experiencing a runaway event, but there's really no 100 percent preventative solution. Diesel engine runaway is rare as it is, and it becomes even rarer under diligent maintenance conditions, but there's always going to be a ghost of a chance that you'll spring a turbo bearing leak at exactly the wrong time.

Stopping diesel engine runaway when it happens, however intense the situation might seem, can be a fairly simple affair. If the car is up on a lift or otherwise not in gear when it happens, just choke off the air intake however you can. Rags from around the garage might work, but they're never the smartest solution -- try and find something rigid and flat to block air from reaching the manifold. Firing a carbon dioxide fire extinguisher into the intake will also do in a pinch.

If you're in the driver's seat when this happens, try and maintain as much control of the vehicle as possible. Never brake hard or attempt any tactical strike on your rebel engine until you and your car are as far out of harm's way as possible.

Shifting the car into neutral will decouple the engine from the transmission, allowing you to brake and guide the vehicle to a neutral location. Once you're safe on the roadside, you'll be able to choke the air intake and get your engine calmed down. Stalling the engine can also stop it, so manual transmission drivers can usually shift into a high gear once they've stopped and convince the runaway thing to quit.

Some vehicles also have special valves installed that can block the airway to the manifold. Consult an owner's or service manual to determine if the engine you're dealing with is equipped with this sort of fail-safe.

How common is a diesel engine runaway?

Quite rare, especially in today's cars and trucks. As stated before, though, the fact that it's rare doesn't mean it doesn't happen. And, when it happens, it can be extraordinarily hazardous to both car and driver. It can definitely cause a collision if it surprises you while you're on the road, and it can do mechanical harm to the engine, transmission and all manner of components under any circumstances.

Also, don't forget to keep up on the maintenance of those turbos. Better safe than speeding uncontrollably down the highway, after all.

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